CHEMICAL TRENDS HENRY 11. BIAAl'.Corning Glass Works, Corning, N . Y.
Chemical developments in glass technology may be logically divided into two groupsthose arising from the chemical relations in the formation or production of glasses, and those associated with the uses of glass products. Glass compositions for general purposes have been evolved by adjustments to meet more exacting requirements in use and the needs of revised production methods. More radical measures embracing many widely different compositions have been taken to meet special technical requirements. For most purposes fused silica glass offers almost ideal properties, which have been approached step by step in commercial glass developments.
ECENT chemical trends in the technology of inorganic glasses have been characterized by a great rate of advancement rather than by changes of direction alone. This may seem surprising in view of the fact that there have probably been as revolutionary developments in the glass industry during the past decade as have occurred in any equivalent period of the long history of this unique material. Such developments may be logically divided into two groupsthose arising from the chemical relations in the formation or production of glasses, and those associated M-ith the uses or applications of-glass articles. Marked changes in glass products and in the methods of fabricating them have given rise to correspondingly radical revisions of the chemical approaches to the resulting problems. Although i n many cases changes of form, requirements, and physical treatments have imposed new or added chemical limitations, in other cases these same factors have extended the ranges of chemical methods and compositions which can be employed. The more extensive use of physical processes in the develop ment of desired properties of glass have required the most intimate correlations of the chemical aspects of these same problems. In addition to these revisions of viewpoint which have originated somewhat outside of its distinctive field, glass chemistry has con-
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tinued in the application of its own initiative and ingenuity in the seeking of old ideals.
General Trends For most purposes the properties of pure silica glass have long been considered as the ultimate goal of the glass technologist. However, the difficulties of making glass consisting of pure silica alone have presented almost insurmountable practical obstacles. Such glasses are difficult and expensive to melt and to free from bubbles of included gases. Their viscous properties not only interfere with the removal of these bubbles, but greatly restrict the fabricating processes so that only limited shapes and perfection of form have been attainable. These limitations, and the associated ones arising from costs of production, have not only greatly r e stricted the use of fused silica glass, but have more commonly necessitated the use of additional oxides which serve to dissolve or flux the silica a t relatively lower temperatures and to adjust the viscosities of the resulting solutions so that they may be freed of gases as well as fabricated commercially. From the earliest times the oxides of the alkalies and the alkaline earths have been used as fluxes in glassmaking. Apparently even the Egyptians and Romans recognized the fact that, although glasses containing higher percentages of the oxides of sodium and potassium were easier and cheaper to melt and form, they also entailed sacrifice of utility and some desirable properties. Although some of these prototypes of modern glasses were fairly satisfactory, most of them showed a marked susceptibility to attack by atmospheric and other agents, which in extreme cases led to the destruction of the glass articles, but in more favorable instances was responsible for the iridescent incrustations so characteristic of museum pieces of ancient glassware. The better glass compositions of these early eras did not differ markedly from the glass compositions used generally for windows, bottles, and even scientific appar a t u s as l a t e as t h e middle of the nineteenth century or even more recently. By this time window glasses were available of somewhat, but not markedly, lower alkali contents and with correspondingly improved chemical stability. Special bottle glass compositions, of very low alkali content with high lime, iron oxide, and alumina contents, had been used for bottles for champagne and other wines. It was recognized that the 1419
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TABLE I. TREND OF GLASSCOMPOSITIONS THROUGH THE
AGES (IN P E R CENT)
Glass
SiOz
NazO
Kz0
MgO
Eg ptian from Thebes (1500B.c.) BagyloniAn-Assyrian, from Nippur (250
67.8 60,s
13.7 13.5
2.3 2.2
2.3 4.1
4.0 5.8
4.4 1.4
Egyptian Pompeian window German window (1849),hand-blown
72.3 69.4 70.5
20.8 17.3 12.0
.... 1.9
....
... ...
...
5.2 7.2 13.0
1.2 3.5 1.9
Belgian window (1898),hand-blown
74.8
13.0
....
...
11.2
French window (1862)
71.9 71.7
13.1 12.0
....
...
71.2
16.8
....
3.2
61.8 63.9 74 1 71.7 77.0 74.1 66.0 80.9 96.0
8.1 9.4 16.6 12.1 5.0 15.1 8.1 4.4
B.C.)
American Lubbers window . ......
(1930).semi-
automatic American Fourcault window full automatic Ordinary green bottle Av. Owens bottle (suction) Bottle for automatic methods ( Bottle’glass (1940),automatic Stas apparatus (1868) Thurinaia apuartltus (1869) Jena afparatbs (1916) U . 6 . a paratua (Pyrex brand), No. 774 967 ai ioa. No. 790 (1940)
f
....
....
...
CaO
-4120%
Bet08
SOs
BzOr
ZnO
....
1.8
1.0 1.0
.... ....
0.5 1.1 0.4
.... .... ....
.... .... ....
7 - i - - .
1.0 L -
13.6 14.2 8.0
1.4 1.8
0.14
0.7
0.11
.... ....
.... 0.6
....
.... .... ....
....
.... .... ....
.... .... .... ....
.... ....
clarity and quality of these liquors were impaired if stored in less chemically resistant containers, although these same glasses had other advantages derived from their colors. However, the properties of these glasses were such that they introduced special problems in melting and forming, even by hand. Shortly after 1860 J. S. Stas recognized that the precision of his work on the determination of atomic weights was impaired by contamination from the available glassware, and he developed a high-silica soda-potash-lime glass which soon found extensive use in other as well as his own laboratories. About this same time i t was recognized that the glasses made in Thuringia were of higher chemical resistance than could be explained in terms of their silica, lime, and alkali contents alone. Although this was a t first attributed to geographical or secret factors, i t was later ascertained that the alumina derived as a n impurity from the sands in use in that district of Germany was largely responsible for these advantages of the finished products and also contributed to the excellent workmanship of the glassware produced in this region. The recognition of these outstanding influences of a previously inconsequential constituent gave an impulse to the investigation of the possibilities of the oxides of other elements. This found its fruition in the work of Schott and Abbe a t Jena. Although their work was primarily devoted to the systematic investigation of the influences of composition on the optical properties of glasses, i t was extended to the consideration of scientific glassware broadly, so as to include chemical- and heat-resisting relations as well. Their work led to the practical production of laboratory glassware containing such oxides as those of zinc, magnesium, barium, boron, and similar elements; but in general their glasses were of relatively low silica content. At about the end of the first World War researches in the United States were responsible for a gigantic stride in the efforts to produce commercial glasses with properties approaching those of fused silica. These glasses were of the borosilicate type and contained slightly over 80 per cent silica with only about 4 per cent alkali oxides. They met wide and ready acceptance in the fields of laboratory, domestic, and industrial glassware as a result of their high chemical, electrical, and thermal resistances. During the past year the development of a new glass was announced which constitutes an even greater advance toward the production of a glass approaching the composition and properties of fused silica. Its outstanding feature is that this has been accomplished with the avoidance of the former limitations of melting and forming, This glass in the finished
...
...
.... ....
.... ....
1.8
....
....
....
8.0 12.6 3.6
.... .... ....
.... .... .... .... .... .... .... .... .... ib:i
.... ....
article consists of approximately 96 per cent silica, boric oxide constituting practically the remainder. The most novel aspect of the production of this new glass arises from the fact that during the melting and even through the molding stages of manufacture, its properties are similar to those of ordinary glasses. Thus most of the previous difficulties associated with melting and fabricating glasses of high silica content have been avoided. As a matter of fact, borosilicate glass compositions of about 75 per cent silica content are used throughout those earlier stages of the process, in which the glasses are melted and molded. After molding and cooling, the glass articles are subjected to heat treatment which induces the glass to separate into two distinct physical phases, with a resulting slightly opalescent or turbid appearance. One of these separated phases is so high in boric and alkali oxides that it is readily soluble in hot acid solutions, whereas the other phase is rich in silica and accordingly quite insoluble in these same solutions. After the glass article has been subjected to this heat treatment and annealed, i t is immersed in a hot acid bath for sufficient time to permit the soluble phase to be virtually all leached out. It is then thoroughly washed to remove traces of the soluble phase as well as impurities, and subjected to a heat treatment which serves to dehydrate the body and to convert its cellular structure into a nonporous, vitreous article. I n the course of these processes the glassware undergoes a shrinkage which amounts to 14 per cent on a linear basis and has led to the resulting product being dubbed “shrunk” glass. The extent to which the properties of this material approach those of fused silica and differ from its predecessors is shown by the data in Table 11.
PROGRESS IN PROPERTIES OF GLASSES TABLE11. RECENT Glass Softening point, a C. Annealin$ point, ’ C. Strain point C. ax. temp.’ for use (for limited periods), 0 ~
CI.
Specific gravity Coefficie?t of linear expansion
Common Lime 696 510 475
...
2.47 92 X 10-7
Pyrex,
No. 774 819 553 510 500-550 2.23 32-33 x 10-7
96% Silica, No. 790 1442 931 857 900-1000
Fused Quartz 1667 1140 1070 1000
2.18 7.8-8
2.21 5.5-5.85 x 10-7
x io-
Although the development of technical glassware has usually served as the forerunner of changes which have intimately influenced the compositions used for more general purposes such as windows and containers, i t would be in-
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positions witb little adjustment were still not only permissible correct to assume that there has always been continuous progbut desirable, in that they permitted higher production as a ress in composition developments independent of other facresult of their more rapid setting or rates of becoming rigid. tors and circumst.ances. The work of Stas and otliers of tlie However, wit,h the more efficient Fourcault and Colbnrn procsame period was reflected in a reduction of alkali content in esses in which the glass is drawn directly from the molten bath window and bottle glasses in the later years of the nineteenth in the form of a flat sheet, it was impossible to employ these century. However, the adoption of mechanical glass-forming methods placed new limitations on the glass compositions compositions without more drastic adjustments. These entailed increasing the ratios of alkali to alkaline earth oxides which could be used successfully. From a thermodynamic to avoid crystallization and to obtain annealing characterviewpoint glasses under conditions of use and through certain istics within the range of possibilities of this type of sheetstages of manufacture are unstable systems. If glass is perdrawing equipment. The resulting decrease in chemical stamitted to remain in these temperature ranges long enough, it bility or resistance to atmospheric attack was offset to some will lose its characteristic amorphous structure and become extent in Europe by the addition of alumina and in this crystalline, at least in part. Unfortunately the temperature country more frequently by the partial substitution of magranges through which glasses must pass in the course of their nesia for calcium oxide. Similarly, the better bottle comfabrication are usually those in which this tendency to cryspositions for hand gathering were not well adapted to m a tallize is most marked, but this is offset by the glasses bechine gathering and forming. Suction gathering by the coming more viscous and thus tending to arrest the mobility Owens method required some changes in t.he direction of nccessary to permit rapid crystallization. The changes of higher alkali contents, but the gravity of gob-feeding methods viscosity with temperature or its correlative, time, also for delivering glass to the machine necessitated even more largely determine the ability by any given method to impart marked deviations. The better command of the mechanical to the glass, the desired form or shape. The selection of a factors, as well as the more and more insistent demands of the suitable glass composition for a particular purpose accordingly users of glass containers, has recently brought a reversal to not only involves its properties in the resulting product but this trend. Auxiliary fluxes, such as fluorides and borates, also its dimensions and the methods by which tbe glassware have been introduced in several instances to further this is to be produced. This results from the fact that combinaprogram. tions of these factors control the time-temperature relations I n other instances new methods and different forms have which, in turn, determine the rates and conditions under increased the ranges of composition possibilities. In the which the glass article may be formed and whether or not forming of glass fibers, the cooling of the glass through those the glass will crystallize. For the most part, hand-forming temperature ranges which permit Crystallization is so rapid processes lend themselves to greater flexibility in adjusting that compositions are permissible which contain little or even the cooling rates of the glasses through these critical temperano alkali. This is particularly fortunate in view of the fact ture zones than is attainable in automatic production. that, when glass is in the form of filaments or fibers such as The adoption of mechanical glass-forming processes bas those used for insulation or thus not only required the textiles, it presents an enormore precise control of those mous surface for atmospheric factors affecting viscosity, or fluid contact. Alkali in such as composition, time, the glass surface tends to and temperature, but also absorb moisture which, in restricted the ranges of comturn, dissolves the alkali. positions which can be emThe resulting solution attacks ployed successfully. I n some the silicate, and this cycle instances this has led to retculminates in the weakening rogression or sacrifice in the or even the destruction of the properties of the resulting fiber. This same mechanism, products and to the need of even when carried only to a mnre exacting chemical solnslight extent, destroys the tiuns to the problems of adability of fibers to serve as justing the glass viscosities electrical insulators. In this and other working properinstance both the process ties to the requirements of and the product permit the the machine, without too use of glasses of extremely great sacrifices of the desired low or no a&& content. characteristics for the final products. The trends resulting from Chemical Properties of these relations may be illusGlass Products trated in both the flat or There is an ever-widening structural glass and container fields. Low alkali and high and more intelligent appre ciation of the relations bealkaline earth silicate comtween glassware and possible positions were used in making contamination arising from window glass by hand in the ita solubility or inadequately form of blown cylinders which low c h e m i c a l s t a b i l i t y . were subsequently split and These have manifested flattened into sheets. In the themselves in laboratory as semiautomatic or Lubbers well as in industrial applicaprocess in which cylinders tions. An outstanding exweredrawn bymachinebefore ample has recently occurred Battening, these same comSTEIJBEN CRYSTAL GLASSVASEBY MUIRHEAD BONE
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in the distilled spirits incontainers for pI3 seta or dustry. Sizable amounts buffer solutions in which of bottled gin were reeven slight changes of turned to one manufacalkalinity must be miniturer because it showed a mized. In other instances bluish opalescent turbidity resistant glasses entirely and, in some instances, free of boron have been floating flakes. Although developed to meet the rethe gin was slightly acidic quirements of research in when originally bottled, the fields of plant nutriit was found to be distion and animal husbandry tinctly alkaline when rein which relatively small turned. An investigation ranges of boron coucentrsrevealed that some of the tions have been found exglass bottles showed alkatremely critical. i n such linity within 3 hours of work as cancer research, contact w i t h d is t illed trials of the 96 per cent water while others did not silica glass as microscope in 3 months. This investislides have shown marked gation has been extended advantages in that dieand has led to more solved material influencing rigorous specifications for the rates of cell or bacteria liquor bottles and to more growth and crystallization enliehtened test methods has been reduced to a for -evaluating the suitminimum. However, the ability of the glassware. problems presented to This constituted another glass technology do not i n s t a n c e demonstrating all arise from chemical that a satisfactory test for sources. evaluating chemical staSpecial Glass bility for a given purpose Developments should approximate the conditions of use rather Modern science, engithan result from an exneering, and industry in b W B E N CRYSTAL GLUS VAS# BY MARIELAWRENOIN tensive extranolation of general have imposed new form (such as occurs and exactins demands on with tests of pulverized glass) or Conditions (such as are glass chemistry. Interesting examples are f o n d in the needs encountered when concentrated solutions or excessively high for glasses for use in connection with the emission, transmistemperatures are employed in certain test methods t o sion, or absorption of radiant energy. Metal-vapor lighting obtain accelerated action). Similarly it was found imtubes have required the development of compositions resistant possible to correlate the tendency of the bottles to form to such highly corrosive vapors as that nf metallic sodium a t flakes or sediment, with the alkali absorption from the elevated temperatures and forced recourse to glasses of exglass alone. Whiskies and other distilled products were tremely low silica as well as a b l i contents, containing found to be affected in much the same way as gin. These preponderant amounts of alumina and alkaline earth oxides. studies led to the recognition of contamination from eonNonsilicateopticalglass, containing for the most part such untact with other equipment. For example, heavy metals such usual oxides as those of lanthanum, zirconium, titanium, tantaas copper and iron caused the formation of flocculent prelum, and tungsten, have been discovered in efforts to obtain cipitates and discolorations. As a result, not only were more glasses combining high indices of refraction with low disperrigorous demands made on the manufacturers of glass eonsions. Other interesting examples of progress are found among tainers, but the use of resistant glass piping, tanks, and other glasses for transmitting or absorbing special ranges of wave equipment is being extended. Such industrial glassware is lengths of the invisible as well as the visible spectrum. This finding ever-widening uses in equipment in the chemical has required not only the development of new compositions industries wherever unusual resistance to corrosion as well by addition and substitution, but in the cases of the ultraas freedom from contamination is essential. Such uses inviolet transmitting glasses, the perfection of techniques clude chlorination, the handling of powerful oxidizing agents, for eliminating minor impurities. To this end, the glass and the conveying of hot brines and acids. chemists’ age-old efforts to eliminate or minimize the conThis critical appreciation of the relation between the quality tents of such impurities as the oxides of iron and titanium of glassware and possible contaminations has been even more have been extended so far that commercial glasses containing noteworthy in the more refined applications. Ampoules and well under 0.01 per cent of the oxides of iron are now availbottles for many pharmaceuticals and serums, such as Inable. The applications of similar techniques to compositions sulin, are made from the more resistant borosilicate glasses. of the general type of heavy English crystal has made posDifficultiesarising from glass apparatus in the laboratory has sible the production nf decorative glass and tableware of taken on new mpeets. In one important instance it was such surpassing clarity and freedom from color that i t offers found necessary to eliminate arsenic, ordinarily introduced a new medium for contemporary artists and craftsmen. in glass to assist in the removal of bubbles, from the comChemical processes have also made possible still another position used for laboratory ware. This move was stimulated form of glass known as cellular or sponge glass as a result nf in part by claims that misleading analytical data had reits unusual structure consisting of many tiny noneonnecting sulted in confusion in litigstion involving fruit growers’ assobubbles, all completely surrounded by glassy walls. This ciations. Special resistant glasses have been developed for material has a density of only 0.17 gram per 00. and a K
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INDUSTRIAL AND ENGINEERING CHEMISTRY
value for thermal conductivity of 0.42 B. t. u. per square foot per hour per O F. in air, as compared with 0.32 for cork brick. It can be sawed, and is impervious to watcr, noninflammable, as well as mold- and vermin-proof. It is producctl by chemical processes by heating finely crushed glass with a material such as calcium carbonate which evolves a gas by its own decomposition, or with one such as carbon, which reacts with the glass itself to form a gas, while the glass is still in suitable temperature ranges for the formation of the desired structures. Chemical developments in glass technology are proceeding so rapidly in the many varied branches of the industry that it is impossible to do more than make a general and cursory survey within the allotted space. Little can be stated here
1423
of the economic limitations which have influenced the revisions of compositions, or of the improvements which have been made possible by developments in refractories, furnace equipment, or fabricating techniques. The use of new materials available as a result of improved mineral purification have met many requirements and opened new approaches. Chemical treatment and purification of sands has made available a source of silica which has permitted the making of cheaper automatic tableware and containers of greatly improved brilliance and color. It is hoped that even so superficial a review may serve to indicate that glass chcmistry is not only meeting the demands of a rapidly changing civilization and cconomy, but is also taking the inittative in the development of new and improved compositions and products.
PHYSICAL TENDENCIES GEORGE W. MOREY, Gcophysical Laboratory, Carnegie Institution of Washington, Washington, D. C.
Physical trends in glass today include the production of essentially new materials of greatly increased strength by controlled heat treatment, of new compositions (also as the result of controlled heat treatment), and of new applications of glass in fields to which its properties make it especially adaptable. These new uses include architectural panels, glass building block, and the several new industries built upon glass fiber. The control and enhancement of physical properties make possible these new uses and applications of glass.
HE physical trends in the development and uses of glass at the present time may be considcred under two dificrent heads: first, the development of new glasses and new uses for glass by modification of physical properties; and scconcily, the wider use of glass in fields in which its propertics have not been fully exploited. The physical propertics of glass are determined primarily by its chemical composition, and the modifications in property that can be obtaincd are usually second-order cffects. These second-order effects, however, are of importance in incrcasing our knowledge of the iiature of glass, and in some cases are of industrial importance. The chicf physical mrthod by which properties of glass can be modified is the control of thermal history by subjecting the glass to a predetermined heat treatment. The heat treatment may affect all the physical properties. Annealed and chilled glasses of the same chemical composition show differences in density, refractive index, expansion, electrical conductivity, elasticity, and strength. These differences are of two kinds: those due to the stress resulting from purely mechanical strain, and those resulting from the freezing in of a n equilibrium condition
T
characteristic of some high temperature. It mill be of interest to consider the theory of these two effects.
Annealing of Glass The heat treatment usually given glass is an annealing treatment, designed to remove most of the mcchariical stress resulting from too rapid cooling and to prevent too lncalizcd a distribution of the residual stress. Suppose that a flat plate or slab of glass, long enough so that end effects may be neglected, originally free from strain, and at a uniform and comparatively low temperature, is heated on both faces-that is, on the two surfaces normal to the shortest dimensions of the piece. The outside layers become hotter and tend to expand more than thc inner layers; that is, the surfaces of the slab are not free to expand the normal amount and will therefore be under longitudinal compression i n all directions parallel to thc surface of the slab. At the same time the innermost layers will be stretched by the outer layers and will be under tension. The longiturlinal stress changes continuously from a compression a t the surface to a tension at the middle, necessarily passing through an intermediate zone of zcro stress. If the heating be continucd until the temperature of the slab again becomes uniform a t some temperature well below the annealing range, the stresses caused by the temperature gradient will have disappeared with that, gradient. Such stresses may be called temporary, for they continue only so long as the temperature gradient is maintaincd. If the same sample of glass, of uniform tempcrature and free from stress, had been cooled from the same original condition, temporary stresses would have been developed of sign opposite to those resulting from heating-that is, tension in the surface and compression in the mitldlc zone. In general, a temperature gradient established by heating will produce longitudinal compression in the outer layers, and a temperature gradient established by cooling will produce longitudinal tension in the outer layers. Conversely, if a temperature gradient, exists in a slab of glass which is free from stress, the removal of the temperature gradient will cause stresses equal and opposite in sign to those that would be produced by the establishment of the
