Symposium on Glass Papers preEented before the Division of Industrial and Engineering Chemistry a t the 85th Meeting of t h e American Chemical Society, Washington, D. C., March 26 t o 31, 1933.
Introduction F. C. FLIIVT,Hazel-Atlas Glass Company, Washington, Pa.
G
LASS is one of the oldest manufactured products of the world, and the industry has existed for thousands of years. Then it was purely as an art, and the workmen were artisans, not mechanics. The quantity produced was limited and the value of the product high, because each man then was limited in his own personal effort. The quality of the glass produced varied because there existed so many unknown factors in the production and use of the materials a t hand. In the last generation by the enlargement of units of manufacture, by the introduction of scientific study of the principles underlying the melting process, and by a greater knowledge of the qualities of the glass, the industry has exhibited an expansion in use of its product and a stability of quality that has amounted virtually to its rebirth. Kow its products are valued a t several hundred million dollars a year, and the amount of glass that is continuously under fire would represent a lake of molten glass waist deep and covering several acres. The largest industrial furnaces in the United States are used in the manufacture of glass. By the introduction of automatic machinery and the increase of the production from each unit, and by the production
of special glasses has a progress in science and engineering been forced on the industry that only reluctantly left its ruleof-thumb methods of former years. This new large-scale production, coupled with the widening use of glass, has demanded a new knowledge which must be accurate and diversified. Though glass might all answer to the same definition of “glassiness,” it must have properties more variable than the alloys of iron. It must be transparent to opaque, color!ees, multicolored, white or black, easily melted or “hard,” brilliant or dull, chemically resistant, or water-soluble, and all this with an accuracy of control worthy of the finest tools. To effect this knowledge and control has necessitated an unusual collaboration of physicists, chemists, and engineers. The problems have included the most difficult analyses, giant questions in fuel application, the construction of special equipment, and the meticulous control of chemical detail. Physical studies have been extremely difficult by the very nature of the substance, and the use of the product has brought to bear on it a phalanx of specialists studying and applying its special value in a multitude of fields.
Phase Equilibrium Relationships Determining Glass Compositions GEORGEW. AIo~EY, Geophysical Laboratory, Carnegie Institution of Washington, Washington, D. C.
M
ODERS glassware is made up of many clifferent types differing in their appearance, properties, uses, and compositions; but of all these types, those which are essentially mixtures of soda, lime, and silica make up an overwhelmingly preponderant proportion of the total production. hloreover, not only is most glassware largely composed of these three oxides, but their relative proportions always remain the same, within rather narrow limits. This has been true of glassware from the earliest days (4). DEFINITION OF GLASS It has not proved possible to set up a definition of glass in terms of specific physical properties which will properly classify divers types of glasslike materials and also place proper limits on the change of these properties with change of composition and temperature. It is feasible to define glass by a comparison with other substances, and in so doing we bring in the concept of phase equilibrium. For any glass composition there exists a temperature above which devitrification is impossible, and at which crystals already formed will be melted. This is the temperature of the beginning of crystallization on cooling or of the completion of melting on heating, and is known as the liquidus temperature. Above this temperature glass cannot
devitrify because the Crystalline products of devitrification dissolve in the liquid; below this temperature the liquid is unstable in respect t o the crystalline phases and, if given time, will change into a crystalline aggregate. From the liquidus temperature down to ordinary temperatures the devitrified condition is the thermodynamically stable one, and the glass persists in the unstable condition of an undercooled liquid only by reason of its great viscosity. There is every range of ease or difficulty with which the undercooled liquid may be obtained and with which it may be retained in the crystalline condition. Some glasses are practically impossible to crystallize even under the most favorable conditions without the aid of a flux or mineralizer; others can be made only in small quantities by the most rapid cooling possible. Similarly, some glasses persist indefinitely in the condition of an undercooled liquid a t ordinary temperatures, while others sooner or later spontaneously crystallize. It should be emphasized that in this discussion the term “liquid” is used in a highly technical sense. I n connection with phase equilibrium studies a liquid is a noncrystalline phase (not a gas) which limits the range of existence with increasing temperature of a crystalline phase, and the idea of fluidity or rigidity plays no part in this definition. The argument as to the propriety of calling a glass an undercooled
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narrow limits. The reason for liquid is pointless. 'There can Glasses ore undercooled liquids, the term this will be evident from a disbe no question that a glass is "liquid" having the technical meaning used in cussion of the phase equilibrium a liquid from fhe point of view phase equilibrium work. Glasses hate passed r e l a t i o n s h i p s in the ternary of phase equilibrium, and the from the f h i n molten condition obtaining during system Sa20-Ca0-SiOi. circumstance that it may a t the their manufucture through a continuously changsame time possess p r o p e r t i e s which, according to some other ing sequence of properties, to a hard, brittle subBIA-ARY SYSTEM definition, are characteristic of a stance of suflcient strength and rigidity to be of In t h e b i n a r y s y s t e m ( 1 ) solid is not pertinent. catue in technology. Glass has been known to SazO?3i02-SiOz (Figure 1) there Glasses, then, are undercooled mankind f r o m the earliest times, and the oldest are formed two compoundsliquids, and a large n u m b e r of sodium m e t a s i l i c a t e (Sa20glasses do not differ greatly in composition f r o m substances can be obtained in Si02) and s o d i u m d i s i l i c a t e the glassy c o n d i t i o n . Those most of the glassware today. This is because the (Sa20.2SiO2). The first of these compositions which have been particular properties required of a practical may be o b t a i n e d as a glass in hit upon for commercial glasses glass composition make it necessary that it be small quantities, though not in have the common characteristic not greatly diflerent in composition f r o m the lots as large as a pound, and that they exhibit a great relucternary eutectic between the compounds is too sluggish a crystallizer to tance to pass from the liquid to enable its melting point to be dea crystalline c o n d i t i o n even ATaz0.2SiO?, ATaz0.3Ca0.6SiOz,and quartz, in termined by the method of coolu n d e r t 11e most favorable cirthe lernary system Na2O-CaO-SiO2, because only ing curves. However, its meltc u m s t a n c es, a characteristic when of such a composition is it both fluid ing is sharp enough to determine possessed in a less or even in a enough at easily attainable temperatures to be the temperature by the heatg r e a t e r degree by many subreadily melted on a commercial scale, and at the ing c u r v e m e t h o d . Sodium s t a n c e s wholly unsuitable for d i s i l i c a t e is more difficult to practical use. Many o r g a n i c same time viscous enough at its freezing point crystallize, and glass of about substances, such as glucose soluso that decitrification is inhibited. The relation this composition for the manutions, fall in this category. Some of earious complex glasses to this simple eutectic facture of water glass is melted complex sulfates can be obtained glass is discussed. While we hate extensive in ton lots without devitrificai:i the glassy form; a r s e n i o u s knowledge of glass composition we know praction. It melts a t 874" C., and oxide and antimony trioxide can the q u e n c h i n g method is the both be obtained as glass, as can tically noihirig about its constitution. only satisfactory means for desome of their compounds; phostermining its melting point. A p h a t es a r e well-known glass few milligrams of the material, formers, a n d i n d e e d the socalled meta- and pyrophosphoric acids probably are known wrapped in thin platinum or gold foic are held a t a known only in the glassy condition; boron oxide has never been ob- constant temperature for a time long enough for equilibrium tained in any but the glassy condition. ?;one of these is to be reached, which may be a matter of hours, days, or suitable for commercial uses because of being too easily de- even weeks. Then the charge is cooled in such a manner as composed by water. i h o n g silicates, glasses of the composi- to freeze the equilibrium, and is examined with the petrotion of the feldspars, albite and orthoclase, are far more re- graphic microscope. If all glass is found, the temperature sistant toward crystallization than commercid glasses. of heat treatment was above the liquidus; if a mixture of Satural glasses exist not very different in composition from glass and crystals is found, it was below the liquidus. By these feldspars; glasses which are highly resistant toward de- measuring the properties of the crystals, they can be positively composition by ordinary agencies, but which are not suitable identified. TJ7ith difficult mixtures it is customary to have for commercial glass compositions because they are so viscous two charges side by side, one initially glass, the other previthat homogeneous and bubble-free melts cannot be obtained. ously crystallized. The heating must be continued long All substances which can be obtained as glasses are viscous enough for both charges to attain the same condition. enough a t their liquidus temperature to inhibit crystallization, Sodium disilicate is noteworthy not only for its low melting and any successful composition must have a high viscosity point, but also for the flatness of the maximum on the melting at its liquidus temperature, yet be fairly fluid a t a temperature point curve. A flat melting point curve is indicative of disnot too high for commercial practice. sociation in the liquid phase; since the compound is largely Glass made from silica itself possesses in the highest degree dissociated, addition of either silica or soda introduces no new the desirable qualities of freedom from devitrification and re- molecular species, but merely shifts the equilibrium between sistance to weathering; and, if i t were not so difficult to those present. Addition of silica to sodium metasilicate melt quartz, to free the glass from bubbles, and to work it, lowers its melting point, as indicated by the curve in Figure silica glass would be the most suitable material for most of 1; similarly, addition of sodium oxide to sodium disilicate the uses to which glass is put. However, the cost of manu- lowers its melting point. The two curves intersect a t 846" facture is prohibitive, and other oxides must be added to C., and a t this point of intersection (or eutectic) three phases lower the melting point and viscosity. In addition, there can coexist, sodium metasilicate, sodium disilicate, and a are some uses in which the modification of properties obtained liquid containing 62.1 per cent silica. When silica is added by the incorporation of other oxides is essential, and the art to sodium disilicate, the melting point is likewise lowered, of the glassmaker lies in the choice and proportioning of the until a t 793" C. and 73.9 per cent silica the sodium disilicateingredients according to the qualities desired in tl-e finished quartz eutectic is reached. On further increase in the silica glass. Of the many oxides which might be added, sodium content the melting point is raised, and one of the forms of oxide and lime are the ones which are used to the greatest crystalline silica remains solid phase, first quartz (up to extent, primarily because of their relative abundance 870" C,), then tridymite (from 870" to 1470°), then cristoand cheapness. From time immemorial glasses have been balite (up to the melting point a t 1710"). composed chiefly of sodium oxide, lime, and silica, and the The low temperature of this eutectic is of great importance proportions of these oxides have always been kept within rather in glass technology and geology. Here is observed a remark-
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able lowering of melting point. The melting point of cristobalite, the stable high-temperature form of crystalline silica, is 1710" C.; this eutectic is a t 793", a Iowering of almost 1000° on the addition of 25 per cent sodium oxide. This eutectic
SO
60
70
60
SOvtp~runt~~o;
FIGURE1. PHASEEOUILIBRIUM DIAGRAM OF THE BINAR; SYSTEM Na20SiO&3iOz
would make an excellent glass were it not for the fact that it is easily attacked by water and hence is not stable in air. I t is of course essential that glasses be resistant not only to the corrosive effects of atmospheric agencies but also to far more drastic treatment, and other oxides must be added to secure such resistance. Most glasses are stabilized by the addition of lime, and the phase equilibrium relationships in the ternary system NazO-CaO-Si02 will be considered next. TERNARY SYSTEM
The phase equilibrium relationships (9) in that portion of the ternary system included within the boundaries NazOSi02-CaO+JiO&Oz are shown in Figures 2 4 . Figures 2 and 3 are parts of the Iarger equilateral triangle whose vertexes represent pure NazO, CaO, and SiOz. I n such an equilateral triangle the position of a point representing a desired composition is given by the center of gravity of the triangle when loaded a t each corner with the proportional quantities of that component. A point on a side represents a mixture containing only two components, the one represented by the opposite apex being absent; the points on a line through any apex represent mixtures in which the ratio of the two other components remains constant; and the points on a line parallel t o a side represent mixtures in which the percentage of the component represented by the apex opposite to that side is constant. For example, points on the side NazO-SiOz represent the same binary mixtures shown in Figure 1; points on a line through SiOz and the compound Na20.2Ca0,3SiOz represent mixtures in which the ratio of NazO to CaO is 1 to 2; and all mixtures lying on a line parallel to the Na20SiOz side have the same CaO content. I n Figure 3 the scale is indicated on the sides and may be transferred into the interior of the diagram by coordinate lines parallel to the sides. This method of representing composition in a system of three components has the advantage of symmetry and is
Vol. 25, No. 7
more convenient except when one component is present in preponderant amount, as is frequently the case with water and salt systems. The other variable, temperature, is represented by erecting a prism on the triangular base, in which the heights from the base are proportional to the temperature. Figure 4 is a sketch of such a solid model. It consists of several mountain peaks and slopes which intersect in valleys, which, in turn, run down to two deep sinks, the two ternary eutectics. In Figures 2 and 3 are shown the projections of these valleys on the triangular base, and in Figure 2 are shown the projections of the curves obtained by passing horizontal planes representing various temperatures, through the solid model. Figure 3 is divided by heavy curved lines (the projections of the boundaries between the various mountain slopes) into several fields, and the direction of falling temperature is indicated by arrows. Within each of these fields a different substance is the first to crystallize on cooling, or is the last to dissolve on heating. For example, from all mixtures whose compositions lie within the area BCSRNML, the compound Na20.2Ca0.3SiO~is primary phase. As the liquid is cooled, and more and more of this compound separates, the composition of the residual liquid changes, and, since the crystals and the residual liquid are being formed out of the original liquid, all three compositions must lie on a straight line, with the composition of the original liquid between the other two; the relative amounts of the two phases will be inversely proportional to the lengths of the two parts of this line. As crystallization continues, the composition of the liquid will be continuously displaced along a straight line radiating out from the composition of Nar0.2Ca0.3Si02until that line intersects one of the boundary lines between the fields, when a second crystalline phase appears. If the original liquid can be made up of the two crystalline phases that separate a t the boundary curve, for example, if it lies on the line joining Naz032Si02 with Naz0.2Ca03sio2, a t M , then it will freeze entirely a t this point, and the mixture may be regarded as a binary system within the ternary system. If it does not happen to occupy this unique position, for example, if it lies a t a higher SiOzcontent than corresponds to the above line, then, as the two kinds of crystals separate together, some liquid will be left over, and the composition of the liquid will follow the boundary curve in the direction of falling temperature, and in the case illustrated the liquid will follow the curve M N . What happens to it after it reaches N will be considered later. It will be observed that the composition of the compound Naz0~2Ca0~3SiOz lies within the field of this compound; a liquid of this composition will freeze completely to this compound when cooled, and the crystalline compound shows a sharp melting point when heated. Such a melting is called congruent. Within the same field is shown the composition of another compound, 2NazOCa0.3Si02, but its field is represented by the area ABLK. Only from mixtures within this field does 2NazOCa0~3SiOz crystallize as primary phase. When the pure crystalline compound is heated, it remains unchanged until the temperature of the point B is reached. Then it begins to decompose into a liquid of composition B and Na20.2CaO,3SiO2; above the temperature and a t the liquidus the latter compound is the crystalline phase coexisting with liquid, Such a melting is called incongruent, and the phenomena taking place at this point are entirely analogous to those which take place a t the transition point of a salt hydrate. When Na2SOd.lOHzOis heated, a t its transition point it decomposes into anhydrous sodium sulfate and saturated solution, I n the binary system H20-Na2S04this is an invariant point, and indeed it is a fixed point whose equilibrium temperature is known to a high degree of accuracy. But if excess of either sulfuric acid or sodium hydroxide is present,
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INDUSTRIAL AND ENGINEERING
the temperature is no longer fixed but is univariant; the transition temperature of the hydrate will be lowered in either acid or alkaline solutions. The case is exactly analogous in the silicate system. B is an invariant point in the binary system Na20Si02-CaOi53iOz but is only one point on the curve BL, the univariant equilibrium 2NazO.Ca0.3SiO~Na20.2Ca0,3Si02-liquid. It is, however, a point of maximum temperature along this curve. Of the two ternary compounds already considered, one has an incongruent, the other, a congruent melting point. But the one with a congruent melting point undergoes unusual dissociation in the liquid phase, as is shown by the shape of its melting point curve. It will be remembered that sodium disilicate also possesses this property; indeed, it is characteristic of all the binary and more complex compounds which are met in the glass field. A third ternary compound is encountered in this system, of the composition h'a~O.3Ca0.6Si02,and it is an extreme case of a compound a i t h an incongruent melting point. The pure compound decomposes a t 1060" C. into liquid and crystals of Ca0,Si02,and the melt does not become entirely liquid until 1325" C. This compound is characterized by an extreme reluctance to crystallize, and every composition in which it is primary phase is a practical g l a s s composition on a manufacturing scale. Its field is s h o w n in F i g u r e 3 by the area NOPQR. Practically all commercial soda-lime glasses are i n c l u d e d within this area. Every mixture within this region w o u l d c r y s t a l l i z e finally a t the ternary eutectic, 0, a t 725" C.; but the crystallization paths by which the liquid would reach this composition differ greatly with the initial composition of the mixture.
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When the liquid phase has followed M N to N , reaction takes place between liquid and NazO 2Ca0 3Sio2 with formation of Na202Si02 and NazO 3Ca0 6Si02; and, since the original composition was assumed to lie within the triangle formed by the three crystalline phases, the liquid will be exhausted while NazO 2Ca0.3SiO2 remains, and the mixture will freeze completely. But if the gross composition does not lie within this triangle, Na20 2Ca0 3sio2 will be used up while liquid still remains, and the resulting mixture of Ka20 2Si02 and Na20 3Ca0 6Si02 will follow the curve NO, to the eutectic a t 0. That would be the sequence of events if equilibrium were reached; but, as the temperature is lowered, the liquid becomes more and more viscous, crystallization and reaction become slower and slower, and to obtain equilibrium is a matter of the greatest difficulty. A mixture whose primary phase is CaOSi02, and whose crystallization path intersects RQ should undergo reaction when this boundary is reached, and, depending on the original composition, the residual liquid will follow this boundary curve either to the invariant point R, in which case it will follow CSRN down to N and thence to 0, or to the invariant point Q, in which case it will follow the boundary QPO,arriving a t the same eutectic. If the original composition lies roo caaStQA
CRYSTALLIZATION PATHS
Consider first a composition within the field of h'a20.2Ca0 3sio2 such that the crystallization path intersects the boundary curve CSRN between S and R. The secondary crystallization will be Ca0.Si02, and the composition of the liquid will follow the boundary line to R. At this point there will be a reaction between liquid and solid phases, with formation of Na20,3CaO6Si02; and, since three solid p h a s e s and one liquid phase a t constant pressure constitute an invariant point, the temperature will r e m a i n constant u n t i l only t w o solid phases, Na20Weight Per cent Sl.4 2Ca0.3Si02and Ka203Ca0.6Si02,remain. Such are the requirements of equilibrium, but in the FIGURE 2. PHASEEQUILIBRIUM DIAGRAM OF THE TERNARY SYSTEM NazO,Si02-Ca0.Si02-Si02, SHOWIXG ISOTHERMS portion of the system under consideration equilibrium is hard to attain, especially when it involves formation of Kaz0.3CaO 6Si02. In practice the boundary within the field of Ka20.3Ca0 6Si02, the crystallization path SRN is easily passed, and the invariant point R still more will intersect R N , in which case Ka20.2Ca0.3Si02 will be easily. To convert all the CaO.SiO2 into the stable l\'a20- secondary phase; the liquid will follow the boundary to A', 3Ca0.6Si02 would require heating for an impractioable time. then react to form Kaz0.2SiO2,and finally run down iV0 to If the conversion is completed, or if the intersection a i t h 0; or the intersection will lie on NO, and Na20.2SiO2will the boundary CSRN takes place between R and N , the liquid be secondary phase; or the intersection will lie on QPO, and will follow the curve to N , the same point previously men- either tridymite, from Q to P , or quartz from P to 0, will be tioned as reached by crystallizing a mixture whose crystalliza- secondary phase. But anywhere within this area crystallization path intersected the boundary M N . This also is an tion is hard to induce, and the glass can be manufactured on invariant point, and equilibrium requires that the tempera- a large scale without devitrification troubles. ture remain constant until all of the Na20.2Ca0,3SiO2 has Any composition more siliceous than the boundary DTQbeen transformed into Nae0.2Si02 and Ka20.3C:a0.6Si02, POF will have a form of crystalline silica as primary phase, with the sole exception of mixtures lying within the triangle and, as the silica separates, the liquid will follow a path on h-az0,2SiO2,Na20.2Ca0.3Si02, and Sa20.3Ca0,6Si02. When the straight line through the Si02 apex and the original solidification is complete in a ternary system, there must composition, until a boundary is reached. If the boundary be three crystalline phases, unless the composition has been intersected is between D and Q, CaO.Si02 will be secondary so chosen that i t lies on a binary join, as was seen to be the phase, and, when Q is reached, equilibrium demands that all case a t M , or unless it, has the composition of a compound. of it be transformed to Na20,3Ca0,6Si02,and that the liquid
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INDUSTRIAL AKD EXGIKEERIE G CHEMISTRY
\-01. 23.
KO.7
4 Pa0
ment of one per cent of lime by alumina lowers the melting point almost 100" C. By lowering the melting point, alumina brings the glass into the field of the Na20.3Ca0,6Si02 compound, which is characterized by small t e n d e n c y toward crystallization. The effect of other oxides which may be cons i d e r e d as adventitious impurities is similar. Some magnesia is usually present, if only as the result of pot corrosion, but small amounts of it have a favorable effect in lowering the liquidus temperature, and it is a common practice in the United States to deliberately introduce magnesia by the use of dolomitic limestone. Iron oxide has a marked effect in lowering the freezing point, and it was present in most of the older glassware in amounts which would not be tolerated today, except when the color was definitely wanted. Boron oxide is always added deliberately to increase the speed of melting, the durability to weathering, the brilliance, and the strength. It has a favorable effect in lowering the liquidus temperature. Many glasses contain part or all of their OF THE TERNARY SYSTEM Pr'ai0.- alkali as K20,but its use is limited by its high FIGTJRE 3. PHASE EQUILIBRIUM DIAGRAM Si02-Ca06i02-Si02, SHOWING BOUNDARY CURVESAND TIE LINES cost, The phase equilibrium diagram (8)of the t e r n a r y system K20-Ca0-Si02 is shown in then follow the curve QPO. If the boundary is met below Figure 5. There are-many more ternary compounds in the Q, Naz0.3Ca0.6SiOz is secondary phase, and the liquid will system formed with KzO than in the corresponding one with follow the same path to the eutectic a t 0. But the remarks Kaa0. In the latter case there was but one compound higher previously made in regard to the difficulty of attaining equilib- in SiOz than the metasilicate ratio, the compound KazOrium apply with added weight to this region. Glasses near 3Ca0 6Si02, while in the system KzO-CaO-Si02 there are QP are difficult to induce to crystallize a t all, and to secure five such compounds-namely, the a- and &forms of the complete equilibrium is next to impossible. Indeed, the compound K20 3Ca0 6Si02, analogous to the compound best composition for a glass composed of KazO, CaO, and formed in the system containing Na20; the two disilicates Si02 is either near t o , and t o the right of, the line PQ, or 4K20 CaO 10Si02 and 2K20 CaO 6Si02; and the trisilicate within the field of Naz0.3Ca0.6SiO2. As has been said, the KzO 2Ca0 9Si02. The glasses formed are more difficult to latter compound is characterized by extreme reluctance to crystallize than is the case with those in the system, Na20crystallize, and the field of practical glass compositions is CaO-Si02, which circumstance is probably to be ascribed almost coincident with the field of Ka20.3Ca0.6SiO2. One largely to the greater viscosity of the potash glasses, although reason for this may be that the field of this compound is far the larger number of dissociating compounds, with consequent removed from its composition, and for crystals to form re- increasing molecular complexity, undoubtedly plays an important part. The addition of potash to a soda glass diminquires a considerable amount of molecular movement. ishes the tendency toward devitrification, increases the r-iscosity, and rendeis the glass more resistant to weathering. REGION OF COJIXERCIAL GLASSCOMPOSITIONS
cQA
We have seen, then, that the region of commercial glass compositions is essentially that of the compound Naz0.3CaO 6Si02, and that the temperatures in this region range from 1060" to 725" C., and we have the answer to why glass behaves like glass. Mixtures within this area are above their freezing temperatures throughout their working range; that is, because of their low liquidus temperature the glass becomes viscous enough for working while still above its freezing point; whereas, by the time the liquidus is reached, the glass is so viscous that crystallization can be induced only by special heat treatment. But depart only a few per cent from a restricted field, and this is no longer true. As soon as the glass enters the field of Si02, of CaOSiOn,or of Na~0.2Ca0.3Si02, the temperature increases rapidly and crystallization takes place much more readily; hence the glassmaker must keep his compositions within narrow limits. He is aided, however, by another factor-he is never working with pure sodalime glasses, even though he may call them such, but always with glasses containing significant amounts of other constituents which are introduced incidentally either as impurities in the batch ingredients or by pot or tank corrosion, or which are introduced deliberately. The effect of each of these impurities is to lower the melting point; for example, replace-
TYPES OF GLASSUSEDIK INDUSTRY Nost of the glass manufactured is essentially of the sodalime-silica type, modified by the inclusion of small percentages of other oxides, and the major uses to which glass is put, including windows, automobile glazing, bottIes, jars, tableware, and electric light bulbs, take a glass of this type. The various types of glass used for chemical ware and laboratory tubing were formerly exclusively derived from this same type. The ordinary "soft" glass was a soda-lime glass, the inferior grades tending to be low in CaO, and high in KanO, which placed them rather near the lower portion of the NazO 2Ca0 3Si02--1-az0 3Ca0 6SiOZ field, and contained little The better grades contained A1203 and were nearer the Ka20 3Ca0 6Si02-Si02 boundary. From this the composition ranged from those which were improved by the inclusion of some RTgO, K20, Bz03, and ZnO, to those, such as the Jena Gerate, in which all of these oxides were present. All of these types, however, are to be considered as derived from the simple Sa20-Ca0-Si02 eutectic by the addition and substitution of additional oxides for the purpose of securing greater resistance to weathering, and a lowered liquidus temperature, These older types of chemical ware, howeve1 ,
have been largely superseded by Pyrex resistant glasa, which (luced are higl~lydissociated in the liquid phase, and rnauy of is of a different type not derived from the Sa20-CaO-Si03 them become unstable on heating and are said to possess ineutectic, or ex-en from the sodium disilicate-quartz eutectic, congruent melting points. It is accordingly a plausible asliiit is one in which the melting point of the SiO, is lowered by suniption that a complex glass a t its liquidus contains reprethe addition of X,O, and, in smaller aniouiit, ALOs, with only the smallest possible amount of alkali. It is intriiisically superior to the best of the soda-lime group in resistaiiee to corrosion and to breakage from heat shock or bad annealing, but is more difficult to manufacture and work. The optical glasses are a group importsiit in their use in instruments but insignificant in their t.onnage and value. They are all rlcrived essentially from the same general type, except that it would be better to consider them as derived not from the ternary eutectic obtained by adding lime to the binary eutectic, sodium disilicate-quartz, but rather from the binary eutectic, alkali silicate-quartz. Many glasses of tlie crown type are essentially the same as ordinary window glass, except that they possess the quality cliaracteristic of all optical glasses of a physical perfection result,ing front freedom from bubbles and inhomogeneity. Other similar glasses contain notable quantities of potassium oxide replacing sirdiurn o d e , and of boron oxide replacing silica in quantity up to, and sometimes esceeding, 10 per cent, giving rise to t i l e borosilicate crowns. AD important series of glasses are the optical flints, ranging from light flints containing 25 to 30 per cent lead oxide and esseiitially the same as the lead glass formerly extensively used for blowpipe work, for lanip biilbs, and for cut glass, up to extra dense flints containing as high as 80 per cent lead oxide. The optical flints usually contain potassium oxide as the only alkali. Another important group of optical glasses are the barium crowns, characterized t,y the presence of barium oside up to as high as 50 per cent, but still essentially derived from tlie same ent,ectic, and owing their important qualities to the high iiiulecular weight sentatives of niany inolecnlar species, with those groupings of barium oside. that give rise to the compounds ~vliichare tlie stable phases dong the liquidus prcsent in comparatively small proportion. The actual proportions of tlie various molecular grwpings, S a ~ u i mOF GLASS transitory though each of them inay be, probahly change with I~roiitthe poirit of view of phase equiliiiriuni, glaca is to lie t,he temperature, in accordance with the usual laws of ntobile considered as a liquid w1,iclr has passed throug!t its freezing equilibrium, with the proportion of the more complex group point or range without crystallizing, and Tvhose viscosity has ings increasing with decreasing temperature. At each increased until it has the physical properties of a rigid solid. temperature there will be a definite equilibrium relatiori beThere is little more that can be said with assurance about tween such molecular Bonpings, and the attainment of this the constitution of glass. .411 of tile binary and ternary com- equilibrium relation takes place wit,h a speed decreasing with pounds n+iich separate from glass d i e n crystallization is in- the temperature. When the viscosity has reached a value of about 1013poises, corresponding to a temperature well below tlie softening temperature, and a little above the upper part of the annealing range, i t becomes possible to detect the change in properties of tlie glass with change in heat treatment., and a t a little lower temperature it becomes n e c e s s a r y to subject the glass to a definite heat treatment in order to obtain i t in a reproducible condition suclt that its properties are not dependent upon the duration of t,he heat treatment. The tiinc required to attain such an equilibriiim condition increases rapidly as the temperature is lowered, dtEough in general tlie attainment of tlie equilibrium condition requires a shorter time than does the removal of mechanical stress. The process, however, seems to be a continuous one, and no actual discontinuities appear in physical properties if the measurement is made on glass which has been held at constant temperature long enough for the given property to take on the constant value pertaining to that temperature. I n spite of some suggestions to tlie contrary, the evidence afforded by x-ray studies of glasses a p
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pears to substantiate this conclusion. Some glasses have been found which give indications of the presence of lines in their x-ray spectrum which might be ascribed to cristobalite, but such lines are weak and not highly diagnostic, and give no suggestion that the other constituents of the glass could be present in groupings even suggestive of crystalline compounds. Other glasses are to be found which give rise to broad and hazy lines suggestive of crystals of colloidal dimensions, but the lines so observed do not correspond to the compounds which would separate if devitrification had taken place. For example, a glass of the composition of the oompound Naz0,2Ca0,3SiOz, the x-ray spectrum of which has been described, and which melts congruently a t 1294" C., gives rise to an x-ray pattern of the type described. Some of this glass was slowly devitrified by long-time heating a t 500" C., and the change in pattern was observed. The new pattern had no relation to the previous hazy lines, and the beginning of crystallization could be observed with the microscope long before a change in the x-ray pattern could be observed. This glass has been crystallized as low as 400' C., and the same crystals separate a t this temperature as a t the liquidus. Most glasses, however, do not give rise even to hazy lines, but give the broad, diffuse band characteristic of the liquid pattern, which has not yet been explained satisfactorily. All lines of evidence point to ,glasses being materials which satisfy the phase equilibrium definition of liquids, which have cooled with continuous change in properties until they have attained the rigidity characteristic of ordinary glass.
Vol. 2.5, No. 7
With respect to the constitution of glass we have little definite knowledge. That such should be the case is not surprising. Glass is a complex solution containing many components, the compounds between which are characterized by a large degree of dissociation in the liquid phase, and as such the problem of its constitution is a more complicated one than that of the constitution of a solution of a salt in water. That is a problem which has attracted the attention of chemists for the past fifty years, and the net result of all their experimental work may be summed up in the statement that we know of no method of establishing beyond a reasonable doubt the constitution of the simplest aqueous solutions. When an adequate theory of the constitution of liquids is formulated, it may be expected to explain the properties of all solutions, not only the solutions of salts in water but also the more complex undercooled solution which is glass. LITERATURE CITED (1) Morey, G. W.,and Bowen, E. L., J. Phys. Chem., 28, 1167 (1924); Kracek, F. C.,Zbid., 34, 1583 (1930). (2) Morey and Bowen, J . SOC.Glass Tech., 9,226 (1925); Morey, J. A n . Ceram. SOC.,13, 683 (1930). (3) Morey, Kracek, and Bowen, J . SOC.Glass Tech., 14, 149 (1930); 15, 57 (1931). (4) Keumann, 2. angew. Chem., 38, 766 (1925); 40,963 (1927). RECEIVED April 11, 1933.
Critical Temperatures in Silicate Glasses J. T. LITTLETON, Corning Glass Works, Corning, X. Y.
S
ILICATE glasses only are to be treated here, not because the organic plastics, which are often included with the silicates, are of insufficient interest, but because it has not as yet been shown that there is any similarity in these two families of materials other than that of appearance. Probably their molecular structures are widely different The question of the existence of any abrupt or abnormally rapid changes in the physical properties of materials as the temperature is changed is often of considerable practical importance. Extrapolation into unexplored temperature regions is never a safe procedure, even with materials of simple composition, but becomes particularly hazardous with materials of several components and of a complex constitution such as the common glasses. Critical temperature points, however, are perhaps of even more interest from the theoretical point of view than from the practical. Any discontinuity or abrupt change in the properties of a material is usually assumed to be associated with, or caused by, a change in the constitution of the material and often a study of such effects will lead to a fairly clear picture of its molecular constitution. The early work on glasses of the Bureau of Standards, particularly the work of Peters and Cragoe and of Tool and his coworkers first pointed out the fact that a change in the con.stitution of glass occurs in the neighborhood of the annealing zone. These observations have been frequently interpreted by others as proving the existence of critical temperatures associated with a discontinuous change in the physical properties of glasses. Accordingly recent glass literature will be Teviewed with the thought of identifying and establishing any real critical temperature points, or to indicate the insufficiency of the data to lead to positive conclusions.
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The search for critical temperatures in glasses is, however, not so simple as it might appear to be. Berger in a series of recent papers has massed a large amount of information proving almost assuredly that the physical constants of glasses depend to a certain extent upon their past thermal histories. We can suppose that a t any temperature there is a certain equilibrium configuration of a minimum potential energy, but, owing possibly to the highly viscous nature of glass a t low temperatures, these configurations do not follow temperature changes rapidly, but seriously lag behind. This is often spoken of as a freezing-in of the properties of a previous state. At low temperatures the glass may be said almost never to have the properties corresponding to its final state of equilibrium a t this temperature. Yet the changes are so extremely slow, because of the practically solid condition of the material, that these properties may be called unchanging, even though the evidence is that there exists a tendency to change. At intermediate temperatures-that is, in the neighborhood of the annealing temperature-the glass is sufficiently fluid so that the changes of properties with time are rapid enough to be observable. These changes depend upon the past thermal history of the glass. A glass suddenly cooled from a higher temperature level tends to retain the properties peculiar to that temperature state, and considerable time may elapse before the new equilibrium configuration is attained. This change of state or of constitution, taking place during the attainment of equilibrium, is not associated with a critical temperature. The physical properties will change when the glass is brought to a temperature either higher or lower than its previous effective equilibrium level, and hence both the magnitude and rate of change depend upon the magnitude of the change of temperature level and the yiscosity of the glass a t the particular instant of
