PRODUCTION AND PROPERTIES OF GLASS CONTAINERS 19 ordinary liquids, and like other liquids will flow under the influence of gravity or other small force. At the temperatures at which they should freeze or begin to crystallise, the glasses are viscous liquids and it is largely because of their great viscosity that glasses can be cooled through their freezing points without devitrification. They then become "undercooled liquids". As undercooling is continued to ordinary temperatures, the glasses become increasingly viscous. The increase in viscosity with decreas- ing temperature is a continuous process from the liquid above the melting or freezing point, to the rigid glass at ordinary temperature. From the freezing point to the ordinary temperatures the material remains as an undercooled liquid with respect to the process of crystallisation. Glass may then best be defined as "any inorganic substance in a condition which is continuous with and analogous to the liquid state of that substance but which, as a result of a reversible change in viscosity during cooling, has attained so high a degree of viscosity as to be for all practical purposes, rigid." This definition, which makes use of only those characteristics which are essential to glass, was oeornnfiated by George W. Morey, a leading American research worker and author on glass technology. COMPOSITION All known glasses are, then, supercooled liquids and that property which makes possible their manufacture and working, namely the property of remaining in the liquid condition while passing through the temperature range immediately below the freezing point and persisting in that condition indefinitely at ordinary temperatures, is the most important and character- istic property of glass. Some silicate glasses, for example those formed by fusing the alkali felspars (soda felspar or albite NasO.Al•03.6SiO• and potash felspar or orthoclose KO•.Al•03.6SiO•) are practically impossible to crystallise by heat treatment alone, while others, for example sodium metasilicate, can be obtained as a glass by rapidly cooling a melt of a few grams or less, whereas larger melts cannot be cooled without devitrification. Some elements can be obtained in the glassy form, notably selenium and tellurium. A number of inorganic salts have been obtained as a glass, notably beryllium fluoride. Zinc chloride, when quickly cooled, forms a glass, which gradually crystallises after standing some weeks or months in air, probably as a result of taking up moisture. Similarly sodium thiosulphite ("hypo"), alums, and some other hydrated salts form glass on rapid cooling. The majority of glasses may be regarded as composed of oxides, and the possibility of glass formation may be intimately connected with the oxygen atoms. Oxygen itself is highly viscous at its melting point and is stated to form a glass if cooled quickly. The outstanding glass forming oxides are

20 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS silicon oxide or silica, (SiO2), boric oxide (B203) and phosphorus pentoxide (P•Os). Germanium oxide (GEO,) is an excellent glass former and if it were not so expensive it might have commercial use. Vanadium pentoxide (V•Os), arsenious oxide (As•O•.) and tellurium dioxide (TeO•.) a/so form glasses. While a large number of substances and mixtures can be obtained as glasses by proper manipulation, only a comparatively few compositions are suitable for glass manufacture on a commercial scale. All such contain a large proportion of the glass forming oxides mentioned above. Of these, phosphoric oxide is little used, because its glass forming properties are not so marked as those of the other two, and because glasses containing a high proportion of it are susceptible to attack by water or weak acids. Boric oxide is used extensively, but only as a constituent of silicate glasses, because when used without silica or in too high a proportion, the resulting glasses lack chemical durability. Commercial glasses are then almost exclusively silicate glasses, and the natural glasses are all silicates. The essential requirements of a commercial glass composition are that it be fluid enough at an industrially accessible temperature to be melted on a commercial scale, viscous enough to be worked above its freezing point, so that devitrification cannot take place, so viscous at its freezing point that it will not devitrify and that the resulting glass has physical properties and chemical durability suitable for the purpose for which it is intended. Silica is the outstanding glass forming oxide, for silica glass possesses in the highest degree the desirable qualities of freedom from devitrification, resistance to attack by water and acids, and low coefficient of expansion, leading to its well known resistance to thermal shock. If it were not so difficult to melt quartz, to free it from bubbles and work it, silica glass would be the most suitable material for most of the purposes to which •lass is put. The difficulty and cost of manufacture makes silica glass impossible to use for general purposes, however, and other oxides must be added to flux the silica and reduce the viscosity. It might be thought that natural glasses would repay imitation, obsidian, for example, being highly resistant to weathering and devitrification, but this cannot be reproduced by the glass manufacturer for various reasons, including the enormous viscosity of the melt. The most effective flux for silica is sodium oxide, usually added to the glass making batch in the form of anhydrous sodium carbonate. Sodium silicate glass (i.e., glass consisting of silica and sodium oxides only) is made in large quantity in the "soluble silicate" industry. It is readily dissolved by water, and some other oxide must be added to give it better chemical durability if a permanent glass is to be obtained. The oxide commonly added is calcium oxide ("lime") because it is cheap and efficacious. If too