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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

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in 2012 with funding from

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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

MOK

TMORI

LLONl

—

-ILL

ITE

♦-KAC

LINIT

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-

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

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.

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.

20.0

18.0

10.8

7.5

19.2

13.5

21.2

13.0

16.8

11.0

6.6

12.5

26.0

14.5

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

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.

MINERAL COMPOSITION AND TEXTURE

TOTAL CLAY

SUB5TANC

CLAY MINERikLS PLUS

LIMONITE

>-
U

QUARTZ

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

LOG DIAMETER IN MM.

Fig. 1 — Frequency Distribution Curves for Sample 1

TOTAL CLAY

SUBSTANC

.^^

CLAY MINER-*

,LS PLUS

LIMONITE

^^^^^..^^^^

~~~^

__^

OUARTZ

^-^..

02 .01

.005 .002 .001 .0005

LOG DIAMETER IN MM.

Fig. 2 — Frequency Distribution Curves for Sample 2.

CLAY SUBSTANCE OF MOLDING SANDS

A /'^

TOTAL CLAY

SUBSTAN

^-^_

CLAY MINER;

LS PLUS

LIMONITE

QUARTZ

005 002 .001

LOG DIAMETER IN MM

.0005

Fig. 3 — Frequency Distribution Curves for Sample 3.

TOTAL CLAY

SUBSTAN (

f-^

--^.__

CLAY MINER/

LS PLUS

LIMONITE

~~"

a.
u

QUARTZ

"■~' —

.02

.005 .002 .001 .0005

LOG DIAMETER IN MM.

Fig. 4 — Frequency Distrihution Curves for Sample 4.

MINERAL COMPOSITION AND TEXTURE

V\,

TOTAL CLAY

SUBSTANCE

/ \

CLAY MINER kLS PLIJS

LIMONITE

QUARTZ

/ ^

.005 .002 .oor

LOG DIAMETER IN MM.

.0005

Fig. 5 — Frequency Distribution Curves for Sample 5.

TOTAL CLAY

SUBSTAN

^Ns

CLAY MINER

kLS PLUS

LIMONITE

UJ
O

QUARTZ

.005 002 .001

LOG DIAMETER IN MM.

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

CLAY SUBSTANCE OF MOLDING SANDS

^-\

TOTAL CLAY

SUBSTANC E

CLAY MINER/

.LS PLUS

LIMONITE

^.^

QUARTZ

.005 002 .001

LOG DIAMETER IN MM .

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

TOTAL CLAY

SUBSTANC

r\.

CLAY MINER

^LS PLUS

LIMONITE

QUARTZ

""^^.^

.005 .002 .001

LOG DIAMETER IN MM

Fig. 8 — Frequency Distribution Curves for Sample 8.

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

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 0 10.8 19.2 21.2 16.8 6.6 26 0 46.6

Total 100 0 100 0 100.0 100.0 100 0 100 0 100 0 99.6

A F A

Fineness No 96 194 125 95 158 95 79 266

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

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.

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

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.

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

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

PHYSICAL AND MINERALOGICAL

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MICRONS

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.

PHYSICAL AND MINERALOGICAL

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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

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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

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

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.

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

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