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Full text of "(1) Elements of the petrographic study of bonding clays and of the clay substance of molding sands"

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STATE OF ILLINOIS 

JOHN STELLE, Governor 

DEPARTMENT OF REGISTRATION AND EDUCATION 

JOHN J. HALLIHAN. Director 

DIVISION OF THE 

STATE GEOLOGICAL SURVEY 

M. M. LEIGHTON, Chief 
URBANA 



REPORT OF INVESTIGATIONS — NO. 69 



(1) Elements of the Petrographic Study oi Bonding Clays 
and of the Clay Substance of Molding Sands 

Ralph E. Grim 

(2) Mineral Composition and Texture of Clay Substance 
of Natural Molding Sands 

Ralph E. Grim and Carl E. Schubert 

(3) The Relationship Between the Physical and Mineralogical 
Characteristics of Bonding Clays 

Ralph E. Grim and Richards A, Rowland 



Reprinted from the Transactions of the American Foundrymen's Association, 
Vol. 47, No. 4, pp. 895-908; 935-953; Vol. 48, No. 1, pp. 211-224, 1940. 




PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS 



URBANA, ILLINOIS 
1940 



STATE OF ILLINOIS 

HON. JOHN STELLE, Governor 

DEPARTMENT OF REGISTRATION AND EDUCATION 

HON. JOHN J. HALLIHAN, Director 



EDSON S. BASTIN, Ph.D 
WILLIAM A. NOYES, Ph.D 

Chemistry 
LOUIS R. HOWSON, C.E., Engineering 



BOARD OF 
NATURAL RESOURCES AND CONSERVATION 

HON. JOHN J. HALLIHAN, Chairman 
Geology WILLIAM TRELEASE 



LL.D., Chcm.D., D.Sc, 



D.Sc., LL.D., Biology 
EZRA JACOB KRAUS, Ph.D., D.Sc., Forestry 
ARTHUR CUTTS WILLARD, D.Engr., LL.D. 
President of the University of Illinois 



STATE GEOLOGICAL SURVEY DIVISION 

Urban a 
M. M. LEIGHTON, Ph.D., Chief 



ENID TOWNLEY, 
JANE TITCOMB, 



M.S., Assistant to the Chief 
M.A., Geological Assistant 



GEOLOGICAL RESOURCES 

Coal 

G. H. CADY, Ph.D., Senior Geologist and Head 
L. C. McCABE, Ph.D., Geologist 
JAMES M. SCHOPF, Ph.D., Asst. Geologist 
J. NORMAN PAYNE, Ph.D., Asst. Geologist 
CHARLES C. BOLEY, M.S., Asst. Mining Eng. 

Industrial Minerals 

J. E. LAMAR, B.S., Geologist and Head 
H. B. WILLMAN, Ph.D., Assoc. Geologist 
DOUGLAS F. STEVENS, M.E., Research Associate 
ROBERT M. GROGAN, Ph.D., Asst. Geologist 
ROBERT R. REYNOLDS, B.S., Research Assistant 

Oil and Gas 

A. H. BELL, Ph.D., Geologist and Head 
G. V. COHEE, Ph.D., Asst. Geologist 
FREDERICK SQUIRES, B.S., Assoc. Petr. Eng. 
CHARLES W. CARTER, Ph.D., Asst. Geologist 
WILLIAM H. EASTON, Ph.D., Asst. Geologist 
ROY B. RALSTON, B.A., Research Assistant 
WAYNE F. MEENTS, Research Assistant 

Areal and Engineering Geology 

GEORGE E. EKBLAW^ Ph.D., Geologist and Head 
RICHARD F. FISHER, B.A., Research Assistant 

Subsurface Geology 

L. E. WORKMAN, M.S., Geologist and Head 
ELWOOD ATHERTON, Ph.D., Asst. Geologist 
MERLYN B. BUHLE, M.S., Asst. Geologist 
I. T. SCHWADE, M.S., Asst. Geologist 
FRANK E. TIPPIE, B.S., Research Assistant 

Stratigraphy and Paleontology 

J. MARVIN W^ELLER, Ph.D., Geologist and Head 
CHALMER L. COOPER, M.S., Assoc. Geologist 

Petrography 

RALPH E. GRIM, Ph.D., Petrographer 

RICHARDS A. ROWLAND, Ph.D., Asst. Petrographer 

Physics 

R. J. PIERSOL, Ph.D., Physicist 

DONALD O. HOLLAND, M.S., Asst. Physicist 

PAUL F. ELARDE, B.S., Research Assistant 



GEOCHEMISTRY 

FRANK H. REED, Ph.D., Chief Chemist 
W. F. BRADLEY, Ph.D., Assoc. Chemist 
G. C. FINGER, Ph.D., Assoc. Chemist 
ROBERTA M. LANGENSTEIN, B.S., Research Assist- 
ant 

Fuels 

G. R. YOHE, Ph.D., Assoc. Chemist in Charge 
CARL HARMAN, M.S., Research Assistant 

Industrial Minerals 

J. S. MACHIN, Ph.D., Chemist and Head 
JAMES F. VANECEK, M.S., Research Assistant 

Analytical 

O. W. REES, Ph.D., Chemist and Head 
L. D. McVICKER, B.S., Asst. Chemist 
GEORGE W. LAND, M.S., Research Assistant 
P. W. HENLINE, M.S., Asst. Chemical Engineer 
MATHEW KALINOWSKI, M.S., Research Assistant 
ARNOLD J. VERAGUTH, M.S., Research Assistant 
WILLIAM F. WAGNER, M.S., Research Assistant 



MINERAL ECONOMICS 

W. H. VOSKUIL. Ph.D.. Mineral Economist 
GRACE N. OLIVER, A. B., Assistant in Mineral Eco- 
nomics 



EDUCATIONAL EXTENSION 

DON L. CARROLL. B.S., Assoc. Geologist 

PUBLICATIONS AND RECORDS 

GEORGE E. EKBLAW, Ph.D.. Geologic Editor 
CHALMER L. COOPER. M.S.. Geologic Editor 
DOROTHY ROSE. B.S.. Technical Editor 
KATHRYN K. DEDMAN, M.A., Asst. Technical 

Editor 
ALMA R. SW^EENY, A.B., Technical Files Clerk 
FRANCES HARPER LEHDE, A.M., Asst. Technical 

Files Clerk 
JEWELL WALCHER, Asst. Technical Files Clerk 
MEREDITH M. CALKINS, Geologic Draftsman 
LESLIE D. VAUGHAN, Asst. Photographer 
DOLORES THOMAS SIMS, B.A., Geologic Clerk 



Consultants: Ceramics, CULLEN W. PARMELEE, M.S., D.Sc, and RALPH K. HURSH, B.S., University of Illinois; 
Pleistocene Invertebrate Paleontology, FRANK COLLINS BAKER, B.S., University of Illinois. 
Topography Mapping in Cooperation with the United States Geological Survey. 

This Report is a Contribution of the Petrography Division. 



ILLINOIS STATE GEOLOGICAL SURVEY 




3 3051 00005 7186 



(A32934— 1500— 10-40) 



November 1, 1940 



^^1 


rvo, G) 


(L, a 



CONTENTS 



PAGE 

Elements of the Petrographic Study of Bonding Clays and of the Clay Substance of Molding Sands: 

Ralph E, Grim 5 

Mineral Composition and Texture of the Clay Substance of Natural Molding Sands: Ralph E. Grim 

and Carl E. Schubert 12 

Relationship Between the Physical and Mineralogical Characteristics of Bonding Clays: Ralph E. Grim 

and Richards A. Rowland 24 



Digitized by the Internet Archive 

in 2012 with funding from 

University of Illinois Urbana-Champaign 



http://archive.org/details/1elementsofpetro69grim 



ELEMENTS OF THE PETROGRAPHIC STUDY OF BONDING 

CLAYS AND OF THE CLAY SUBSTANCE 

OF MOLDING SANDS* 

By Ralph E. Grim 

ABSTRACT 

This paper briefly outlines the modern methods for the study of clay materials, the pre- 
vailing concept of the composition of clays, and the application of modern clay researches 
to studies of natural molding sands and bonding clays. An appended bibliography of a 
selected list of reports giving details and theoretical considerations of points discussed is 
included. 



INTRODUCTION 

It has been known for a long time that 
molding sands are essentially mixtures of 
silica sand and clay substance with more or 
less yellow or red hydrated ferric iron oxide. 
This fact is recognized in the preparation of 
synthetic sands by mixing silica sand and 
bonding clay. 

Papers^ have been published in the A.F.A. 
Transactions describing in detail the char- 
acteristics of the silica : shape of grains, grain 
size distribution, etc. Detailed information 
on the clay substance has not been obtained 
and investigations of molding sands state 
only that the quality of the clay substance 
varies in different sands without making any 
attempt to study it in detail. Some reports 
give chemical analyses of the clay, but it 
is generally recognized that chemical data 
alone tell very little about the character of 
a clay. 

Within the last ten years a large amount 
of work has been done on the development 
of methods for the study of clays and in 
actually studying them by these new meth- 
ods. A large number of publications in 
many scientific journals have reported new 
and important data on the composition and 
properties of clay materials. As a conse- 
quence of this recent work, the character- 
istics of the clay substance of a molding 



*Reprinted from Trans. Am. Foundrymen's Assoc, 
VoL 47, No. 4, pp. 895-908, 1940. 

iRies, H., and Conant, G. D., The character of sand 
grains: Trans. Am. Foundrymen's Assoc, VoL 39. pp. 
353-392, 193L 

Ries, H., and Lee, H. V., Relation between shape of grain 
and strength of sand: Trans. Am. Foundrymen's Assoc, 
VoL 39, pp. 857-860, 193L 

Note: This paper was presented at the Sand Research 
session of the 43rd annual A.F.A. Convention, Cincinnati, 
O., May 17, 1939. 



sand can now be determined as thoroughly 
as the characteristics of the silica sand por- 
tion. 

It is proposed in the present paper to 
briefly outline the modern methods for the 
study of clay materials, to state the prevail- 
ing concept of the composition of clays, and 
to point out the application of modern clay 
researches to studies of natural molding 
sands and bonding clays. For details and 
theoretical considerations of any points con- 
sidered herein, reference should be made to 
the reports of the work on which this paper 
is based. The appended bibliography is a 
selected list of such reports. 

CLAY COMPOSITION 

If a pure clay^ could be examined with 
a microscope magnifying many thousand 
times, it would be found that it was nothing 
but an aggregation of flake shaped particles. 
The actual size of these flakes would vary 
from several microns^ to less than 0.1 mi- 
cron in diameter. A working picture of the 
makeup of clays may be had if one starts 
with large flakes of mica and then reduces 
the size of the flakes until each flake is about 
one micron in size. A mass composed of 
such flakes approximates the makeup of a 
clay. If the flakes composing clays are ana- 
lyzed, it is found that they are composed of 
atoms of aluminum, silicon, oxygen, and 
hydrogen. Potassium, magnesium, and/or 
iron would be found in the flakes of some 
clays. 

'■^The discussion of clay composition holds for almost all 
clays. There may be, however, a few relatively unim- 
portant clay materials which have a composition slightly 
different from that presented. 

^One micron is one thousandth of a millimeter (0.001 mm.) 
or about one twenty-five thousandth of an inch (0.00004 in.). 



[5] 



PETROGRAPHIC STUDY 



The atoms have a definite arrangement 
in the flakes ; e.g., the silicon atoms have 
fixed positions with respect to the oxygen 
atoms, the aluminum atoms have definite 
positions with respect to the oxygen atoms, 
and so on for the other elements. Substances 
composed of atoms arranged in a definite 
pattern are crystalline, and hence the flakes 
which compose clays are crystalline. The 
flakes are minute fragments of crystals. 

That fraction of clay composed of parti- 
cles smaller than a given size (±1 micron) 
is the so-called colloid fraction. The col- 
loidal material in clay is made up of crystal- 
line clay mineral flakes, and it is not an 
amorphous heterogeneous mixture of silica, 
alumina, etc. 

There are several important kinds of 
flakes which make up clays. All the different 
kinds of flakes are composed of about the 
same atoms, but for each kind of flake there 
is a distinctive and different arrangement of 
the atoms. From the viewpoint of mineral- 
ogy, the different kinds of flakes are different 
mineral species and warrant different min- 
eral names. These minerals, which are the 
essential constituents of clays, are called clay 
minerals. Extensive analysis of clays has 
shown that there are only three important 
clay minerals, and that almost all clays are 
composed essentially of extremely minute 
flake-shaped particles of one or more of these 
three minerals (Table 1). In addition to 
the clay minerals, minor amounts of quartz, 
organic material, limonite, and other min- 
erals, are also found in many clays. Some 
examples of the composition of clay mate- 
rials are as follows : Bentonites are made up 
of extremely minute flakes of montmoril- 
lonite, shales are composed usually of par- 
ticles of illite frequently with quartz and 
other minor constituents, fireclays are usual- 
ly mixtures of flakes of kaolinite and illite, 
and kaolins and china clays are made up 
essentially of particles of kaolinite. 

Table 1. — Important Clay Minerals 

Name Chemical Composition 

Kaolinite (OH)8Al4Si40io 

Illite (0H)4Ky (Al4-Fe4-Mg4-Mg6) 

(Sis-yAly) O20 
Montmorillonite, . (OH)4Al4Si8O20-XH2O 

Since the atoms are arranged differently 
in the three important species of clay min- 
erals, it follows that their characteristics, 



and the physical properties of the clay which 
they make up will be different; e.g., the 
characteristics of kaolinite will be unlike 
those of montmorillonite and the physical 
properties of a clay composed of kaolinite 
will be different from those of a clay com- 
posed of montmorillonite. 

CLAY MINERAL PROPERTIES 

In the following discussion some of the 
characteristics of the three important clay 
minerals which are related to the properties 
of natural molding sands and bonding clays 
are considered. 

The montmorillonite clay minerals usual- 
ly occur in particles less than 1 to 0.1 micron 
in diameter, or in larger particles which 
are easily reduced to this size when the clay 
is worked with water. Kaolinite occurs in 
particles which are rarely smaller than 1 
micron and which are not easily broken 
down by working in water. Most illite 
occurs in particles about the same size as 
kaolinite, but there are some clays in which 
the illite flakes are much smaller. It fol- 
lows, therefore, that if a clay composed of 
montmorillonite is compared with one com- 
posed of kaolinite, the montmorillonite clay 
will be made up of smaller flakes than the 
kaolinite clay. As a consequence, a given 
amount of montmorillonite clay will contain 
a larger number of flakes, and a larger total 
flake surface than will a kaolinite clay. As 
many of the properties of clays are closely 
related to the size of their component parti- 
cles, it follows that this difference between 
montmorillonite and kaolinite would cause 
clays composed of montmorillonite to have 
properties that differ from those composed 
of kaolinite. 

All the clay minerals have the power to 
adsorb certain ions. Thus, if a solution con- 
taining lime is passed through a clay some 
of the lime will be taken out of solution by 
the clay unless the clay already has all the 
lime it can adsorb or the lime solution is 
too dilute. The adsorbed ions are exchange- 
able, e.g., if a clay carrying lime is treated 
with a potash solution, some of the adsorbed 
lime will be replaced by potassium ions. 
Hydrogen, sodium, potassium, calcium, and 
magnesium are the common exchangeable 
ions held by clays. Montmorillonite has 
about ten times as much capacity as kaolinite 
to hold adsorbed ions. The capacity of illite 
varies; some illite has the capacity of kaolin- 



OF BONDING CLAYS 



ite, whereas other illite has several times 
this capacity. The difference in the adsorp- 
tive capacity of clay minerals is illustrated 
by their dye adsorption; a montmorillonite 
clay will adsorb much more dye than a 
kaolinite clay. 

It is known in a general way that the 
physical properties of clays vary with the 
ion which the clay carries. An example will 
illustrate the point; if the green and dry 
compression strengths of two montmorillo- 
nite clays are compared, one of which carries 
hydrogen and the other sodium as the ex- 
changeable ion, it will be found that the 
hydrogen clay has higher green strength 
than the sodium clay, and that the hydrogen 
clay has lower dry strength than the sodium 
clay. The exact variation of bonding prop- 
erties caused by various exchangeable ions 
is a promising field of future research which 
remains to be worked out. 

The clay minerals differ from each other 
in their refractoriness and in their dehydra- 
tion characteristics. Kaolinite fuses at a 
much higher temperature than either mont- 
morillonite or illite. Kaolinite loses all of 
its water when it is heated to about 900° F., 
and after subjection to this temperature does 
not again regain its moisture or its physical 
properties when cooled to ordinary tempera- 
tures. Montmorillonite may be heated to 
about 1025°F. before its moisture is perma- 
nently removed and its physical properties 
are destroyed. The dehydration character- 
istics of illite are not well known. The 
above temperatures are equilibrium temper- 
atures. Montmorillonite, for example, must 
be held at 1025°F. for a considerable period 
of time before it is completely changed. 
When montmorillonite is heated to 1025°F. 
and immediately cooled, only a small amount 
of it is irreversibly dehydrated. 

It is clear from the foregoing considera- 
tions that clays composed of different clay 
minerals must have different properties. A 
large amount of work must be done before 
the relation between the various clay min- 
erals and bonding properties are well under- 
stood, but some information on this subject 
is available. It is known, for example, that 
montmorillonite clays have higher compres- 
sion strengths than kaolinite clays, and that 
some illite clays are weak, whereas others 
have high strength. Thus, a mixture of 95 
per cent sand and 5 per cent clay composed 
of montmorillonite will have greater green 



and dry strengths than a mixture of 95 per 
cent sand and 5 per cent kaolinite clay at 
their optimum moistures. The properties of 
bonding clays depend, therefore, on the clay 
minerals of which they are composed. Also, 
as brought out before, the properties will 
vary depending on the exchangeable ions 
which they contain. 

Similarly, it is clear that two natural 
molding sands with the same fineness char- 
acteristics, and the same amount of clay will 
not necessarily have the same strength. In 
fact the natural sands can not have the same 
strength unless their clay is made up of the 
same clay mineral carrying the same ex- 
changeable base. It is obvious, then, that 
the properties of two sands cannot well be 
compared without information on the char- 
acter of their clay mineral content. 

DETERMINATION OF MINERAL 
COMPOSITION 

It is easy to study the characteristics of 
the sand grains and of the coarse silt in 
molding sands with the microscope. Clays 
generally or the clay substance of molding 
sands cannot be studied so easily because 
they are composed of particles which are so 
small that they cannot readily be seen with 
the microscope even using very high mag- 
nification, much less identified and studied. 
These very small particles are mostly the 
clay mineral flakes just mentioned, and only 
lately has it been possible to devise tech- 
niques for their adequate study. In the fol- 
lowing paragraphs these techniques are re- 
viewed briefly. 

X-RAY Method 

When a beam of x-rays is passed through 
a crystalline substance, the beam is reflected 
and refracted from the planes of atoms 
which make up the crystal. The x-rays 
emerge from the crystal as a series of beams 
which can be recorded on a photographic 
film as a series of lines (fig. 1 ) or dots, 
depending on the details of the procedure 
followed and the character of the material. 
The position, intensity, and number of 
beams emerging from any crystalline sub- 
stance depend on the character of its atoms 
and their arrangement in the substance. 
Thus, it follows that if two crystalline sub- 
stances with different atomic structures are 
placed in front of x-ray beams and the 



PETROGRAPHIC STUDY 






KAOLJNITE 




MONTMGRILLONITE 




ILLITE 




r-'-%| ■if'^. 



Fig. 1— X-ray Diffraction Patterns. (After W. Noll., Ber. Deut. Keram. Qes. 19, p. 181, 1938) 



emerging beams are recorded on photo- 
graphic films, the pattern of the lines on the 
films will be different. 

The clay minerals have different atomic 
structures and, therefore, yield different 
x-ray patterns. Thus, if a beam of x-rays 
is sent through a clay sample and the emerg- 
ing beams recorded, it is possible to deter- 
mine from the recorded beams the minerals 
which make up the clay, regardless of the 
fact that the particles composing the clay 
are extremely small. The identification of 
the minerals is made by comparing the pat- 
tern from the clay with patterns of known 
pure mineral material. 

One of the greatest difficulties in the iden- 
tification of the constituents of clays has 
been to obtain pure samples of many of the 
minerals found in clay for determining 
standard analytical data. Thus, it is difficult 
to obtain pure illite for the study of its 
x-ray, optical, chemical, and other proper- 
ties which can be used as a basis of compari- 
son and hence for the identification of illite 
in clays generally. 

It is frequently possible to work out the 
exact arrangement of the atoms within a 
crystal from the pattern of the emerging 
beams of x-rays. In recent years a large 
body of data has become available on the 
arrangement of the atoms within the various 
clay minerals. This work is providing, per- 
haps for the first time, a fundamental, basic 
explanation of the physical properties of 
clays. 



Microscopic Method 

One difference between crystalline and 
noncrystalline substances is that in crystals 
the velocity of light traveling through them 
depends on the direction of the path of light. 
In a flake of mica for example, light passing 
through the flake at right angles to the flake 
surface has a different velocity than light 
passing through parallel to the flake. An- 
other character of crystalline material is 
that light passing through it is polarized, 
i.e., broken up into light vibrating in only 
one plane. As a result of these phenomena 
crystalline substances have certain optical 
properties. The optical properties of crys- 
tals are dependent on their atomic structure 
and, therefore, materials with different crys- 
tal structures, such as the clay minerals, 
have different optical properties. The petro- 
graphic microscope is constructed so that 
optical properties can be measured and, as a 
consequence, minerals can be identified. Sat- 
isfactory determinations, however, can only 
be made on individual particles coarser than 
about 1 micron, which is larger than many 
of the clay mineral particles in clays. This 
limitation to the application of petrographic 
microscopic technique in the study of clays 
has been overcome by taking advantage of 
the flake shape of the clay mineral particles. 
Aggregates of clay minerals can be prepared 
in such a way that the flakes rest on top of 
each other in the same relative crystallo- 
graphic position. The optical properties of 
the aggregates can be measured as if they 



OF BONDING CLAtS 



12 



















, 












MOK 


TMORI 


LLONl 


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/^ 












^ 




— 


^ 






z' 


^ 


i 


-ILL 


ITE 








/ 


/ 


/ 


1 












^ 


^ 


^ 




♦-KAC 


LINIT 


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200 300 400 500 600 700 
TEMPERATURE - DEGREES C 

Fig. 2- — ^Dehydration Curves 



800 900 



were large individual cr3^stals. In this way 
the component particles can be identified 
even though they are too small to be seen 
individually. The aggregates are prepared 
by carefully drying suspensions of the clay. 

Dehydration Method 

The clay minerals contain different 
amounts of water, and they lose their water 
at different temperatures when heated (fig. 
2). Thus, it would seem possible to deter- 
mine which clay minerals are present in a 
clay by determining the amount of water 
present and the loss of water as the clay is 
heated. The method has been used in clay 
studies, but it must be used with caution 
chiefly because the dehydration character- 
istics of the clay minerals vary with the size 
of the particles in which they occur. It has 
been shown, for example, that extremely 
finely ground mica has dehydration proper- 
ties that differ from those of the same mica 
composed of coarser particles. 

Chemical Method 

Past studies of clay materials have fre- 
quently included chemical analyses. In gen- 
eral, chemical analyses alone do not permit 



an identification of the mineral components 
of clay substances. Such determinations give 
values for the amount of silica, alumina, 
etc., in the substance but do not indicate 
in what minerals they are present. There- 
fore, the chemical data do not give informa- 
tion on the units making up the clay which 
largely determine its properties, i.e., the clay 
minerals. Along with x-ray and optical 
analyses, chemical data are helpful ; alone 
they have little value. 

Application of Present Methods 

The study of clay materials has shown 
that they are frequently composed of mix- 
tures of clay minerals. Clay minerals occur- 
ring in mixtures are particularly difficult to 
study and identify because, although the 
optical and x-ray properties of the clay min- 
erals on which identification is based are 
different, the differences are not great and 
frequently the analytical data for mixtures 
cannot be interpreted readily. For example, 
x-ray and optical analytical data for a clay 
composed of a large proportion of one clay 
mineral, and a minor amount of another clay 
mineral, may not positively indicate the 
presence of the minor component. To over- 
come this difficulty, a fractionation proced- 



10 



PETROGRAPHIC STUDY 



ure has been developed which literally takes 
the cla}^ apart into fractions of its component 
minerals. The object is to isolate the clay 
minerals in fractions of sufficient purity so 
that they can be identified positively. 

The general procedure is as follows: The 
material is disaggregated and placed in sus- 
pension in water, using ammonia as the 
dispersing agent. The suspension is allowed 
to stand until the material coarser than 2 
microns has settled out. The suspension 
carrying the — 2 micron particles then is 
removed and saved, and the settled material 
again is placed in suspension and allowed to 
stand until particles coarser than 2 microns 
have settled. This process is repeated until 
the material coarser than 2 microns has been 
freed of particles smaller than this size, 
i.e., the sample has been split into a fraction 
carrying -\-2 micron particles and a suspen- 
sion carrying finer particles. The particles 
in the coarse fraction can be identified and 
studied individually by means of the petro- 
graphic microscope. 

Oriented aggregates are prepared of the 
material in the suspension and they are 
studied with the petrographic microscope. 
If the finest fraction of the clay is composed 
of only one clay mineral, it can be studied 
adequately by x-ray and optical analyses of 
the entire — 2 micron grade size. If the 
finest grade size is a mixture of clay min- 
erals, it may be necessary to fractionate the 
suspension by sedimentation (e.g., pipette 
analysis) or by supercentrifuge processes. 
By running the suspension through a super- 
centrifuge, it is possible to separate the parti- 
cles of the suspension into size fractions. 
Frequently, fractions containing particles 2 
to 1 microns, 1 to 0.1 micron, and — 0.1 
micron are obtained. Because the clay min- 
erals tend to break down into different sizes, 
a concentration of clay minerals will be 
effected by such a fractionation. For exam- 
ple, if the suspension contained kaolinite and 
montmorillonite, the kaolinite would tend to 
be concentrated in the +1 micron fraction 
and the montmorillonite would be concen- 
treted in the.-—] micron fractions. Frac- 
tions of relatively pure clay minerals would 
be obtained which would permit accurate 
identification of the constituents of the clay. 

Thus by taking a clay material apart a 
complete picture of its makeup can be ob- 
tained. Also the technique gives some infor- 
mation on the size distribution of the min- 



erals making up the material. Such data on 
the makeup of clays and molding sands arc 
the fundamentals on which studies of the 
causes of variation in properties must be 
based. They are the necessary starting point 
for investigations of why molding sands and 
bonding clays have the properties they do, 
and what determines their variations. Until 
the factors controlling the properties of 
sands are understood, the sands themselves 
cannot be controlled with complete satis- 
faction. 



SELECTED BIBLIOGRAPHY 

Bragg, W., Clay: Royal Institution of Great 
Britain, Nov. 19, 1937. 

Bray, R. H., Grim, R. E., and Kerr, P. F., Appli- 
cation of clay mineral technique to Illinois' clay and 
shale: Bull. Geol. Soc. of Am. 46, pp. 1909-1926, 
1935. 

Correns, C. W., The petrography of clay: Natur- 
wiss. 24, pp. 117-124, 1936. 

Correns, C. W., and Mehmel, M., On the optical 
and X-ray data for kaolinite, halloysite, and mont- 
morillonite: Zeit. f. Krist. 94, pp. 337-348, 1936. 

Endell, K., Hofmann, U., and Wilm, D., The 
nature of ceramic clav: Ber. deut. keram. Ges. 
14, pp. 407-438, 1933.' 

von Engelhardt, W., The silicate clay minerals: 
Fort. Min. Krist. u. Pet. 21, pp. 276-337, 1937. 

Grim, R. E., Relation of the composition to the 
properties of clays: Jour. Am. Cer. Soc. 22, pp. 
141-151, 1939. 

Grim, R. E., and Bray, R. H., The mineral con- 
stitution of various ceramic clays: Jour. Am. Cer. 
Soc. 19, pp. 307-315, 1936. 

Grim, R. E., Bray, R. H., and Bradley, W. F., 
The constitution of bond clays and its influence on 
bonding properties: Trans. Am. Foundrymen's 
Assoc. 44, pp. 211-228, 1936. 

Grim, R. E., Bray, R. H., and Bradley, W. F., 
The mica in argillaceous sediments: Am. Min. 22, 
pp. 813-829, 1937. 

Hofmann, U., Endell, K., and Wilm, D., X-ray 
and colloid chemical study of clav: Angew. chem. 
14, pp. 539-547, 1934. 

Hendricks, S. B., and Fry, W. H., The results of 
X-ray and microscopical examinations of soil 
colloids: Soil Sci. 29, pp. 457-478, 1930. 

Kelley, W. P., Jenny, H., and Brown, S. M., 
Hydration of minerals and soil colloids in relation to 
crystal structure: Soil Sci. 41, pp. 259-274, 1936. 

Kerr, P. F., A decade of research on the nature 
of clay: Jour. Am. Cer. Soc. 21, pp. 267-286, 1938. 

de Lapparent, J., Structural formulae and classi- 
fication of clays: Zeit. f. Krist. 98, pp. 233-258, 
1937. 

Marshall, C. E., The chemical constitution as 
related to the physical properties of clavs: Trans. 
Cer. Soc. (Eng.) 35, pp. 401-411, 1936. 

Mehmel, M., Water content of kaolinite, hallo- 
site, and montmorillonite: Chem. d. Erde. 11, pp. 
1-16, 1937. 

Noll, W., Minerals of the system Al203-Si02-H20: 
Neues Jahrb. f. Min. Beilage Bd. 70 Abt. A., pp. 
65-115, 1935. 



OF BONDING CLAYS 



11 



Orcel, J., The use of differential thermal analysis 
in determining the constituents of clays, laterites, 
and bauxites: Int. Congress Min. Met. I, pp. 359- 
371,1935. 

Pauling, L., The structure of micas and related 
minerals: Proc. Nat. Acad, of Sci. 16, pp. 123-129, 
1930. 

Ross, C. S., and Kerr, P. F., The clay minerals 
and their identity: Jour. Sed. Petrog. 1, pp. 55-65, 
1931. 

Ross, C. S., and Kerr, P. F., The kaolin minerals: 
U. S. Geol. Survey, Prof. Paper 165 E, 1931. 

Ross, C. S., and Shannon, E. V., Minerals of 
bentonite and related clays and their physical 
properties: Jour. Am. Cer. Soc. 9, pp. 77-96, 1926. 



DISCUSSION 



Presiding: H. S. Washburn, Plainville Casting 
Co., Plainville, Conn. 

Dr. H. Ries^: We all realize that this subject is a 
very deep one and it may seem intensely theoretical, 
but I believe it brings out one very important point, 
and that is that as we go farther with the study and 
the research on molding sands, that we are getting 
down into finer and finer details. I presume when 
we first started on molding sands back in 1921, at a 
time when I think all of us will admit now we did 
not know anything about them- — we thought we 
did, but we have found out since how little we ac- 
tually did know — we did not think of these very 
small details. We knew, of course, that sands had 
bonds and that these bonds behaved in different 
ways. Now, we are coming to a point where we are 
beginning to study these bonds in greater detail. 
Of course, the study of them requires a considerable 
expertness, particularly when you have to use these 
high-powered, complicated microscopes, but, as 
Dr. Grim has pointed out, we do have these different 
clay minerals in the bond, and it has been shown 
that they have different bonding properties, and so 
that would tend to explain why certain clays might 
be more efficient as bonds than others. 

But there is another interesting point which I 
think he has brought out — I am not sure whether he 
mentioned these by name — namely base changes, 
the possibility of kicking out one ion in a clay and 
substituting another ion for it, as, for instance, the 
comparison he drew in the case of this water soften- 
ing material, the zeolites. 

To put it in plain language, we might say we give 
the clay a dose of salts and, as a result of that, it 
behaves differently from what it did before. 

That opens up an interesting field. It is possible 
that if a bonding clay does not behave just right, 
perhaps by treating it with some chemical and 
getting the ions of this chemical to take the place 
of certain ones which were in the clay, we may im- 
prove its properties. Of course, whether we do it or 
not may be influenced somewhat by the expense 
which might be Incurred by doing so, but I think 
it is something that will be worth trying in the 
future. 



^Cornell University, Ithaca, N. Y. 



Member: In the very descriptive information 
as to the constitution of these various layers, Mr. 
Grim pointed out that in certain clays the struc- 
ture shows a contact point of, in one place OH and 
the other point, O. Does he by these terms indicate 
that there is actually atomic OH or atomic O? 
That is, is this contact point based on a chemical 
reaction? 

Dr. Grim: The OH and the O are not in contact. 
They are parts of different units that tend to make 
up the whole lattice structure. The theory of the 
thing is that because you have the OH in the top 
layer of one kaolinite unit and O in the bottom 
layer of the next unit, there is a tendency to hold 
the units in a relatively fixed position which is more 
secure than you would have if O and O were in 
adjacent layers of two units (e.g. montmorillonite). 

Member: That supposedly is based on the affinity 
of those elements for each other? 

Dr. Grim: Yes, that is the basis for it. 

Member: Dr. Grim could you show by a molecu- 
lar diagram what happens when the clay substance 
loses its water? In other words, dehydration. 

Dr. Grim: That depends on what sort of clay 
you are talking about. In the case of montmorillon- 
ite, the lattice structure seems to be retained in all 
of its attributes up to about 550°C. That is about 
1025°F. There may be a considerable amount of 
water present in montmorillonite between the 
structural units but that is lost at relatively low 
temperatures, perhaps of the order of magnitude 
of 220 or 250°F. Above 550°C. the whole lattice 
tends to break down and different constituents are 
formed before eventual fusion. Exactly what the 
mineralogical changes are that take place when 
montmorillonite and illite break down, are not 
known, but it is quite well known for kaolinite be- 
cause the ceramists have worked it out in their study 
of China clay. There is a change to cristobalite, 
which is a high temperature form of quartz, and 
mullite, which is aluminum silicate. As the temper- 
ature is raised above the temperature where all the 
water is lost, mullite and cristobalite form eventually 
and then later fusion takes place. To be perfectly 
correct, there is some dispute among ceramists as to 
the exact sequence of changes that take place within 
that range when all the water is gone and before 
the new crystalline material develops. It is difficult 
to get any positive evidence on which to identify the 
material. X-ray pictures provide patterns that are 
very difficult to interpret and you can not see much 
under the microscope. 

Member: Dr. Grim showed the difference be- 
tween the adjacent layers of kaolinite as differing 
from montmorillonite and their apparent attraction 
for each other. What would be the case there in the 
instance of the illite as regards the adjacent oxygen 
atoms? 

Dr. Grim: Illite differs from montmorillonite in 
that some of the silicon atoms are replaced by 
aluminum atoms in the silica tetrahedral sheet. In 
the lattice structure, silicon carries four charges and 
aluminum carries three. Every time a silicon is re- 
placed by an aluminum, there is one excess charge 
in the lattice and that excess charge is usually com- 
pensated by an atom of potash that occurs on top 
of the silica sheet. The potash ions in between the 
silica sheets act as sort of a bridge that binds them 
together, so that they do not swell and come apart 
easilv. 



12 



MINERAL COMPOSITION AND TEXTURE 



MINERAL COMPOSITION AND TEXTURE OF THE CLAY 
SUBSTANCE OF NATURAL MOLDING SANDS f 



By Ralph E. G 



RIM^ AN 



D Ci 



E. Schubert** 



ABSTRACT 

The authors experimented with samples of eight different molding sands. Six of these 
sands are in commercial use and two are potential Illinois molding sands. The size-grade 
distribution of the clay substance is determined, the minerals making up the clay sub- 
stance of each sand were identified, and the distribution of the important mineral con- 
stituents with respect to particle size was determined. Curves of the frequency distribu- 
tion of the various samples are included. 



INTRODUCTION 

The clay substance of a molding sand is 
defined by the American Foundrymen's 
Association^ as that part occurring in parti- 
cles less than 0.02 mm. in diameter. The 
physical properties of any sand are closely 
related to the detailed characteristics of its 
clay substance' and, consequently, properties 
cannot be well understood until the clay 
substance has been studied in detail. 

In the researches herein reported, the size- 
grade distribution of the clay substance was 
determined for each one of a number of 
molding sands selected because the general 
character of their clay was thought to vary. 
Also, the minerals which make up the clay 
substance of each molding sand were identi- 
fied and the distribution of the important 
mineral constituents with respect to particle 
size was determined. 

Objective 

The objective of the present study was 
to obtain detailed analytical data for the 
clay substance of various natural molding 
sands and thereby to develop a basis for a 
study of the factors controlling some of the 
physical properties of natural sands. 



iTesting and grading molding sands and clays: Am. 
Foundrymen's Assoc, 1938 edition, pp. 26-27, 157-158. 

^Throughout this report "clay substance" is used as 
defined by the American Foundrymen's Association. 

tReprinted from Trans. Am. Foundrymen's Assoc, 
Vol. 47, No. 4, pp. 935-53, 1940. 

*Petrographer, Illinois State Geological Survey. 

**Associate in Mechanical Engineering, University of 
Illinois. 

Note: This paper was presented before the Sand Re- 
search Session of the 43rd Annual A.F.A. Convention, 
Cincinnati, O., May 17, 1939. 



Sands Investigated 

The molding sands investigated are listed 
in Table 1, together with their content of 
clay substance and their green compression 
strength at optimum moisture. Six of the 
samples are molding sands in commercial 
use, and two samples are potential Illinois 
molding sands obtained by the Illinois Geo- 
logical Survey in a recent study of the mold- 
ing sand resources of Illinois, 

PROCEDURE 

The amount of clay substance was deter- 
mined by the standard method of the Ameri- 
can Foundrvmen's Association.^ 



'able 1, 



-Clay Substance and Green Compression 
Strength of Molding Sands 







Green compression 


Sample 


Clay substance 


at optimum 


No. 


Per cent 


moisture 
lb. per sq. in. 


1 


20.0 


18.0 


2 


10.8 


7.5 


3 


19.2 


13.5 


4 


21.2 


13.0 


5 


16.8 


11.0 


6 


6.6 


12.5 


7 


26.0 


14.5 


8 


46.6 


21.0 



The size-grade distribution within the 
clay substance was determined on an aliquot 
of the original sample by pipette method, as 
applied to molding sands." 

^Jackson, C. E., and Saeger, C. M. Jr., Use of pipette 
in the fineness test of molding sands: U. S. Bur. Standards, 
Jour, of Research 14, 1935, pp. 59-66. 



CLAY SUBSTANCE OF MOLDING SANDS 



13 



In order to obtain samples for mineralog- 
ical study, another aliquot of the original 
sample was dispersed in water using 
NH4OH as the dispersing agent. The sus- 
pension was then allowed to stand until 
particles larger than 0.02 mm. had settled 
out. The suspension carrying — 0.02 mm. 
material was then removed and allowed to 
stand until the -|-0-01 mm. particles had 
settled out. The particles settling out of 
this suspension ranged in size from 0.02 
mm. to 0.01 mm. contaminated by some 
finer material. By repeatedly placing this 
settled material into suspension and remov- 
ing the —0.01 mm. particles, a fraction 
containing only grains ranging from 0.02 
mm. to 0.01 mm. was obtained. By a sim- 
ilar procedure, but with different settling 
times, fractions containing particles from 
0.01 to 0.005 mm., 0.005 to 0.002 mm., 
— 0.002 mm., and — 0.001 mm. were ob- 
tained. NH4OH was used as the dispersing 
agent, because on evaporation no salt is left 
as a residue. 

The minerals making up the fractions 
coarser than 0.002 mm. were identified on 
the basis of their optical characteristics using 
the petrographic microscope. The compo- 
nents of the fractions finer than 0.002 mm. 
were identified on the basis of their optical 
and x-ray characteristics. 

Determinations of green compression 
strength and fineness characteristics were 
made by the standard A. F. A. procedures. 

PARTICLE SIZE ANALYSES 

The results of the determinations of size- 
grade distribution were plotted in the form 
of cumulative curves on semi-logarithmic 
paper. From the cumulative curve of each 
sample, a frequency distribution curve (figs. 
lA to 8A) was constructed by the graphic 
diiiferentiation method described by Krum- 
bein.* The frequency curves show the rela- 
tive abundance of various size grades by the 
area under the curve. For example, the per- 
centage of material between 0.005 and 0.002 
mm. in any sample is obtained by dividing 
that portion of the area under the curve 
which is bounded by vertical lines construct- 
ed at the 0.005 and 0.002 mm. divisions of 
the horizontal axis by the total area under 
the curve; e.g., in figure lA, 15 per cent 
of the total area under the curve lies be- 



tween the 0.005 and 0.002 mm. verticals, 
and therefore 15 per cent of the sample 
occurs in the 0.005 to 0.002 mm. grade size. 
Thus the relative abundance of any size can 
easily be visualized or accurately deter- 
mined. 

The relative amount of — 0.0005 mm. 
material is represented by the area of the 
rectogram thus, 



Per cent material- 
0.0005 mm. 



Area of rectogram 



Area beneath curve including 
area of rectogram 



If the — 0.0005 mm. fractions were pre- 
sented as a continuation of the curve rather 
than as a rectogram, the curves would ex- 
tend to infinity since the — 0.0005 mm. 
fraction contains all material from 0.0005 
mm. to an infinitely small particle size. 

The chief components of the clay sub- 
stance of the molding sands studied are 
quartz, clay minerals, and limonite.'"' The 
distribution and relative abundance of the 
quartz and clay minerals plus limonite are 
shown by figures IB, C to 8B, C. The clay 
minerals and limonite are shown together 
because together they are chiefly responsible 
for the strength properties of the sand, and 
because they cannot well be separated in 
such material on an accurate quantitative 
basis. 

In figures IB, C to 8B, C the distribution 
curves of the entire clay substance are 
broken down into two curves representing, 
respectively, the distribution of quartz and 
the clay minerals plus limonite. The dis- 
tribution curves for the total clay substance 
are constructed on the basis of weight analy- 
ses. The curves for the component mineral 
are constructed on the basis of numerical 
values. The shape and specific gravity of 
the particles are sufficiently alike so that the 
curves are comparable. 

An analysis of figures lA, B, C will 
illustrate the data contained in the curves. 
The area beneath the curve for the clay 
minerals plus limonite; added to the area 
beneath the curve for quartz is equal to the 
area beneath the curve for the total clay 
substance, and thus in the claj^ substance : 

Per cent Area beneath quartz curve 

quartz = 

Area beneath curve for total clay 
substance 



^Krumbein, W. C, Size frequency distribution of sedi- 
ments: Jour. Sed. Petrology, vol. 4, 1934, pp. 65-77. 



■"'Limonite is used througliout the report for the hydrated 
ferric iron oxide compounds present in tlie clay substance. 



14 



MINERAL COMPOSITION AND TEXTURE 



A 




TOTAL CLAY 


SUB5TANC 


E 






r^ 


V 










1 















B 




CLAY MINERikLS PLUS 


LIMONITE 




>- 
U 




UJ 

a 




s. 












r^ 













C 




QUARTZ 








^ 














.02 .01 .005 .00 2 .001 .0005 

LOG DIAMETER IN MM. 

Fig. 1 — Frequency Distribution Curves for Sample 1 



"^ 


^N 


TOTAL CLAY 


SUBSTANC 


E 




/ 


\ 










/ 


\ 


V 












\^ 












^^ 












1 



>- 


B 


.^^ 


CLAY MINER-* 


,LS PLUS 


LIMONITE 




UJ 

a 


/ 


\ 










a. 


/ 




^^^^^..^^^^ 










~~~^ 






1 



c 


__^ 


1 

OUARTZ 








/^ 


\ 










1 




^-^.. 









02 .01 



.005 .002 .001 .0005 

LOG DIAMETER IN MM. 



Fig. 2 — Frequency Distribution Curves for Sample 2. 



CLAY SUBSTANCE OF MOLDING SANDS 



15 



A /'^ 




TOTAL CLAY 


SUBSTAN 


CE 




/ 












/ 












/ 












/ 












\ 




^-^_ 









1 



B 




CLAY MINER; 


LS PLUS 


LIMONITE 






\ 












\ 












\, 






-^ 




1 



c 




QUARTZ 










\ 










1 













005 002 .001 

LOG DIAMETER IN MM 



.0005 



Fig. 3 — Frequency Distribution Curves for Sample 3. 



A 




TOTAL CLAY 


SUBSTAN ( 


:e 




f-^ 


^^ 










1 




--^.__ 





























o 


B 




CLAY MINER/ 


LS PLUS 


LIMONITE 




~~" 


u 






o 

UJ 

a. 
u 


/ 















c 




QUARTZ 








/x 












/ 


"■~' — 












.02 



.005 .002 .001 .0005 

LOG DIAMETER IN MM. 



Fig. 4 — Frequency Distrihution Curves for Sample 4. 



16 



MINERAL COMPOSITION AND TEXTURE 



V\, 




TOTAL CLAY 


SUBSTANCE 




/ \ 


y^ 











































B 




CLAY MINER kLS PLIJS 


LIMONITE 










^ 















A 




QUARTZ 








/ ^ 




















, 





.005 .002 .oor 

LOG DIAMETER IN MM. 



.0005 



Fig. 5 — Frequency Distribution Curves for Sample 5. 



A 




TOTAL CLAY 


SUBSTAN 


CE 
























/ 




^Ns 








/ 



























B 




CLAY MINER 


kLS PLUS 


LIMONITE 




O 

z 

UJ 
O 

UJ 

a. 










/ 












/ 












' 


/ 






^ 







c 




QUARTZ 





















.005 002 .001 

LOG DIAMETER IN MM. 



Fig. 6^ — Frequency Distribution Curves for Sample 6. 



CLAY SUBSTANCE OF MOLDING SANDS 



17 



A 


^-\ 


TOTAL CLAY 


SUBSTANC E 




/ 


\ 










/ 


V 






" 






/ 















B 




CLAY MINER/ 


.LS PLUS 


LIMONITE 










^.^ 









^ 















c 


^ 


QUARTZ 








/ 


V 










/ 


V 











02 



.005 002 .001 

LOG DIAMETER IN MM . 



Fig. 7 — Frequency Distribution Curves fo^ Sample 7. 



A 




TOTAL CLAY 


SUBSTANC 


E 




r\. 












^ 


\ 


































1 



B 




CLAY MINER 


^LS PLUS 


LIMONITE 




f 










1 



W 




QUARTZ 








\ 












1 


\^ 














""^^.^ 









.005 .002 .001 

LOG DIAMETER IN MM 



Fig. 8 — Frequency Distribution Curves for Sample 8. 



18 



MINERAL COMPOSITION AND TEXTURE 



The frequency curves for the quartz and 
clay minerals plus limonite represent the 
size-grade distribution of these constituents 
in the same manner that the curve for the 
entire clay substance represents the size- 
grade distribution of the entire clay sub- 
stance. For example, in the curve for the 
clay minerals plus limonite (fig. IB), the 
area under the curve and between vertical 
lines drawn at 0.02 and 0.01 mm. is 13 per 
cent of the total area under this curve, and 
therefore 13 per cent of the total clay min- 
erals plus limonite occurs in the size grade 
0.02 to 0.01 mm. The curves are so con- 
structed that the area under curve B be- 
tween any grade size, plus the area under 
curve C between the same grade size is equal 
to the area beneath curve A between these 
grade sizes. Thus, in figure 1 the area under 
curve B between 0.02 and 0.01 mm. is two 
thirds of the area under curve A between 
0.02 and 0.01 mm., and therefore two thirds 
of the clay substance occurring between 
0.02 and 0.01 mm. is clay mineral plus 
limonite. It is possible, therefore, from the 
above curves to determine the relative 
amount of the mineral components in the 
total clay substance and in any size grade. 

ANALYTICAL RESULTS 

The clay substance of sample 1 shows a 
concentration of material in the 0.02 to 0.01 
mm. and the — 0.0005 mm. grade sizes. 
Only a small proportion of the clay sub- 
stance is quartz, and it is concentrated 
slightly in the coarser fractions. The clay 
mineral and limonite fraction shows two 
points of concentration ; in the coarsest frac- 
tion (0.02 to 0.01 mm.) and the finest 
fraction (-0.0005 mm.). 

In sample 2 there is a pronounced con- 
centration of the total clay substance in the 
0.02 to 0.005 mm. grade size. Only about 
10 per cent of the total clay substance occurs 
in particles smaller than 0.002 mm. which 
is frequently considered the upper size limit 
of true clay. Quartz is almost as abundant 
as the clay minerals plus limonite, and all 
constituents show a great concentration in 
the sizes coarser than 0.005 mm. 

The clay substance of sample 3, like 
sample 2, is chiefly composed of particles 
coarser than 0.005 mm. Only about 15 per 



cent of the clay substance is found in parti- 
cles finer than 0.002 mm. Quartz is con- 
siderably less abundant than the clay min- 
erals plus limonite, and all constituents are 
concentrated in the sizes coarser than 0.005 
mm. 

The clay substance of sample 4 is com- 
posed chiefly of particles coarser than about 
0.002 mm. There is a minor concentration 
in the —0.0005 mm. grade size. Most of 
the clay substance is composed of clay min- 
erals plus limonite concentrated slightly in 
the coarsest and finest sizes. A considerable 
amount of quartz is present, and it is con- 
centrated in the coarsest sizes. 

In sample 5, a large part of the clay sub- 
stance is concentrated in particles larger 
than 0.005 mm. Clay mineral plus limonite 
is more abundant than quartz and is evenly 
distributed except for a slight concentration 
of the finer sizes. The quartz is concen- 
trated in the coarsest grades. 

A large part of the clay substance of 
sample 6 is found in particles smaller than 
0.001 mm. There is a small concentration 
in the 0.02 to 0.01 mm. grade size, which 
reflects the concentration of the minor 
amount of quartz in this size. The sample 
is distinctive because of the small amount 
of limonite plus clay mineral in particles 
coarser than 0.001 mm., and the great con- 
centration of this material in particles finer 
than this size. 

The clay substance of sample 7 shows 
pronounced concentrations in the 0.02 to 
0.005 mm. and the finest grades. Quartz 
is slightly less abundant than clay mineral 
plus limonite, and it is highly concentrated 
in particles coarser than 0.005 mm. There 
is little clay mineral plus limonite in the 
coarser sizes, this material being concen- 
trated in the sizes finer than about 0.002 
mm. 

The clay substance of sample 8 shows a 
high concentration in the coarsest size with 
decreasing amounts in successively finer 
sizes. About half of the clay substance is 
quartz in particles concentrated in the coars- 
est grade sizes. The limonite plus clay min- 
eral is evenly distributed throughout the 
entire clay substance except for a slight 
concentration in sizes between about 0,005 
to 0.0005 mm. 



CLAY SUBSTANCE OF MOLDING SANDS 



19 



DISCUSSION OF RESULTS 

The analytical data show that in all 
samples very little quartz occurs in particles 
smaller than 0.002 mm, and that most of it 
is present in grains coarser than 0.005 mm. 
Particles of quartz are angular in shape, and 
they do not add to the bond strength of a 
sand. If the clay substance were defined 
with an upper limit of 0.002 mm., quartz 
grains would be largely eliminated and the 
clay substance would be more nearly com- 
posed of clay minerals and limonite, which 
are the materials determining bonding 
strength. 

The clay mineral plus limonite component 
of the clay substance usually is found in 
particles smaller than 0.002 mm. However, 
in some samples much of the clay mineral 
and limonite is present in particles larger 
than 0.002 mm. These larger particles are 
aggregates of smaller clay mineral flakes 
bound rather loosely together by limonite. 
In general these materials add little to the 
strength of a sand when they are present in 
aggregates. When a sand is mulled or re- 
vivified there is a tendency to break up the 
aggregates thereby releasing the component 
particles which then add to the strength of 
the sand. Thus, a sand containing aggre- 
gates may maintain its strength or even 
increase it for the first several heats. As 
the sand is used, aggregates are broken up 
and bond is developed which may more than 
compensate for the bond burned out during 
the first several heats. 

It has long been known that all sands 
with the same amount of clay substance do 
not have the same strength. The absence of 
a close correlation between amount of clav 



substance and green compression strength is 
shown by the data in table 1. Thus samples 
1 and 3 have about the same amount of clay 
substance whereas the green compression 
strength of sample 1 is 18 lb. per sq. in. as 
compared with 13.5 lb. per sq. in. for sample 
3. Sample 6 has only one third as much clay 
substance as sample 3, but has a green com- 
pression strength about equal to that of sam- 
ple 3. 

The variations in size-grade distribution 
and in relative amounts of quartz and clay 
minerals plus limonite within the clay sub- 
stance explain to a considerable extent the 
absence of a close correlation between physi- 
cal properties and amount of clay substance. 
For example, the clay substance of sample 
1 contains only a small amount of a con- 
stituent (quartz) which does not add to its 
bonding power. Also in sample 1 a large 
amount of the clay mineral plus limonite 
is in the finest size grade ( — 0.0005 mm.) 
and it is well recognized that the finer the 
size of the particles of a given type of clay 
mineral the greater its bonding power. In 
comparison the clay substance of sample 3 
contains a large proportion of quartz and 
a comparitively small amount of the clay 
mineral plus limonite in a fine size. It fol- 
lows, therefore, that the bonding power of 
the clay substance of sample 3 would be 
less than that of sample 1. This correlation 
between the strength of the sand and the 
constitution of the clay substance is further 
substantiated by the fineness test results in 



tabl( 



The fineness test shows that sand 



No. 1 is coarser grained than sand No. 3. 
Because of the composition of its clay sub- 
stance, the coarser sand (Sample 1) has the 
higher green compression strength. 



Table 2. — Clay Substance and Fineness Tests of the Molding Sands 



Per cent 



Sieve Sample Sample Sample Sample Sample Sample Sample Sample 

Size 12 3 4 5 6 7 8 

40 0.8 0.8 0.4 2.0 0.0 0.4 2.8 0.2 

70 3.4 2.8 0.6 22.2 0.8 7.8 20.2 0.4 

100 510 8.4 21.6 37.0 21.6 55.8 41.0 2.4 

140 12.0 4.0 34.6 3.6 9.2 12.6 2.4 1.2 

200 6.0 19.4 19.4 2.6 12.0 8.0 1.6 2.6 

270 1.6 27.0 1.0 1.2 13.0 3.0 0.8 4.0 

—270 5.2 26.8 3.2 10.2 26.6 5.8 5.2 42.2 

Clay 20 10.8 19.2 21.2 16.8 6.6 26 46.6 

Total 100 100 100.0 100.0 100 100 100 99.6 

A F A 

Fineness No 96 194 125 95 158 95 79 266 



20 



MINERAL COMPOSITION AND TEXTURE 



Sample 6 shows a very large proportion 
of its clay substance in the finest grade size 
and comparatively little quartz whereas the 
clay substance of sample 3 has a large 
amount of quartz and a small amount of 
material in the — 0.0005 mm. grade size. 
This explains why sample 6 has a green 
compression strength about equal to that of 
sample 3 which has several times as much 
total clay substance. Again, the correlation 
is supported by the fineness test results in 
table 2 which show that sample 3 is a finer 
grained sand than sample 6. On the basis 
of fineness tests alone, sample 3 should have 
a much higher green compression strength 
than sample 6. The fact that it does not 
have much higher strength emphasizes the 
important effect of the composition of the 
clay substance on bonding properties. 

Sample 2 has a small amount of clay sub- 
stance, and the clay substance has a large 
amount of quartz and a small amount of 
clay mineral and limonite in the finest sizes. 
The low green compression strength of this 
sand is, therefore, explained. The impor- 
tance of the composition of the clay sub- 
stance is again emphasized by the fineness 
tests of sample 2 (table 2). Sand No. 2 is 
a fine-grained molding sand and on this basis 
alone a greater green compression strength 
would be expected. The size of the clay 
particles and the large amount of quartz 
in the clay substance, however, cause the 
strength to be low. A further check of tables 
1 and 2 and figures 1 to 8 shows that all the 
molding sands investigated exhibit the same 
general correlation between their green com- 
pression strength and composition of clay 
substance as the ones selected and quoted 
herein. 

Table 3. — Composition of the Clay Mineral Plus 
Limonite Portion of the Clay Substance 

Sample Composition 

1. Kaolinite VA; illite A; limonite A. 

2. Kaolinite A; illite A; limonite A. 

3. Kaolinite VA; illite (?); limonite A. 

4. Kaolinite VA; illite A; limonite VA. 

5. Kaolinite A; illite A; limonite VA, 

6. Illite VA; kaolinite (?); montmorillonite (?); 
limonite C. 

7. Kaolinite A; illite A; montmorillonite (?); 
limonite A. 

8. Illite VA; kaolinite A; limonite C. 
VA=40 per cent +; A=40-20 per cent; 

C^20 per cent — . 



Variations in the relative abundance of 
quartz and clay minerals plus limonite and 
in the size-grade distribution within the 
clay substance are not the only factors tend- 
ing to prevent a close correlation between 
amount of clay substance and physical prop- 
erties. Another factor is the variation in the 
type of clay mineral present. In a previous 
paper, ^ it has been shown that the bonding 
power of a clay will vary depending on 
whether illite, kaolinite, or montmorillonite 
is the constituent of the clay. The identity 
of the clay minerals, their relative abun- 
dance, and the relative abundance of the 
limonite are given in table 3. It is evident 
from this table that the clay mineral in all 
the samples is essentially a mixture of kao- 
linite and illite. Samples 6 and 7 also appear 
to contain montmorillonite. The relative 
amounts of these clay minerals and the char- 
acter of the illite would have to be deter- 
mined in considerable detail before a close 
correlation with ph5^sical properties could be 
made. 

SUMMARY 

It has been shown that there is consider- 
able variation in the size-grade distribution 
within the clay substance of molding sands, 
in the relative amounts of quartz, clay min- 
erals, and limonite which compose the clay 
substance of different sands, and in the size- 
grade distribution of these mineral constitu- 
ents in the clay substance of various sands. 
These variations explain to a considerable 
degree the absence of a close correlation 
between amount of clay substance and physi- 
cal properties of natural molding sands. 

The quartz grains are concentrated in the 
coarser fractions of the clay substance, and 
only a very small amount of quartz is found 
in particles smaller than 0.002 mm. The 
clay minerals and limonite occur in all sizes 
within the clay grade, and are frequently 
concentrated in the coarsest (0.02 to 0.01 
mm.) and finest (—0.0005 mm.) fractions. 
These materials in the coarsest fraction are 
present as aggregates which are broken 
down on using so that strength is developed 
in the sand during the first few times the 
sand is used. 



''Grim. R. E., Elements of the petrographic study of 
bonding clays and of the clay substance of molding sands: 
Trans. Am. Foundrymen's Assoc, vol. 47, No. 4, pp. 
89.S-908^ 1940; see this Rept. Inv., pp. 5-11. 



CLAY SUBSTANCE OF MOLDING SANDS 



21 



The clay minerals in the sands investi- 
gated are kaolinite and illite. Two of the 
sands also appear to contain small amounts 
of montmorillonite. 

ACKNOWLEDGMENTS 

The x-ray analyses were made by W. F. 
Bradley of the Illinois State Geological 
Survey. R. A. Rowland, also of the Illinois 
Geological Survey, assisted in making the 
determinations of size-grade distribution and 
green compression strength. It is desired 
also to acknowledge the counsel of W. F. 
Krumbein of the University of Chicago in 
preparing the graphs portraying some of the 
analytical data. 



DISCUSSION 



Presiding'. H. S. Washburn, Plainville Casting 
Co., Plainville, Conn. 

H. L. Daasch^ {Written discussion): The conten- 
tions of the authors might be stated: (1) quartz 
particles should not be considered effective in bond 
strengths and (2) equally coarse aggregates of true 
clay minerals are ineffective in bond strength. 
These lead to (3) sub-micron clay minerals plus 
limonite are "chiefly responsible for strength prop- 
erties of the sand." A number of comparisons will 
be made on these premises. 

Samples 1 and 3 are noted in paragraph 25 of the 
paper. Table 3 shows quite similar clay mineral 
composition. If less than one micron size clay 
mineral content is considered for each, we find a 
ratio of approximately 4:1; a ratio much different 
than the strength ratio of 18:13.5. 

Samples 6 and 3 are also compared in paragraphs 
25 and 27 (pp. 19, 20). Note however, that if 
clay which is smaller than one micron is consid- 
ered, a ratio of such clay contents is practically 
2:1. Yet strength ratio is 12.5:13.5. 

Again, compare samples 2 and 5. The below one 
micron size clay content is approximately in the 
ratio of 1:5 while the strength ratio is 7.5:11.0. 
Table 3 shows very similar clay composition. 

The writer does not feel that conclusions should 
be too quickly drawn on the data now available on 
clay. For example, kaolinite should, according to 
Grim^, occur in particle sizes larger than 1 micron. 
Grim indicates further that illite is most likely to 
occur in similarly 1 micron and larger particles. 
Let us now consider samples 1, 2, 3 and 5 which have 
been compared in previous paragraphs. If kaolonite 
is eliminated and illite similarly but reasonably not 
considered because of the likelihood of greater than 
1 micron size; we have left primarily limonite in 
sub-micron size. In three of the four cases the per- 
centage of this limonite is 20-40 per cent. According 
to the premises of the authors, we would conclude 
that such limonite would have to account for 
strength property characteristics. 

1 Associate Professor, Dept. of Mechanical Engineering, 
Iowa State College, Ames, Iowa. 



Again, let us consider statements of Casberg and 
Schubert^ and Grim- which indicate that base ex- 
change capacity is a criteria of strength properties. 
Consider further the statements of Grim^ that 
kaolinite has relatively low ionic exchange and that 
illite may often be similarly low. This would bring 
us to the peculiar conclusion that base exchange 
variations for the samples 1, 2, 3 and 5 would be ac- 
counted for by (1) low exchange capacity material or 
(2) by limonite. 

The writer does not, of course, concur in these con- 
clusions. They are based on generalized statements 
propounded by the authors in the present paper and 
in the references quoted. 

The point the writer would make is that a "gen- 
eral correlation between green compression strength 
and composition of clay substance" is not necessar- 
ily shown by the data submitted. When viewed in 
the light of preceding paragraphs we do not "ex- 
plain to a considerable degree the absence of a close 
correlation between amount of clay substance and 
physical properties." 

The writer has produced a wide variety of molding 
sand strength properties by adjustment or change in 
sand component without any change in type or 
amount of bond material. Variations in molding 
sand strength properties need not involve ipso facto 
any differences in the clay. 

Finally, the writer would like to ask the authors 
if any control is offered in the use of the pipette size 
analysis and the A.F.A. strength tests which would 
permit a correlation study as made in the paper. 
This query is prompted by a recent report by 
Grim*. After discussing the effect of water in fine- 
ness and use tests, Doctor Grim writes: "In such 
correlation work, the objective should to deter- 
mine the effective particle size i.e., the particle size 
of the clay as it is usually used." So far as the 
writer can determine, the authors have neglected 
this previous admonition in the present correlation 
study. 

A. Samuel Berlin^ {Written discussion): This 
paper, like others by the same authors, is extremely 
interesting. It constitutes a valuable contribution 
on the influence of clay particle size on green strength 
of molding sands. Since the increase in the green 
strength depends, to a certain extent, upon the 
breaking down of the large size particles into smaller 
ones as for example, the mineral kaolinite, 1 think 
it would be advantageous to find a rapid method to 
determine the chemical and mineral content of the 
clays so that we would be able to control the green 
strength of the molding sands in those cases where 
these factors are important. 

Having an economical and positive method of 
controlling the particle size of the clay so that it 
would approach the ( — 0.0005 mm.) fraction, I think 
would be the solution of quite a few of our molding 
difficulties that are attributed to green strength 
failure. 



'R. E. Grim, Elements of the petrographic study of 
bonding clays and of the clay substance foundry sands: 
Trans. Am. Foundrymen's Assoc, vol. 47, 1940; see this 
Rept. Inv., pp. 5-11. 

•■'Casberg, C. H., and Schubert, C. E., An investigation 
of the durability of molding sands: Illinois Eng. Exp. Sta., 
Bui., 281, April, 1936. 

^Grim, R. E., Relation of the'compositionjtorthe proper- 
ties of clays: Jour. Am. Cer. Soc. 22, pp. 141-151, 1939. 

■''American Manganese Steel Division, New Castle, Del. 



22 



MINERAL COMPOSITION AND TEXTURE 



R. E. Grim and C. E. Schubert {Reply to written 
discussions): Mr. Daasch has arrived at conclusions 
from our data that are obviously in error because he 
has failed to understand our statements or has read 
into them meanings which they do not contain. 

We state that "particles of quartz ... do not 
add to the bond strength of a sand." The point is 
made that the bonding power of the clay substance 
rests in the clay mineral and limonite component 
and not in the quartz component. We realize, and 
in fact point out, that the total bond strength of a 
natural bonded sand is partly dependent on the 
size of quartz grains which the clay mineral and 
limonite must bond. This, in no way, argues against 
the idea that the seat of the bonding power is in the 
clay mineral and limonite and not in the quartz. 

Daasch has somehow read into our statements the 
idea that only the portion of the clay mineral and 
limonite occurring in particles less than 0.001 mm. 
in diameter has bond strength. He then proceeds to 
show that the ratio of the — 0.001 mm. clay mineral 
and limonite fractions of various sands is not the 
same as the ratio of the bond strength of the same 
sands. W'e state clearly (in par. 23 for example, 
p. 19) that bond strength rests in the clay mineral 
and limonite component. Nothing is said anywhere 
that only — 0.001 mm. clay mineral and limonite has 
bond strength. What is stated is that the bonding 
power of this (Component of the clay substance tends 
to increase as the particle size decreases. The ratio 
between the — 0.001 mm. clay mineral and limonite 
for different sands should, of course, not be the 
same as the ratio of the bond strength of the same 
sands because this — 0.001 mm. material does not 
alone determine strength of the sands even when 
the clay minerals are the same. As pointed out all 
the clay mineral plus limonite component has 
strength, the fineness of the quartz must be con- 
sidered, and the character of the exchangeable base 
also exerts an influence. What we believe our data 
show convincingly is that the relative total amount 
of clay mineral and limonite in the clay substance 
of a molding sand is important in determining its 
strength, and also that the initial strength of two 
molding sands, equivalent in every way except in 
the size distribution of the clay mineral and limonite 
particles and aggregates will be different — the one 
containing these components in the finer size having 
the higher strength. 

Grim states that kaolinite occurs in particles 
rarely smaller than — 0.001 mm. and that most 
illite occurs in particles about this same size. Grim 
also states that some illite occurs in finer particles. 
Daach concludes, as the clay minerals in the samples 
are chiefly kaolinite and illite, that there are no clay 
minerals in the — 0.001 mm. size grades and that it 
is all limonite. As Daasch has thought that only — 
0.001 mm. material has bonding power, he concludes 
that the bonding power is only possessed by the li- 
monite. This is, of course, an erroneous conclusion. 

Some of the kaolinite and i llite occur in the 
— 0.001 mm. fractions and the entire clay mineral 
plus limonite fraction is responsible for bonding 
power. Grim is perhaps responsible for Daasch's 
false conclusion here, by not stating that kaolinite 
rarely occurs in particles smaller than about 0.001 
mm. The point that Grim wished to emphasize 
was that kaolinite and most illite tend to occur in 
particles larger than about QX)Q\ mm., and that they 
are not easily broken down by agitation in water 
into particles much smaller than this size. This is 



a generality encountered by all students of clay 
mineralogy. 

Kaolinite is shown to have a low base-exchange 
capacity, and illite is known to have low or moderate 
base-exchange capacity. These are the clay mineral 
components of the samples studied. We have not 
determined the base-exchange characteristics of 
our samples, and consequently do not know whether 
such determination would substantiate or deny the 
findings of Casberg and Schubert. Because the 
clay minerals are those having low capacity seems 
to us to be no reason to conclude, as Daasch has 
done, before any determinations are made that 
such determinations would not agree with Casberg 
and Schubert. 

In reply to Daasch's question about particle size 
analysis, we would say that we followed the Bureau 
of Standards procedure to which reference is made. 
We point out that the pipette analyses show some 
clay mineral and limonite in all size grades and that 
it is frequently concentrated in the coarse (because 
of aggregates) and in the finest size grades. These 
fractionations were made to show first the particle 
size distributions of quartz in the clay substance. 
It should be emphasized that Grim's statements of 
"effective particle size" referred to clay minerals 
only and not to quartz. The fractionations were 
made also to determine the particle size of the clay 
minerals and limonite when prepared by a method 
suggested for molding sands. The authors made no 
attempts to correlate the size grade determinations 
in detail with the bonding properties. This cannot 
be done in detail because of the "effective particle 
size" concept, and, as pointed out before, because 
other factors control bond strength. The justifica- 
tions for the statement of Daasch that some general 
conclusions cannot be recognized because details 
cannot be worked out is not clear to us. In our 
minds the discussion of the break down of the 
aggregates with the liberation of bonding power as 
the sand is used is an illustration of the importance 
of "effective particle size" rather than a neglect of it. 

Finally we feel that the data presented by us show 
that the abundance and size grade distribution of 
the quartz and clay mineral plus limonite "explain 
to a considerable degree the absence of a close corre- 
lation between amount of clay substance and 
physical properties of natural molding." We in- 
serted the expression "to a considerable degree" be- 
cause these "are not the only factors tending to 
prevent a close correlation" (paragraph 29, p. 20). 

In reply to Mr. Berlin we wish to say that at the 
present time the method for determining the mineral 
composition of the clay substance of molding sands 
and for studying the size grade distribution of the 
clay mineral particles is long and somewhat in- 
volved. The method embodies a combination of 
size-grade fractionation and petrographic analysis. 
The petrographic analysis identifies the minerals by 
X-ray and optical analytical data. Mr. Berlin is 
quite correct, it would be very advantageous to find 
a shorter method for this work, and we hope that 
this may be possible. 

Member: Is it possible to separate the limonite 
from the clay minerals? 

Dr. Grim: There are ways of getting rid of the 
iron oxide. You can remove it, but it is frequently 
impossible to remove it quantitatively and still re- 
tain the clay minerals with their original character. 

H. RiEs"; In paragraph 6, the authors state that 
the size-grade distribution of the grains was de- 



CLAY SUBSTANCE OF MOLDING SANDS 



23 



termined with a pipette. Have the authors in their 
laboratories ever used the hydrometer method? 

Dr. Grim: Yes, we have used the hydrometer 
method. The chief reason for the pipette method 
here was to obtain fractions for microscopic study. 
In the case of the pipette, you draw off a certain 
amount that represents the material finer than a 
given size grade and then you have something to 
study for the identification of the constituents. 
We have no objection to the hydrometer method. 
We chose to use the pipette method simply because 
it gave us samples for mineralogical analysis. 

Member: Would the amount of limonite present 
be indicative of the effective life of the clay? 

Mr. Schubert: According to the hydration and 
dehydration curve, I might answer it in this way. 
Limonite breaks up at a very low temperature and 
gives up its water of crystallization or what might be 
called water crystallization. Therefore, a lot of 
limonite in any natural sand or any bonding sub- 
stance should cut down the life of the sand. Now, 
we have not yet determined the durability or life 



of sands of the natural variety. We reported^ at the 
1937 convention on what we call synthetic sands. 
Those were of the montmorillonite and kaolinite 
type and we did not include plain limonite and 
silicia sand. From the hydration curves alone, we 
know that limonite, being a hydrated ferric oxide, 
does give off its water at very low temperatures and 
in ordinary casting work, you would expect that 
sand, if it had an appreciable amount of it in there, 
to burn out very readily, and therefore its life 
would be cut down to some extent. 

Member: In these two comparative cases, 1 
and 3, could the author give us an indication as to 
the difference in the amount of limonite? 

Mr. Schubert: That is contained in Table 3. 
The limonite was about 20 per cent in both samples. 



'•Cornell University, Ithaca, N. Y. 

^Schubert, C. E., A correlation of the physical and chemi- 
cal properties of clays with the durability of molding 
sands: Trans. Am. Foundrymen's Assoc, vol. 45, pp. 661- 
688, 1937. 



24 



PHYSICAL AND MINERALOGICAL 



THE RELATIONSHIP BETWEEN THE PHYSICAL AND 
MINERALOGICAL CHARACTERISTICS OF BONDING CLAYSf 

By Ralph E. Grim and Richards A. Rowland 

ABSTRACT 

Data are presented to show that the kind and amount of clay mineral in a clay are 
the most important factors determining its bonding strength. The relation between 
these factors is discussed. The problem of the break up of the clay mineral in clays, 
such as might be brought about by muUing, is analyzed in relation to bonding strength. 



INTRODUCTION 

The object of the investigation reported 
herein was to study the fundamental factors 
controlling the variations in the bonding 
properties of clays. It is a well recognized 
fact that not all clays possess the same 
bonding characteristics, but the factors that 
determine the variations have not been well 
understood. This report is the fourth^- ■'' ^* 
of a series reporting the results of the Illi- 
nois State Geological Survey's project for 
the investigation of the fundamental proper- 
ties of bonding clays and molding sands. 

In recent 3^ears, it has been shown that 
most clays and shales are made up of ex- 
tremely small particles, frequently less than 
0.001 mm. (1/25000 in.), of a limited 
number of minerals known as the clay min- 
erals. Stated another way, clays and shales 
are essentially aggregates of extremely mi- 
nute particles of one or more of the clay 
minerals. The most common clay minerals 
are noted in table 1. 



fReprinted from Trans. Am. Foundrymen's Assoc, VoL 
48, No. 1, pp. 211-24, 1940. 

^Superior numbers refer to bibliography at end of 
paper. 

Note: This paper was presented at the Foundry Sand 
Research Session during the 44th annual A.F.A. conven- 
tion held in Chicago, May 8, 1940. 



In addition to clay minerals, clays and 
shales also may contain varying amounts of 
quartz, organic material, limonite (hydrat- 
ed ferric iron oxide), etc. Of the non-clay 
mineral constituents, quartz, in the form of 
grains, is by far the most abundant and 
most common. Clays differ from each other 
fundamentally in the clay minerals that 
compose them, in the amount of admixed 
quartz or other non-clay mineral constitu- 
ents, and in their texture (size of particles, 
etc.).^ It is the purpose of this report to 
present the results of a study of the influence 
of these fundamental differences between 
clays on their bonding characteristics. 

PROCEDURE 

In the past eight years, the complete min- 
eral composition of a large number of clays 
and shales has been determined by x-ray and 
microscopic methods in the laboratory of 
the Illinois State Geological Survey. From 
these clays and shales, a group was selected 
for the present investigation that exhibited 
wide variations in composition and texture. 
The clays selected are not all commercial 
bond clays. The clays used were chosen so 



Table 1. — Chemical Composition and Occurrence of Common Clay Minerals 
Name Chemical Composition* Occurrence 

Montmorillonite.. . (0H)4 AI4 Sig O20 XH2O Bentonites, gumbotils, etc. 

IlHte (0H)4 Ky (Al4-Fe4-Mg4-Mg6) (Si (g-y). Aly) O20. .Shales, gumbotil, underclays, etc. 

Kaolinite (0H)8 AI4 Si4 Oio Underclays, fire clays, china clays, etc. 



*Certain substitutions are possible in these general formulae. For details see Grim, R. E., Relation of the composition to 
the properties of clay: Jour. Am. Ceramic Society, vol. 22, pp. 141-151, 1939; reprinted as Illinois Geological Survey, Cir- 
cular 45, 1939. 



CHARACTERISTICS OF BONDING CLAYS 



25 



that the results would provide a broad pic- 
ture of the effect of different characteristics 
of clays on their bonding properties. 

The green compression strength of each 
sample, at several different moisture con- 
tents, was determined using 8 per cent clay 
and 92 per cent sand (except for the benton- 
ite sample when 4 per cent clay and 96 per 
cent sand were used). These determinations 
were carried out according to A. F. A. stand- 
ard procedure except that the sand* used 
had a fineness number of 56 instead of 50. 

For each sample, the amount of material 
coarser than the A. F. A. clay grade^ and 
the size grade distribution within the A. F. 
A. clay grade were determined by the pipette 
method (modified after Jackson and Sae- 
ger^). Any method for the determination 
of the size grade distribution of a clay or 
shale requires that the clay or shale be first 
broken down in water into a suspension. 
The results obtained are always dependent 
to some degree on the amount the clay or 
shale has been disaggregated in water prior 
to the analysis. In other words, widely 
different size grade distribution determina- 
tions may be obtained from a single clay by 
the use of different amounts and kinds of 
disaggregation. 

In the present research, great care was 
used to carry out the disaggregation of all 
samples in exactly the same manner so that 
the results would be comparable. The dis- 
aggregation procedure followed was mild, 
/. e., no attempt was made to break the clays 
or shales down to anything like their ulti- 
mate particle size. The mild procedure was 
followed because in the actual use of clays 
or shales for rebonding, they are in general 
not immediately broken down to their ulti- 
mate particle size. It was felt that the fol- 
lowing procedure disaggregated the clay to 
about the same degree as in the actual use 
of the clay for rebonding and, therefore, 
that the results would be particularly sig- 
nificant in an understanding of the varia- 
tions of the bonding properties of clays or 
shales. 

The clay or shale was first ground to pass 
a 70-mesh screen and then soaked in water 
for 7 days. During the soaking period, the 
clay and water were agitated about once 
every 12 hours. Ammonium hydroxide 
(NH_jOH) was used as the dispersing 
agent. 



*The sand used comes from the Ottawa, 111., district 
and is regularly sold as "bond" sand. 



ANALYTICAL DATA 

The determinations of maximum green 
compression strength, moisture content at 
maximum strength, quantity of clay min- 
erals, clay mineral composition, and quantity 
of material in the A. F. A. clay grade are 
given in table 2. The per cent of clay min- 
eral in the entire sample was determined by 
means of microscopic examination. In some 
clays and shales, the clay minerals occur in 
individual particles and aggregate masses 
that are larger than the upper size limit of 
the A. F. A. clay grade, i.e., 0.020 mm. 

The quantity of A. F. A. clay given in 
table 2 is that portion of the sample shown 
by the pipette analyses to be less than 0.020 
mm. in diameter. The disaggregation pro- 
cedure preliminary to the pipette analyses 
was a very mild one which did not reduce 
all the clay mineral aggregates or large indi- 
vidual particles in all samples to a size 
smaller than 0.020 mm. As a consequence, 
the table shows the total amount of clay 
mineral to be more than the A. F. A. clay 
for some samples. 

The determinations of the size grade dis- 
tribution of each sample within the A. F. A. 
clay grade are given in figure 1. The dis- 
tribution curves in figure 1 were constructed 
by the graphic differentiation method^ from 
cumulative curves plotted from the results 
of the pipette analyses. The distribution 
curves show the amount of material between 
any given size limits by the proportion of 
the area under the curve between the given 
size limits to the area beneath the entire 
curve. 

For example in curve No. 2, the area 
(EFSC) beneath the curve and between 
vertical lines constructed at the 0.020 mm. 
and 0.010 mm. points is 11.2 per cent of the 
area beneath curve (EJNCJ, plus the rec- 
tangular areas (ABCD and KLMN ) and 
therefore 11.2 per cent of the sample is in 
the 0.020 mm. to 0.010 mm. grade size. 
The area of the rectangle (KLMN J, on 
the right, in proportion to the area under 
the curve (EJNC) plus the area of the 
rectangles (ABCD and KLMN) represents 
that portion of the sample smaller than 
0.005 mm. in size (12.7 per cent for sample 
No. 2). The rectangle (ABCD), on the 
left for sample No. 2, has an area equal 
to 33.9 per cent of the area under the curve 
(EJNC) plus the areas of rectangles 



26 



PHYSICAL AND MINERALOGICAL 






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CHARACTERISTICS OF BONDING CLAYS 



27 









1 














































































































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A 


B 




2 








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^rTTT77 




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20 



10 



5 2 

MICRONS 



05 



Q^CLAY MINERAL fTZI QUARTZ 

Fig. 1 — Size Grade Distribution Curves. 1 Micron is Equal to 0.001 mm. For the Significance of the "XX' 

See Text Below. 



(ABCD and KLMN ) and, therefore, indi- 
cates that 33.9 per cent of the sample is 
composed of particles larger than 0.020 
mm. in size. 

In the curves in figure 1, the cross-hatched 
areas represent the amount of calcite, the 
diagonal ruled areas represent the amount 
of quartz, and the remaining area represents 
the amount of clay mineral. The amount of 
these minerals in the samples as a whole 



or in the individual size grades can be ob- 
tained from the curves by comparing areas. 
The clay mineral material occurring in the 
coarser size grades, for example -]-0.005 
mm., may be either aggregates of smaller 
particles or individual particles of the size 
indicated. Where the size grade is com- 
posed primarily of individual particles of 
clay minerals rather than aggregates, the 
designation "XX" is used. 



28 



PHYSICAL AND MINERALOGICAL 



^Tm. 






5 








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20 



10 



5 2 

MICRONS 



05 



I I CLAY MINERAL r//J QUARTZ 
Fig. 1 Continued. 



SIGNIFICANCE OF 
DISTRIBUTION CURVES 

The size grade distribution curves in 
figure 1 show that there may be a tremen- 
dous variation in the size distribution of 
particles for different clays. Thus, one clay 
material (No. 1) has most of its particles 
smaller than 0.005 mm. whereas another 
(No. 12) has very few particles finer than 



this size. Some samples (Nos. 1 and 11) 
have very few particles coarser than the 
A. F. A. clay grade, i.e., 0.020 mm., whereas 
other samples (Nos. 4 and 6) have many 
particles coarser than this size. One clay 
(No. 2) has a very even distribution of 
particles between 0.020 mm. and 0.0005 
mm., whereas another one (No. 11) has an 
uneven distribution of particles between 
these sizes. 



CHARACTERISTICS OF BONDING CLAYS 



29 









10 








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X X 


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5 2 

MICRONS 

[^CLAY MINERAL [^^ QUARTZ 
Fig. 1 Continued. 



CALCITE 



The distribution curves in figure 1 are 
arranged in the order of decreasing green 
compression strength of the clay materials 
they represent, No. 1 having the greatest 
and No, 14 the least strength. A critical 
analysis of the size distribution data of these 
samples in relation to their bonding strength, 
brings out the fact that there is no precise 
correlation between any single size distribu- 
tion characteristic, or combination of them, 
and the bonding strength of the clay ma- 



terials. For example, there is no correlation 
between the amount of A. F. A. clay grade 
as determined in this study and the green 
compression strength ; also there is no cor- 
relation between the amount of material 
finer than 0.0005 mm., and the green com- 
pression strength. This means that some 
other factors are dominant in controlling 
the bonding strength of clay materials. It 
will be shown presently that the amount 



30 



PHYSICAL AND MINERALOGICAL 



and character of the clay mineral content 
are the dominant factors. 

One might conclude, on the basis of 
theoretical considerations and microscopic 
study of clay-bonded sands, that a clay com- 
posed chiefly of extremely minute particles 
(less than 0.001 mm.), or of larger particles 
that broke down immediately to such minute 
particles when used, would have the high- 
est bonding power, other factors being equal. 
This conclusion would seem to follow, be- 
cause smaller particles would give a greater 
total surface area in the clay for bonding 
purposes, and microscopic study shows that 
a very fine-grained clay evenly coats the 
sand grains while another clay of larger 
particle size coats the quartz grains irregu- 
larly with many large clay lumps between 
grains that can have little influence on the 
strength of the sand-clay mixture. 

In the clays studied, the sample composed 
of the finest particles has the greatest 
strength (No. 1). However, sample No. 2 
with about the same clay mineral composi- 
tion is composed of much larger particles 
and yet has about the same strength as sam- 
ple No. 1. Data from other samples support 
the conclusion that a raw clay need not 
necessarily be composed of extremely minute 
particles ( — 0.001 mm.) to be a good com- 
mercial bonding clay. In fact a clay com- 
posed of somewhat coarser particles of clay 
mineral may have certain advantages over 
one composed only of extremely minute 
particles. If the coarser clay contains clay 
mineral particles that break down into mi- 
nute particles with fair rapidity when the 
clay is worked, the clay will probably have 
better durability because breaking down of 
the clay mineral particles will continue to 
provide new bonding surfaces as the clay is 
being used. 

It should be emphasized that only the 
clay mineral particles in a clay possess bond- 
ing strength'', and that only the clay mineral 
particles can easily and readily be broken 
into smaller sizes when the clay is worked, 
e.ff., in mulling. The clay mineral particles 
are flake-shaped and their breakdown tends 
to take place by cleavage of the flakes. Al- 
though clay mineral particles tend to occur 
in clays generally in particles less than 
0.002 mm., individuals and aggregates are 
frequently 0.020 mm. or more in diameter 
in many clays and shales. 



In general the clays studied that have a 
relatively even distribution of particles, be- 
tween 0.020 mm. and 0.0005 mm., also 
have high green compression strength. The 
explanation seems to be that such clays do 
not contain large amounts of non-clay min- 
eral material in particles smaller than 0.020 
mm. and also that they are composed of clay 
mineral particles and aggregates that break 
down with reasonable rapidity into extreme- 
ly minute particles. A critical consideration 
of the data indicates that a clay composed of 
large clay mineral particles that does not 
break down into very fine particles is not 
a satisfactory bonding clay. On the other 
hand a satisfactory bonding clay is not neces- 
sarily one composed only of very fine parti- 
cles, or of larger particles that break down 
immediately when the clay is first used. A 
clay composed of particles that break down 
with intermediate ease is apt to be a most 
satisfactory bonding clay (other factors of 
clay mineral composition, etc., being equal). 

Those clays studied that show an irregu- 
ular distribution between 0.020 mm. and 
0.005 mm. are also those that contain a large 
amount of non-clay mineral material, or an 
abundance of large clay mineral particles 
that do not break down easily. In either 
case the sample has low bonding value be- 
cause it contains a large proportion of ma- 
terial wnth little or no bonding power. 

SIGNIFICANCE OF CLAY 
MINERAL COMPOSITION 

Data obtained in the present work sub- 
stantiate the conclusion of an earlier paper" 
that the clay mineral composition is the 
most important factor in determining the 
bonding strength of a clay. The data also 
verify the conclusion that montmorillonite 
is the clay mineral providing the highest 
strength (samples 1 and 2), and that the 
presence of a small amount of montmorillo- 
nite in a clay will raise the bonding strength 
out of all proportion to the actual amount 
of the montmorillonite. This is illustrated 
by comparing the green compression strength 
of samples 4 and 5, which contain small 
amounts of montmorillonite in addition to 
kaolinite and illite, with samples 7 to 12 
which contain only kaolinite and illite. 

The present data show that kaolinite and 
most illite materials have much lower bond- 
ing strength than clays composed of mont- 
morillonite, and that kaolinite clays have 



CHARACTERISTICS OF BONDING CLAYS 



31 



higher strength than most illite materials. 
The characteristics of illite vary within wide 
limits and the data indicates that occasion- 
ally an illite clay may have very high bond- 
ing power. For example, sample No. 3 com- 
posed largely of illite has very high strength. 
Chemical data, obtained in another study, 
show that there is a range in the potassium 
oxide (KgO) content of the illite clay min- 
erals, and that in general clay materials 
composed of illite with relatively high 
K2O content have low green compression 
strength. 

It is significant that one of the attributes 
of montmorillonite and the certain illite 
{e.ff., sample No. 3) is that they either 
occur in extremely minute particles or in 
larger particles that break down easily into 
very small particles. Kaolinite and illite, 
particularly illite with a high K^O content, 
tend to occur in particles of larger size that 
break down with difficulty into particles of 
very small size. One, but probably not the 
only reason for the difference in bonding 
power of the different clay minerals, is the 
variation in the ease with which they break 
down into very fine particle size. 

A comparison of samples 2 and 8 empha- 
sizes the importance of the clay mineral com- 
position in determining bonding strength. 
Sample No. 2 has much less material finer 
than 0.0005 mm. and much more material 
coarser than 0.020 mm. than has sample 
No. 8, yet sample 2 is the stronger clay 
because it is composed of montmorillonite. 
In both samples the actual amount of clay 
mineral is about the same. 

INFLUENCE OF THE AMOUNT 
OF CLAY MINERAL 

The data in table 2 illustrate that there 
is a relation between the amount of clay 
mineral in clay materials of the same clay 
mineral composition and their bonding 
strength. Thus samples 13 and 14 with low 
clay mineral content have very low bond 
strength. The data suggest further that the 



non-clay mineral content does not reduce 
the bond strength of a clay or shale very 
much unless there is more than about 30 
per cent of it present. In other words non- 
clay mineral matter tends to have little 
effect on bond strength unless it makes up 
more than 30 per cent of the total clay 
material. 

SUMMARY 

The green compression strength of four- 
teen clays of widely different compositions 
was determined, at 8 per cent clay and 92 
per cent sand (except bentonite — 4 per cent 
clay and 96 per cent sand). 

Pipette analyses of these clays were made 
using mild disaggregation procedure. These 
analyses show approximately the effective 
particle sizes of the clays as they exist when 
the clay is used in the foundry. 

An analysis of the size distribution of the 
clays in conjunction with their green com- 
pression strength shows that those clays 
which break down with intermediate ease 
are apt to be most satisfactory (other fac- 
tors of clay mineral composition, etc., being 
equal). 

A comparison of the kind of clay min- 
eral with bonding strength indicates that 
clays composed of montmorillonite have the 
greatest green compression strength. Clays 
composed of kaolinite and illite have lower 
bonding strength. An example of an unus- 
ual illite clay is given that has high green 
compression strength. A small amount of 
montmorillonite present in a mixture with 
either kaolinite or illite yields a bonding 
strength out of all proportion to the amount 
of montmorillonite present 

The presence, in amounts greater than 
about 30 per cent, of such materials as 
quartz, calcite and/or large clay mineral 
flakes which do not break down easily causes 
low bonding strength. 

There is a relation between the amount 
of clay mineral and the bonding strength 
in clays of the same clay mineral content. 



32 



PHYSICAL AND MINERALOGICAL 



BIBLIOGRAPHY 



1. Grim, R. E., Elements of the petrographic 
study of bonding clays and of the clay substance 
of bonding sands: Trans. Am. Eoundrymen's Assoc, 
vol. 47, pp. 895-908 (1940). See this Rept. Inv., 
pp. 5-11. 

2. Testing and Grading Foundry Sands and 
Clays: Am. Eoundrymen's Assoc, 1938 edition, 
pp. 26-27. 

3. Jackson, C. E., and Saeger, C. M., Jr., Use 
of pipette in the fineness test of molding sands: 
Jour, of Research, U. S. Bureau of Standards, pp. 
59-66, 1935. 



4. Krumbein, W. C, Size frequency distribution 
of sediments: Jour. Sedimentary Petrology, vol. 4, 
pp. 65-77, 1934. 

5. Grim, R. E., and Schubert, C. E., Mineral 
composition and texture of the clay substances of 
molding sands: Trans. Am. Eoundrymen's Assoc, 
vol. 47 pp. 935-953, 1940; see this Rept. Inv. pp. 
12-23. 

6. Grim, R. E., Bray, R. H., and Bradley, W. F., 
The constitution of bond clays and its influence on 
bonding properties: Trans. Am. Eoundrymen's 
Assoc, vol. 44, pp. 211-228, 1936. 



ILLINOIS STATE GEOLOGICAL SURVEY 

Report of Investgations No. 69, 1940