B E fibT
UNIVERSITY OF [ILLINOIS BULLETIN
[Vol. XVIII December 13, 1920 No. 15
i Entered M second-olaes matter December 11. 1912. at the post office at Urbana. Illinois, under the act of Augmt
24. 1912. Acceptance for mailing at the special rate of postage provided for in section 1103
Act of October 3. 1917. authoriied July 31. 1918)
DISSOLVED GASES IN GLASS
EDWARD W. WASHBURN
FRANK F. FOOTITT
ELMER N. BUNTING
i 10 1956
UNIVERSITY OF CALIFORNIA
BULLETIN No. 118
ENGINEERING EXPERIMENT STATION
PUBLISHED BY THH UNIVKBSITT or ILLINOIS, UBBANA
PRICE: TWENTY CENT*
CHAPMAN & HALL, LTD., LONDON
THE Engineering Experiment Station was established by act of
the Board of Trustees, December 8, 1903. It is the purpose
of the Station to carry on investigations along various lines of
engineering and to study problems of importance to professional engi-
neers and to the manufacturing, railway, mining, constructional, and
industrial interests of the State.
The control of the Engineering Experiment Station is vested in
the heads of the several departments of the College of Engineering.
These constitute the Station Staff and, with the Director, determine
the character of the investigations to be undertaken. The work is
carried on under the supervision of the staff, sometimes by research
fellows as graduate work, sometimes by members of the instructional
staff of the College of Engineering, but more frequently by investiga-
tors belonging to the Station corps.
The results of these investigations are published in the form of
bulletins, which record mostly the experiments of the Station's own
staff of investigators. There will also be issued from time to time, in
the form of circulars, compilations giving the results of the experi-
ments of engineers, industrial works, technical institutions, and gov-
ernmental testing departments.
The volume and number at the top of the front cover page are
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For copies of bulletins, circulars, or other information address the
ENGINEERING EXPERIMENT STATION,
UNIVERSITY OF ILLINOIS
ENGINEERING EXPERIMENT STATION
BULLETIN No. 118 DECEMBER, 1920
DISSOLVED GASES IN GLASS
EDWARD W. WASHBURN
PROFESSOR OF CERAMIC CHEMISTRY
FRANK F. FOOTITT
SGT. 6th SERVICE COMPANY, SIGNAL CORPS, U. S. A.
ELMER N. BUNTING
RESEARCH ASSOCIATE IN THE ENGINEERING EXPERIMENT STATION
ENGINEERING EXPERIMENT STATION
PUBLISHED BY THE UNIVERSITY OF ILLINOIS, URBANA
I. INTRODUCTION 7
1. Foreword . '. 7
2. Purpose of the Investigation 7
3. Acknowledgments , 8
II. DEMONSTRATION OF THE EXISTENCE OF DISSOLVED GASES
IN FINISHED GLASS . . . .... . . 11
4. The Method Employed . .....'.. . . 11
5. The Glass 11
6. The Furnace ... . 11
7. Experimental Procedure . 11
8. The Result . 14
III. PARTIAL ANALYSIS OF THE GASES EVOLVED FROM THE
GLASS . . " . . . . . 17
9. The Apparatus and Method . . . . <" .. . 17
10. The Analysing Train ... . . ... . 17
11. Flushing the Furnace .......... 17
12. Melting the Glass . . 18
13. The Results 18
IV. A SPECIAL APPARATUS FOR BOTH MEASURING AND ANA-
LYSING THE DISSOLVED GASES IN GLASS . . .21
14. Description of the Apparatus 21
15. Determination of Free Volume of Furnace ... 21
16. Experimental Procedure 23
V. THE GAS CONTENT OF THREE TYPES OF COMMERCIAL GLASS 24
17. A Barium-Flint Optical Glass . . '. . . - . 24
18. A Light Flint Bulb Glass . 24
19. A Borosilicate Laboratory Glass 24
20. Discussion of the Results 25
VI. THE SIGNIFICANCE OF DISSOLVED GASES IN GLASS ... 27
21. The Relation between Adsorbed and Dissolved Gas 27
22. The Influence of Dissolved Gases upon the Proper-
ties and Behavior of Glass 29
23. The Use of Vacuum Furnaces in the Manufacture
of Glass 30
VII. SUMMARY 32
24. Summary of Results 32
LIST OF FIGURES
1. Melting Pot, with Block of Glass before Melting 9
2. Detail of Vacuum Furnace 12
3. Detail of Pot, Eesistor, and Insulation 13
4. Vacuum Furnace and Large Vacuum Tank 15
5. Melting Pot, with Block of Glass after Melting and Evacuating ... 16
6. Apparatus for Measuring and Analysing the Dissolved Gases in Glass . 19
7. Detail of Apparatus for Measuring and Analysing the Dissolved Gases in
Glass . \ . , 22
LIST OF TABLES
1. Per Cent by Weight of Oxygen and of Carbon Dioxide Dissolved in a
Barium-Flint Optical Glass 18
2. Summary of the Eesults on the Amounts of Dissolved Gases in Finished
DISSOLVED GASES IN GLASS
1. Foreword. The work described in the following pages was
begun in June, 1917, as part of a program of research on some of the
problems connected with the manufacture of optical glass. The first
experiments were carried out by Mr. FRANK F. FOOTITT, at that time
Research Assistant in the Engineering Experiment Station. Mr.
Footitt later joined the Signal Corps of the United States Army and
was detailed at the University to assist in the continuation of the re-
search. His part in the work continued until he was honorably dis-
charged from the service in February, 1919. The results given in
the first three chapters of the present paper are based upon Sgt.
Footitt 's experiments, an account of which was given before the
Pittsburgh meeting of the American Ceramic Society in February,
1919. After Sgt. Footitt 's discharge the investigation was dropped
until January, 1920, when it was again taken up with the assistance
of DR. ELMER N. BUNTING, who is continuing it at the present time.
2. Purpose of the Investigation. All varieties of glass, even at
ordinary temperatures, are in the liquid state of aggregation. They
are liquids which have been cooled through their normal crystalliza-
tion interval so rapidly that there has not been time for crystallization
("devitrification") to occur. Instead, the viscosity of the liquid has
been increased to such a large value that the molecules do not have
sufficient freedom of motion to permit the rearrangements necessary
for the formation and growth of crystals. The liquid has thus been
supercooled until it has become a solid. In principle any liquid can
by supercooling be brought into the condition of a glass, but since
it still remains a liquid, it should possess the characteristic properties
of liquids, including the power to hold gases in a state of solution.
During the process of manufacturing glass, large quantities of
gas, mainly carbon dioxide, oxygen, and nitrogen, are evolved from
"8 ILLINOIS ENGINEERING EXPERIMENT STATION
the batch owing to the occurrence of chemical reactions such as the
Na 2 C0 3 + Si0 2 = Na 2 Si0 3 + C0 2
4 KN0 3 + 2 Si0 2 = 2 K 2 Si0 3 + 2 N 2 + 50 2
2 Pb0 2 + 2 Si0 2 = 2 PbSi0 3 + 2
If ammonium nitrate, NH 4 N0 3 , is employed in "blocking"* the
glass, water vapor will also be evolved during the fining. The glass
will thus be saturated with these gases at the partial pressures which
prevail at the end of the "fining" operation.
On cooling the glass, these gases should remain in solution, and
glass in the finished state may therefore be expected to contain appreci-
able quantities of these dissolved gases. Since no actual data concern-
ing the nature or amounts of such dissolved gases were available, the
experiments described below were undertaken for the purpose of
throwing some light on this question. These experiments are to be
regarded as preliminary to a more extended investigation of these
dissolved gases, and of their influence upon the properties of the
finished glass, and its behavior during use.
In addition to the account of the experiments conducted to date,
and their results, there will be found in the following pages some dis-
cussion of the relation between adsorbed and dissolved gas, the in-
fluence of dissolved gases upon the properties and behavior of glass,
and the use of the vacuum furnace in the manufacture of glass.
3. Acknowledgments. For samples of glass used in the present
investigation we are indebted to the United States Bureau of Stand-
ards, and to the Pittsburgh Plate Glass Company. The Signal Corps,
and later the Aircraft Production Board, made possible the prosecu-
tion of the work during the war by the detail of Sgt. Footitt as Re-
* The term "fining" or "plaining" is applied to the operation of eliminating bubbles
from the molten glass. This may be accomplished by heating the glass to a sufficiently high
temperature to cause the bubbles to expand and rise to the top of the melt. If this method
is not effective the operation of "blocking" is employed. This consists in inserting into the
melt, with the aid of an iron rod, a potato, a piece of green wood, a pellet of ammonium
nitrate, or in general any material which will give a copious evolution of gas in the form
of large bubbles which will rise through the melt and gather up the small bubbles in their
FIG. 1. MELTING POT, WITH BLOCK OF GLASS BEFORE MELTING
DISSOLVED GASES IN GLASS 11
II. DEMONSTRATION OF THE EXISTENCE OF DISSOLVED
GASES IN FINISHED GLASS
4. The Method Employed. In order to demonstrate the ex-
istence of dissolved gases in considerable quantity in a piece of per-
fectly clear homogeneous glass, the method of " sudden evacuation"
may be employed. In this method the piece of glass to be investigated
is melted under atmospheric pressure in a vacuum furnace which can
be connected through a valve to a large - evacuated tank. When the
temperature of the glass has reached about 1200 deg. C. the valve is
opened quickly, thus causing a sudden drop of pressure within the
This experiment is similar to the opening of a siphon of soda
water, and if the glass contains dissolved gases a similar result would
be expected, that is, there should be a sudden evolution of gas from
the glass, causing it to expand in volume and to effervesce vigorously.
5. The Glass. The glass employed in the first experiment was
a piece of barium flint optical (1.6053-43.6) having the following
composition, as determined by the Bureau of Standards:
Oxide . . . Si0 2 As 2 5 PbO ZnO BaO K 2
Mole (per cent) 64.5 0.15 9.73 9.22 10.2 6.24
A piece free from bubbles was selected, placed on a table beside
an inverted melting pot, and photographed. (See Fig. 1.)
6. The Furnace. The vacuum furnace and the details of the
heating element and thermocouple installation are shown in Figs.
2 and 3, which are self explanatory. The outlet tube was connected
to a Nelson rotary vacuum pump and also, through a valve, to a
large vacuum tank (A, in Fig. 4) having a capacity about 100 times
that of the furnace chamber.
7. Experimental Procedure. The melting pot containing the
piece of glass was placed inside the heating chamber (Fig. 2) and
this in turn placed within an insulating cylinder supported on the
ILLINOIS ENGINEERING EXPERIMENT STATION
FIG. 2. DETAIL OF VACUUM FURNACE
DISSOLVED GASES IN GLASS
Porce/a/n Thermocouple Tube*
; '. ^-P/atinum He&f/rtg Co/7
Una /a zed ' Porce/ar/r? Pof
FIG. 3. DETAIL OF POT, EESISTOR, AND INSULATION
furnace base as shown (Fig. 3). The water cooled iron dome (see
Fig. 4) was then lowered into place on its rubber gasket and the cur-
rent started in the heating coil. When the glass had attained a tem-
perature of about 1200 deg. C. the valve connecting the outlet with
the large vacuum tank was quickly opened.
This tank had been previously evacuated to a pressure of 1 inch
of mercury, and as soon as pressure equalization had taken place, as
indicated by the manometer, the valve was quickly closed, the Nelson
pump started, and the pressure in the furnace chamber brought
down rapidly to less than 1 cm. of mercury. The heating current-
was then cut off and the furnace allowed to cool with the vacuum on.
14 ILLINOIS ENGINEERING EXPERIMENT STATION
8. The Result. On opening the furnace most of the glass was
found outside of the pot, standing above it in the form of a large
white mass of foam. This was broken away from the pot and photo-
graphed as before, beside the inverted pot. The result is shown iu
Fig. 5. By comparing Figs. 1 and 5 an idea of the increase in volume
associated with the evolution of the dissolved gases may be obtained.
This amounted to about six times the volume of the original piece.
The existence of considerable quantities of gas in a state of solution
in the glass was thus demonstrated.
FIG. 4. VACUUM FURNACE AND LARGE VACUUM TANK
FIG. 5. MELTING POT, WITH BLOCK OF GLASS AFTER MELTING AND EVACUATING
DISSOLVED GASES IN GLASS 17
III. PARTIAL ANALYSIS OP THE GASES EVOLVED
FROM THE GLASS
9. The Apparatus and Method. The furnace employed was
that used in the preceding experiment: The large vacuum tank was
disconnected, however, and the outlet tube V was connected to a Gaede
high vacuum pump, through an analysing train. The method con-
sisted briefly in evacuating the furnace until all adsorbed gases were
removed, melting the glass, drawing the evolved gases out through
the analysing train, and finally washing out the furnace with pure
nitrogen. All connections throughout the system were sealed glass
joints, or glass-to-glass joints covered with heavy rubber tubing and
coated with a beeswax-rosin mixture.
10. The Analysing Train. The analysing train consisted of the
following elements in the order named, starting from the furnace end :
(1) a series of six gas wash bottles containing standard barium
hydroxide solution, and having their delivery tubes drawn down to
capillary openings so as to produce a stream of small bubbles through
the solution when in operation:
(2) a drying tower containing pumice and sulphuric acid:
(3) a glazed porcelain combustion tube containing copper gauze
and provided with a heating coil. Two pieces of copper gauze previ-
ously reduced in hydrogen and then weighed were placed in series
in the combustion tube, which was kept at 700 deg. C. during the run.
Before the experiment was begun the analysing train was thor-
oughly washed out with pure nitrogen in order to remove all air.
The nitrogen used for this purpose was purified by passing it over
hot copper, and through wash bottles containing barium hydroxide
solution. The nitrogen thus purified gave zero test for both carbon
dioxide and oxygen.
11. Flushing the Furnace. In order to remove adsorbed gases
from the glass pot and the insulating materials, the following proce-
dure was adopted. The furnace was assembled as shown in Fig. 3
with the melting pot in place. The Gaede pump was started and the
pressure in the furnace reduced to 0.02 mm. At the same time the
current was started in the heating coil and the pot heated to a
temperature several hundred degrees higher than that employed in
the melting operation. The furnace was kept hot and the Gaede
ILLINOIS ENGINEERING EXPERIMENT STATION
pump in operation for several hours. Pure nitrogen was then ad-
mitted to the furnace chamber until atmospheric pressure was
attained, after which the nitrogen was pumped out. This wash-
ing with nitrogen was repeated several times and the furnace finally
allowed to cool while filled with nitrogen.
12. Melting the GlassWhen the nitrogen filled furnace was
entirely cold, the water cooled dome was hoisted sufficiently to permit
a weighed quantity (about 350 grams) of glass to be dropped into
the melting pot, after which the dome was immediately lowered into
place and the Gaede pump started. At the same time the resistor was
heated to just below red heat, and after the pressure had fallen to
0.02 mm. the furnace was again flushed two times with pure nitrogen.
Finally, with a vacuum of 0.01 to 0.02 mm. in the furnace, it was
sealed by closing a stop-cock, and the temperature of the pot was
raised to about 1000 deg. C. and kept there for one hour. The pres-
sure was then noted and the temperature of the pot allowed to drop
to about 650 deg. C. after which pure nitrogen was admitted until
atmospheric pressure had been reached.
With the resistor maintained at about 650 deg. C. the contents
of the furnace were then pumped out through the analysing train and
the furnace washed out with nitrogen, the washings being also pumped
out through the analysing* train. The furnace was finally allowed to
stand full of pure nitrogen until time for the next experiment.
13. The Results. The results obtained in four separate experi-
ments, using pieces of the same block of glass, are shown in Table 1.
It will be noticed that oxygen and carbon dioxide are present in solu-
tion itf the glass to the extent of 0.1 per cent of its weight. Part,
perhaps the greater part, of the carbon dioxide is present in the com-
bined state as carbonate, and some of it would therefore be retained
in the glass even under a vacuum of 0.02 mm.
PER CENT BY WEIGHT OF OXYGEN AND OF CARBON
DIOXIDE DISSOLVED IN A BARIUM-FLINT OPTICAL GLASS
WEIGHT PER CENT
MOLES PER LITER
o a .
FIG. 6. APPARATUS FOR MEASURING AND ANALYSING THE DISSOLVED GASES
DISSOLVED GASES IN GLASS 21
IV. A SPECIAL APPARATUS FOR BOTH MEASURING AND ANALYSING
THE DISSOLVED GASES IN GLASS
14. Description of the Apparatus. The experiments described
in the preceding section gave satisfactory evidence of the existence
of dissolved oxygen and carbon dioxide in considerable amounts in
finished glass. The apparatus and the method employed in these ex-
periments were, however, rather cumbersome and complicated, and it
was very difficult to make sure that no leakage of atmospheric gases
into the evacuated furnace took place. The method, moreover, did
not yield a measure of the total amount of the dissolved gases.
In order to eliminate these drawbacks a new vacuum furnace
was designed, constructed entirely of glass and porcelain, which could
readily be made perfectly gas tight, and which also permitted all of
the gas evolved by the glass to be both measured and analysed. The
final form of this apparatus is shown in Figs. 6 and 7.
The vacuum casing of the furnace consisted of a pyrex glass
tube 5 cm. in diameter and 13 cm. high, provided with a ground glass
stopper having a mercury seal at the joint. The melting pot was a
cylindrical porcelain tube, 3 cm. in diameter and 13 cm. high. It was
wound with platinum wire and slipped into a tightly fitting porcelain
protecting tube. An outer more loosely fitting protecting tube com-
pleted this portion of the apparatus, which was suspended inside of
the glass tube by means of two heavy copper leads which passed out
through the capillary tubes, T and T 2 . The joint between these lead
wires and the top of the capillary tubes was made tight by a rubber
plug covered with a beeswax-rosin mixture.
15. Determination of Free Volume of Furnace. For this pur-
pose a calibrated 230 cu. cm. flask containing air at atmospheric
pressure was attached at M and the stop-cock S was closed. The
furnace, containing the melting pot and its protecting tubes, was then
evacuated to a pressure of 0.1 mm. of mercury and, after the connec-
tion to the pump had been closed, stop-cock S t was opened and the
manometer reading again taken. The volume of the flask being
known, and the change in pressure which occurred on connecting it
to the evacuated apparatus, the free volume of the latter was calcu-
lated to be 475 cu. cm.
ILLINOIS ENGINEERING EXPERIMENT STATION
C/ay Protecting PoK
a 14 Copper Wire
o. /8 P/atinvm Wire
a 25 Platinum W/re
FIG. 7. DETAIL OF APPARATUS FOR MEASURING AND ANALYSING THE DISSOLVED
GASES IN GLASS
DISSOLVED GASES IN GLASS 23
16. Experimental Procedure. The following procedure was em-
ployed in measuring and analysing the dissolved gases in glass. A
weighed sample of glass in some cases, 25 grams, in others, 50
grams, was placed in the melting pot and the whole apparatus as-
sembled as shown in the figure. With stop-cock S x closed, the ap-
paratus was evacuated to a pressure of 0.1 mm. of mercury, and at
the same time a sufficient current was passed through the heating
wire to heat the pot and protecting tubes to about 400 deg. C., at
which temperature no dissolved gas is given up by the glass.' This
preliminary heating and evacuating was necessary in order to remove
adsorbed moisture from the porcelain. The connection to the pump
was then closed and the whole apparatus allowed to stand for several
hours in order to make sure that it was perfectly tight, this fact, of
course, being indicated by an absolutely constant manometer reading.
Sufficient current was then passed through the heating coil to
raise the temperature of the glass to 1400 deg. C. and the heating was
continued till no more gas was evolved from the molten glass, as shown
by a steady manometer reading. During this heating a blast of air
was directed on the ground glass joint. The apparatus was then
cooled to room temperature and the manometer reading was recorded.
The free volume of the furnace being known, the total amount of gas
evolved by the glass could be calculated. The total time required
for a run was from two to three hours.
After the final manometer reading had been taken, the stop-cock
leading to the manometer was closed, the manometer disconnected,
and a small Orsat apparatus connected in its place. The tube M was
then connected to an adjustable mercury reservoir, the mercury filling
the tube completely up to the stop-cock. This stop-cock was opened
and the furnace completely filled with mercury, all of the gas being
driven out ahead of the mercury into the Orsat apparatus, where it
was analysed for carbon dioxide and oxygen, any residual gas being
considered nitrogen. The accuracy of the chemical analysis was
about one per cent.
24 ILLINOIS ENGINEERING EXPERIMENT STATION
V. THE GAS CONTENT OF THREE TYPES OF COMMERCIAL GLASS
17. A Barium Flint Optical Glass. This was the same type of
glass as that used in the experiments described in Chapter III, but
was obtained from the Pittsburgh Plate Glass Company, and may
have differed somewhat in composition. Its index of refraction was
given by Dr. Hostetter as "about 1.605, and its v value, about 43.6.'*
Two experiments on 50 gram portions, and one on a 25 gram
portion, of one block of glass gave total volumes of dissolved gases
(measured under standard conditions) of 15.6, 14.3 and 8.04 cu. cm.
respectively. The average is 15.3 cu. cm. for 50 grams of glass,
amounting to 1.1 times the volume of the glass itself. Two samples of
another block of the glass gave a volume of gas (under standard con-
ditions) equal to half the volume of the glass. The history of the
two blocks used in these experiments is not known. They were taken
from a 25 Ib. lot of cullet, and may have come from two entirely dif-
ferent melts. It is, of course, to be expected that the gas content of
finished glass will depend very materially upon the melting and fining
procedure which has been followed.
The gas from the second block of glass was analysed, and was
found to consist of 25 per cent carbon dioxide and 75 per cent oxygen.
If any nitrogen was present it was less than one per cent.
18. A Light Flint Bulb Glass. The sample of glass used had the
following composition according to the manufacturer's analysis:
Oxide . . * . Si0 2 PbO A1 2 3 CaO Na 2
Mole (per cent) 75.0 7.15 0.45 0.60 16.76
Two melts of 50 grams each were made and they gave respectively
3.2 and 3.5 cu. cm. of gas under standard conditions. The dissolved
gas thus amounted to 0.2 times the volume of the glass itself. Analysis
of the gas gave 58 per cent carbon dioxide, 24 per cent oxygen, and
18 per cent nitrogen. The density of the glass was 2.89.
19. A Borosilicate Laboratory Glass. The glass investigated had
approximately the following composition:
Oxide . . . Si0 2 B,0 3 As 2 5 A1 2 3 Fe 2 3 CaO MgONa 2 O K 2
Mole(percent) 83.0 10.5 0,2 1,2 0.1 0.3 0,1 4,4 0.1
DISSOLVED GASES IN GLASS
Twenty-five grams of glass gave, on melting, a volume of gas
(under standard conditions) equal to 0.2 times the volume of the
glass. On analysis the gas was found to consist of 26 per cent carbon
dioxide, 37 per cent oxygen, and 37 per cent nitrogen.
SUMMARY OF RESULTS ON THE AMOUNTS OF DISSOLVED GASES IN FINISHED GLASS
VOLUME PER CENT S.T.P.
WEIGHT PEB CENT
MOLES PER LITER
Barium flint .1
Barium flint .2
Water at deg. C .
20. Discussion of the Results. The results obtained with the
above three varieties of glass are summarized in Table 2. Owing to
lack of data concerning the melting schedule and finishing operation
used in the melts from which the samples studied originated, it is
impossible to correlate the results obtained with the manufacturing
The quantity and nature of the gases present in the finished glass
must obviously depend upon the batch composition and the melting
and finishing procedures. The influence of the latter factor is prob-
ably responsible for the different results obtained with the different
samples of the barium flint optical.
It is not probable that any appreciable quantities of gas are ab-
sorbed by the glass from the atmosphere of the furnace, except possibly
in the case of glasses which are mechanically stirred for a long period.
This conclusion seems to be borne out by the absence of nitrogen from
the barium flint glass. The dissolved gas must therefore originate
from the gases given off by the batch itself during the melting and
At the end of the melting period, just before the fining operation
begins, the glass usually contains numbers of small bubbles in which,
owing to the high surface tension of molten glass, the gas is under
a pressure greater than atmospheric. At the end of the fining opera-
26 ILLINOIS ENGINEERING EXPERIMENT STATION
tion the glass is therefore probably still somewhat supersaturated with
gas, since, owing to its high viscosity, it cannot very rapidly give up
this extra dissolved gas. The higher the finishing temperature, and
the longer the glass is held at high temperatures, the smaller should
be the amount of dissolved gas remaining in the finished glass. This
conclusion seems to be borne out by the results obtained with the boro-
silicate glass, which is a glass requiring very high finishing and work-
ing temperatures. The great preponderance of acidic constituents in
this glass may, however, be partially responsible for the small quantity
of carbon dioxide found, since as shown by Niggli,* a good part of
the dissolved carbon dioxide in glass is probably combined with the
* Niggli, Paul, "The Phenomena of Equilibria between Silica and the Alkali Carbonates,"
Jour. Araer. Chem. Soc., Vol. 35, 1706 (1913),
DISSOLVED GASES IN GLASS 27
VI. THE SIGNIFICANCE OF DISSOLVED GASES IN GLASS
21. The Relation between Adsorbed and Dissolved Gas. It has
long been recognized that glass in'common with many other substances
displays a strong tendency to adsorb, that is, to condense upon its
surface, gases with which it is in contact.*
Adsorption may indeed be regarded as a type of solution in which
the dissolved molecules do not penetrate below the surface layer of
the adsorbent. Such a " surf ace solution" will therefore ordinarily
be saturated when the surface of the adsorbent is covered with a layer
of the adsorbed material one molecule deep and with its molecules
close-packed laterally, t
Adsorption may sometimes be accompanied by a gradual penetra-
tion of the adsorbed material beneath the surface layer of the ad-
sorbent, that is, it may be accompanied by ordinary or " volume " solu-
tion; but in the case of glass at low temperatures, such solution will
probably be confined to the superficial layers. Adsorbed or superfici-
ally dissolved gases are thus to be distinguished from the dissolved
gases studied in the present investigation, which are more or less
uniformly disseminated throughout the whole mass of the glass. Lang-
muirj has found that water vapor is adsorbed and then slowly dis-
solved by glass. He also found that lamp bulbs when heated in vacuo
evolved adsorbed carbon dioxide and nitrogen in addition to water
Recently Sherwood has devised a dynamic method for studying
the gases evolved by glass when heated in vacuo to temperatures be-
low its softening point. He found that adsorbed gases could be re-
moved completely by heating to 200 deg. C. in vacuo and that the
amount of such gases corresponded to a layer about one molecule
* Of. Guichard, M., "Sur les gaz degage des parois des tubes de verre." Bull. Soc.
Chim. 100, 440 (1911).
t For a more detailed discussion of the relation between adsorption and solution see
Washburn, E. W., "Introduction to the Principles of Physical Chemistry," Ed. 2, Chap. XXV,
The McGraw-Hill Book Company, New York, 1921.
$ Langmuir, I.. "The Adsorption of Gases on Plane Surface's of Glass. Mica and
Platinum," Jour. Amer. Chem. Soc., Vol. 38, 2283-4 (1916); Ibid., Vol. 40, 1387 (1918).
Sherwood, R. G., "Gases and Vapors from Glass," Phys. Rev., Vol. 12, 448 (1918).
28 ILLINOIS ENGINEERING EXPERIMENT STATION
deep over the surface of the glass. On subsequent heating to 500 deg.
C. a further evolution of gas occurred, which he attributed to " chem-
ical reactions " occurring within the glass.
The well known jump in pressure within an exhausted glass
vessel which occurs when it is " sealed off" and the subsequent deteri-
oration of the vacuum with time has been studied by Shrader.* He
concludes that : ' ' The vacuum in sealed vessels deteriorates with time,
rapidly at first, and then more slowly, and subsequent heating, even
at temperatures lower than the heat-treating temperature, results in
increase of pressure due to further liberation of the gases and vapors
from the glass. No connection between different samples of the same
glass or different glasses can be established. It is quite probable that
there are variations in the properties of different samples of the same
glass quite as great as the variations between different glasses of about
the same grade. ' '
There is every reason to believe that the dissolved gases in glass
play an important role in the behavior described by Shrader. Cer-
tainly the jump in pressure which occurs during the sealing-off
process can be ascribed to this source, and it seems entirely probable
that gas-free glass would be superior in many respects to ordinary
glass for the manufacture of high vacuum apparatus.
The adsorption of various gases by glass and their subsequent
evolution with vacuum-heat treatment have been studied, particularly
with respect to the production and maintenance of high vacua, by
Ulreyt in a recent investigation which at the time of writing is avail-
able only in abstract. His conclusions in the main substantiate those
already referred to, but the following may be mentioned :
(1) "Glass from which practically all absorbed gases
have been removed by melting in vacuo subsequently reab-
sorbed gases from the atmosphere at room temperature."
(2) "At temperatures up to the softening point, diffusion
of gases of the atmosphere through glass does not take place."
With reference to his first conclusion, a distinction should be
drawn between absorbed or dissolved gases and adsorbed gases. The
removal of the former by melting in vacuo does not, of course, affect
* Shrader, J. E., "Residual Gases and Vapors in Glass Bulbs," Phys. Rev., Vol. 13, 437
t Ulrey, D., "Evolution and Absorption of Gases by Glass," Abstract in Phys. Rev,, Vol.
14, 160 (1919).
DISSOLVED GASES IN GLASS 29
the ability of the glass to adsorb gases from the atmosphere, the latter
being an entirely independent process.
The evidence for his second conclusion not being available, its
exact significance is not entirely clear. The fact that the atmospheric
gases seem able to diffuse througji quartz glass at comparatively low
temperatures renders it not improbable* that a similar behavior might
be exhibited by some of the ordinary commercial glasses under some
circumstances, although doubtless to a considerably less extent.
22. The Influence of Dissolved Gases upon the Properties and
Behavior of Glass. Evolution of dissolved gas in the form of bubbles
tends to occur whenever the pressure, on glass, while in a fluid condi-
tion, is decreased. Such a decrease in pressure will occur during the
manufacturing operation whenever there is a marked fall in the
barometer, and it would be interesting to know whether there is any
record of troubles with "seedy" glass accompanying periods of
A condition of reduced pressure with a consequent evolution of
bubbles of gas also results from the strains set up by the contraction
of the glass itself. If the outside of a mass of glass be allowed to
solidify while the interior is still in a fluid condition, it is evident
that the gradual solidification of the remaining glass must bring about
a tension upon the still fluid portions; and this decrease in pressure
will cause them to evolve their dissolved gases, with the consequent
formation of a mass of bubbles in those portions of the glass which
remain longest in the fluid condition. Some interesting examples
of the formation of seed from this cause have been described by
Still another instance of the occurrence of reduced pressure dur-
ing manufacturing operations is met with in cases where the glass is
"gathered" by suction, as in the case of the Owens machine. If the
glass when it reaches the gathering machine is supersaturated with
dissolved gases, the operation of gathering will evidently result in
the formation of seed, some of which will not disappear again when
the suction is released. If the glass at the moment of gathering is
* Of. Le Chatelier, "La Silice et les Silicates," p. 94, Hermann et Fils, Paris 1914;
Mayer, E. C., "Leakage of Gases through Quartz Tubes," Phys. Rev., Vol. 4, 283 (1915),
t Williams, A. E., "Observations on the Formation of Seed in Optical Glass Melts,"
Jour. Amer. Ceramic Soc., Vol. I, 134 (1918).
30 ILLINOIS ENGINEERING EXPERIMENT STATION
undersaturated with the dissolved gases, as will be the case if it has
been kept long enough at a sufficiently high temperature before reach-
ing the gathering machine, the suction may still result in the momen-
tary appearance of seeds, but these will be largely of the vacuum type
and not permanent; if permanent, they will probably be exceed-
ingly small, after the glass is finished.
Since both the appearance and disappearance of the seeds under
these conditions is a process requiring a certain amount of time for
the attainment of equilibrium, the viscosity of the glass, the time
during which it is under the reduced pressure, and the subsequent
cooling and annealing operations will evidently all have an influence
on the final state of the glass as regards freedom from seeds. Seeds
formed from glass which is supersaturated or practically completely
saturated with dissolved gases cannot be removed by annealing, but
seeds resulting from reduced pressure upon glass which is under-
saturated with dissolved gases will disappear or be greatly reduced in
size by proper annealing.
The complete story of the effect of dissolved gases upon the prop-
erties and behavior of glass must await the results of further in-
vestigation. It seems entirely probable that the presence of dissolved
gases, and especially of microscopic seeds, may materially increase
the tendency of the glass to devitrify, and the entire removal of this
constituent might make it possible to obtain results which are not
possible under ordinary conditions owing to the rapidity of devitrifica-
tion. Certainly the known facts concerning the behavior of other
supercooled solutions point in this direction.*
23. The Use of Vacuum Furnaces in the Manufacture of Glass.
The results described in the preceding pages are to be regarded as
preliminary only. It is intended to continue the investigation not
only for the purpose of determining the influence of dissolved gases
upon the properties and behavior of glass, but also for the purpose of
determining the practicability and value of a commercial process for
the production of gas free glass.
The investigation of this subject was started early in 1918, and
the results thus far attained indicate that a vacuum furnace process
for the manufacture of certain types of glass is entirely feasible on
* In this connection see Germann, A. F. O., "The Devitrification of Glass, a Surface
Phenomenon. The Repair of Crystallized Glass Apparatus." Jour. Amer. Chem. Soc., Vol. 43,
DISSOLVED GASES IN GLASS 31
an industrial scale, and that it offers a number of pronounced ad-
vantages over current methods. It eliminates entirely the fining
operation as ordinarily understood, materially reduces the high finish-
ing temperatures required with some glasses, produces in all cases a
product absolutely free from even the smallest seeds, and these re-
sults suggest the possibility of considerably increasing the yield of
perfect glass. Its main field of usefulness will probably be in the
manufacture of certain types of optical glasses and of glass for high
32 ILLINOIS ENGINEERING EXPERIMENT STATION
24. Summary of Results. The results of the investigation to date
may be summarized as follows:
(1) All varieties of glass in the finished state contain dis-
(2) The amount of this dissolved gas is sufficient to cause
the glass to effervesce violently if the pressure upon it be sud-
denly reduced while it is in a fluid condition.
(3) The amount and composition of the dissolved gas
varies greatly with the type of glass and the detail of the
melting and fining procedures.
(4) In the three types of industrial glass examined, the
volume of the dissolved gases (measured under standard con-
ditions) varied from 0.2 to 2 times the volume of the glass it-
(5) Carbon dioxide, oxygen, and nitrogen were found in
varying amounts in the gas.
In addition, as a result of this experimental work, a convenient
apparatus for measuring and analysing the dissolved gases was de-
veloped, and an improved type of vacuum furnace for the manufac-
ture of gas-free glass was constructed.
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34 PUBLICATIONS OF THE ENGINEERING EXPERIMENT STATION
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36 PUBLICATIONS OF THE ENGINEERING EXPERIMENT STATION
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38 PUBLICATIONS OF THE ENGINEERING EXPERIMENT STATION
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