INDUSTRIAL A N D ENGINEERING CHEMISTRY
392
Vol. 17, No. 4
glass compositions stable enough to be used in optical instruments, either with or without protection, as to the types of glass that have proved serviceable as window and gage glasses, and as to those compositions that have proved serviceable for chemical laboratory glass. In the same group of methods are to be included the many tests that have been carried out on the relative resistance to attack by various common laboratory reagents, such as the work of Walker and Smither,Bthe results of which are directly applicable to laboratory work. The methods based on the use of sensitive indicators are also to be included in this class, though the results of such tests are to be accepted with caution. Foremost among these methods is the iodeosine test developed by Mylius. I n this method a fresh surface of known area of the glass is immersed in an ether solution of iodeosine. The alkali in the glass reacts with the dye, forming a soluble salt, which is dissolved in water, and the color compared with a standard. This gives excellent results with the usual soda-lime and potash-lime glasses, but may give wholly misleading results with glasses rich in boric oxide or with glasses containing little alkali but much lead oxide, or barium oxide, whose salts with iodeosine are not soluble in water. Results obtained by treating powdered glasses with water or with dilute acid are also to be accepted with caution. I n all such cases the reactions taking place are decomposition reactions, in which the end results will be complete decomposition of the glasses. What is determined is roughly a rate of reaction, and of a reaction particularly susceptible to variations in the experimental conditions. This is the clue to the lack of concordance in the observations recorded in the literature. The data given refer, not to solubility, but to rates of reaction, frequently determined under poorly defined conditions. The second type of testing method, in which the process is accelerated by increasing the temperature, is subject to the same limitations. I n addition, we have here the possibility that the type of reaction between glass and water may be completely altered, as is the case with potassium disilicates in contact with water. Although such temperatures have been reached in high pressure bombs, it is not probable that they will be reached with complex glasses in the type of autoclaves used in testing, but nevertheless it must be born in mind that the processes taking place a t high temperatures and pressures differ greatly from those that take place under more ordinary conditions. One important difference is the tendency of the silica to pass into colloidal solution. This is marked a t low temperature, but when the temperature is raised to 200”-300”C. there seems to be no tendency to form colloidal solutions, but rather for the silica to crystallize as quartz.
noteworthy in their deleterious effect on the resistivity of glass to corrosion, and it may be stated as another rough generalization that the power of a glass to resist corrosion is inversely proportional to the alkali content. Of the two oxides, potash usually exerts the greater effect, but a mixture of the two oxides is superior to either alone. This has been especially well brought out by the work of Peddle,’ who showed that in the alkali-lead oxide glasses “to obtain the maximum of durability in an alkali-lead-silicate glass the best proportions in which to mix the alkalies is the ratio of 7 parts of the potash to 3 parts of soda.” No explanation has been offered for this remarkable circumstance. Of the three oxides, magnesia, lime, and barium oxide, the last is inferior to the others in the resistivity that it confers upon glass. Although glasses high in barium oxide and excellent in resistivity are well known, the excellence is to be ascribed to other ingredients. Between magnesia and lime there is little choice, each surpassing the other under certain conditions, but the difference never being marked. As far as the writer is aware, no work has been done on a possible advantage of mixed magnesia-lime glasses, similar to the marked advantage found with the mixed sodapotash glasses. Lead oxide is superior to the alkali oxides but slightly inferior to lime from the point of view of resistance to corrosion. However, lead oxide can be introduced in much larger quantities than can lime without devitrification, and the high lead glasses thus obtained are excellent for chemical resistivity, until the very heavy lead glasses are reached, and even these are good as regards resistance to water alone. It seems to be a general rule that glasses complex in composition are superior to the simpler ones, as, for example, the mixed alkali glasses. Certain substances rarely present in large quantities seem to be of special value in this connection, among which are zinc oxide, boric oxide, and alumina. Pure zinc oxide glasses are not highly stable themselves; yet the addition of zinc appears to have a distinctive favorable effect in certain cases. Boric oxide is a constituent of many, if not most of the best glasses; indeed, Pyrex glass, which is distinctly superior in this respect, contains a considerable quantity of boric oxide. Alumina is also of great value in enhancing the stability of glasses, a content of as little as 1 or 2 per cent having a markedly beneficial effect. The dense barium crowns would probably be much inferior glasses were it not for their very considerable content of alumina, but as manufactured they rank with the best.
Effect of Chemical Composition
Manuscript for the National Directory of Commodity Specifications has been sent to the Government Printing Office. It is to be followed by an Encyclopedia of Specifications, giving in loose pamphlet form complete copies of the more important specifications, Dr. A. S. McAllister, of the Bureau of Standards, is in charge of the work. The directory, formerly called the Dictionary of Specifications, is to contain classified lists of all the commodity specifications in general use in the United States, including those employed by the United States Government, by state and city purchasing agents, and by large industries; and those prepared by the American Society for Testing Materials and by the Federal Specifications Board. For each a brief description will be given, and directions for obtaining copies. A cross-indexed finding list will be included. I n qll about six thousand commodities are included. These are divided into ten groups, covering respectively animals and animal products; vegetable food products, oil, seeds, expressed oil, and beverages; other vegetable products except fibers and wood ; textiles; wood and paper; nonmetallic minerals; ores, metals, and manufactures (except machinery and vehicles) ; machinery and vehicles; chemicals and allied products; and miscellaneous.
The correlation of the ability of glasses to withstand the corrosive action of water with their chemical composition is difficult, as is to be expected from the complex character of the processes involved. The one outstanding feature is the superiority of silica glass over all others; indeed it may be said, as a rough generalization, that the chemical resistivity of a glass is proportional to its silica content. Addition of other substances is necessary, however, to lower the temperature required in the melting process to one conveniently attainable in practice, and in glasses used for certain special purposes, such as optical glasses, for the attainment of other desirable qualities. Of the substances copmonly introduced, the alkalies, soda and potash, occupy a special place in the increased fusibility they give to glass, and few glasses are made which do not contain one of these oxides They also are especially 6
Bur. Standards, Tech. Paper 107.
7
J. Soc. Gloss Tech., 5, 195 (1921).
National Directory of Commodity Specifications
