PRODUCTION AND PROPERTIES OF GLASS CONTAINERS 25 with a straight gas-air flame, and is thus suitable for ampoules, and in general glasses with coefficients greater than this can be similarly treated. This includes the well known Wembley M6 White Neutral glass and its related Amber Neutral, for use where protection against U.V. light is required. (At the other extreme, Vycor and fused silica can only be worked in an oxy- hydrogen or similar flame, and the difficulty is further increased by the fact that quartz appears to volatilise appreciably in this temperature range, so that heating cannot be too prolonged.) Finally, optical glasses may be mentioned briefly, these having the greatest diversity of composition because of the lens designers' need for the greatest range of optical properties. They are characterised by almost perfect freedom from physical defects and their quality represents the highest achievement of the glass maker. CHEMICAL DURABILITY The resistance which glass offers to the corroding action of water, of atmospheric agencies (mainly water and carbon dioxide), and of aqueous solutions of acids, bases and salts, is a property of great practical significance and is denoted by the term "chemical durability." In a large proportion of the uses to which glass is put, its power of resisting such attack is the chief reason for its preference over competing materials. An example is the use of glass containers in the distribution of commodities ranging from milk to medicine and acids. In this field the superiority of glass leaves it without. a competitor. Even in chemical manufacturing where the requirements are: more exacting, glass is being used to an increasing extent because of the resistance it offers to surface attack under extreme conditions. In other uses of glass, chemical durability is a secondary factor the requirement of a chemical durability sufficient for the service contemplated, however, places a limit on the composition which may be employed. Examples of such uses are those in •vhich glass is chosen for its optical properties, uses ranging from windows to lens systems. Although glass used for such purposes is not subjected to as drastic treatment as in the preceding cases, nevertheless it is essential that there be no appreciable amount of surface alteration. So important is this property of withstanding corroding agencies that many of the early workers in glass technology, from Merrett onwards, included such resistance in their definitions of glass. While such a definition seems too restrictive, it is well to remember that the earliest users of glass on any scale, peoples of the Mediterranean area replacing earthenware wine containers with those of glass, were much more concerned with the imper- viousness of the surface to attack by the contents than any consideration of transparency or other property.

26 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Before considering the methods in use for testing chemical durability, it is as well to mention briefly the nature of the processes involved in the decomposition of glass by water. It is not a simple case of solution, and the testing of glass is not merely the determination of its solubility in water. On the contrary, the process is a highly complex one, involving the penetration of the glass by water and the subsequent decomposition of the complex silicate mixture, with formation of substances wholly different from those originally present. The attacking medium is not always water, however, and frequently the resistance of glass to the action of aqueous solutions is of importance. In many cases, solutions are dilute enough for differences caused by the dissolved material to be insignificant, but this is not always true. Whether or not such substances are present, the action will tend to go on until decomposition of the glass is complete. The presence of an acid or alkali will often profoundly affect the rate of decomposition and the effect will be dependent both on the composition of the glass and of the dissolved material. Decomposition of most glasses by water results in the liberation of alkali and the liberated alkali may accelerate further decomposition of the glass by dissolving away the silica framework and thus exposing fresh glass to attack, If, however, the reagent be continuously removed and replenished, or if it remains acid, the attack will slow down owing to the formation of a surface film, richer in silica than the original surface. This film may be indicated by faint iridescence. For these reasons, some types of glass, notably those containing a high percentage of silica and boric oxide, are more resistant to acid solutions than to water, and more resistant to water than to alkalis. On the other hand, glasses containing a low percentage of silica, such as are encountered in the optical glass range, are more rapidly attacked by acid solutions. In general, however, the rate of decomposition of glasses by alkaline solutions is much greater than by water, or by acids, and the glass may, in prolonged contact with hot caustic alkali solutions, appear to be unaffected yet be dissolving as a whole. The alkali silicates formed by the reaction of glass silica and reagent alkali are soluble in water and dissolve, thus in no way forming a protective coating. But ff the solution becomes loaded with extracted salts, or if certain salts are added deliberately, the reaction can be slowed down. Silicates, aluminares, and zinc salts exert this influence, and are used in commerce to decrease the attack of alkaline cleaning agents on glass. In a similar manner, concentrated solutions of acids as well as salts have been considered to have no direct action as such, the attack being due exclusively to water. This view naturally leads to the conclusion that a large amount of acid weakens the attack by diminishing the concentration of water, a conclusion borne out by experimental results. It is evident, therefore, that the term "solubility" has no meaning in the case of glass and there can be no quantitative figure for the solubility of even