JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS who showed that the superiority of certain glasses, made in the Thuringian Forest, for table and blowpipe working, was caused by the presence of almost 4 % of alumina in the Martinsroda sand used, an d th at the addition of alumina to the usual glass batches greatly diminished the tendency towards devitri- fication of the resulting product. Too much alumina, however, increased the viscosity of the glass, making it difficult to melt and work. The high alumina content of some special glasses such as for fibres and discharge lamps (Table 1) greatly improves chemical and physical durability where ease of melting and appearance of the finished product are of secondary importance. The outstanding durability of natural glasses of igneous origin, referred to above, is largely the result of their being composed almost entirely of silica and alumina, obsidian usually being found to contain up to 20% of the latter. "Neutral" glass for ampoules, etc., occupies a position midway with respect to alumina contents, representing a compromise between conflicting require- merits. Another frequent constituent in commercial glasses, not so much in this country as in the U.S.A., is magnesium oxide. It can be substituted, with advantage, for lime provided the percentage is not too large. Its appear- ance in the American container glass in Table 1 is mainly on economic grounds, dolomitic limestone (approximate composition CaCOaMgCOa) of high purity frequently being easier to obtain than ordinary limestone, the normal source of lime in this country. As it usefully extends the glass working range and increases the durability and resistance to devitrification, magnesia frequently appears in "flat" glass (sheet and plate) and glass for tubing, etc. (Table 1.) The use of a small proportion of potassium oxide can be beneficial, but its appearance in container glasses is usually due to the use of potash felspar on economic grounds, in order to introduce alumina. Small amounts of barium oxide, boric oxide or fluorides are often added for various glass making purposes, e.g., controlling the rate of refining of the molten glass or altering the viscosity characteristics for manipulative purposes. These are comparatively expensive ingredients, however, and a large proportion of any one of them produces some unfavourable results. The composition of the glass used for all the great mass production industries is much the same. Window and p!ate glass is very similar to container glass with only minor modifications in the proportions of the major constituents to satisfy the requirements of the differing manufacturing processes. It follows then that the greatest volume of glass melted today is of the soda-lime-silica-magnesia type, but there are several other basically different groups, each of supreme importance in its own field of application. One such group which should be mentioned in passing is that based on lead oxide, usually in conjunction with potassium oxide. Container glasses, for

PRODUCTION AND PROPERTIES OF GLASS CONTAINERS 23 obvious reasons, rarely if ever contain lead except as a trace impurity, but Table I gives an example of the use of lead glass for electrical purposes and high quality table ware (English crystal cut glass). Glass for laboratory ware represents a type which must be especially resistant to decomposition by water and acids. The earlier types developed were, in general, improvements of existing bottle glasses, for example small amounts of boric oxide were added and magnesia and zinc oxide substituted for lime, ("Apparatus glass"). The improvement of glass melting tech- niques, leading to the easier attainment of the necessary high founding temperatures, led, however, to the appearance of the borosilicate glasses, of which the earliest was the well-known Pyrex range (1916). Pyrex, it must be noted, is not one glass alone but is a trade mark of the Corning Glass Works of U.S.A., covering a range of thermally, electrically or chemically resistant glasses. They are all fundamentally different from the usual soda-lime- silica composition, the type commonly used for chemical ware and cooking utensils having the typical analysis shown in Table 2. It can be considered as a glass, in which the melting point of the silica has been lowered by the addition of boric oxide and in smaller amounts, of alumina, with only the smallest possible amounts of alkali. It is intrinsically superior to the best glasses of the soda-lime group in its chemical durability and resistance to breakage from heat shock or mechanical strain, but is more difficult to manufacture and work. While Pyrex glasses occupy such an unassailed position in laboratory ware there are a number of other types available, both in this country and the U.S.A. These are characterised, however, by being generally more complex than Pyrex, with lower silica and boric oxide contents but higher alumina ("aluminosilicates" and "aluminoborosilicates"). Some of these are set out in Table 2, which also includes the composition of l•vcor, the product of an interesting new' technique in glass making, again due to Corning Glass Works. Glass chemical ware is moulded from an easily melted and worked glass of a special composition, such that on continued heating below its softening point, it splits up into two phases. The soluble phase is then leached out by the action of dilute acid, leaving behind a rigid but porous structure consisting almost entirely of silica. Reheating the article at 900-1,000øC results in the "skeleton" shrinking some 20% to gix'e a clear material approximating to fused silica, but which has been produced without the need for extremely high fusing and working temperatures. Ampoule glasses can be regarded as soft borosilicate glasses, the composi- tions shown in Table 2 containing less boric oxide and silica than Pyrex chemical ware but more alumina and alkali. In addition, the alkalis consist of part soda and part potash, which has a marked effect in increasing chemical durability, up to an optimum ratio of about 7:3 potash to soda by weight.