MAKING OF GLASS

Greymouth Evening Star, 18 April 1934, Page 9

MAKING OF GLASS

A UNIQUE INDUSTRY. Definite knowledge of many facts about the history of glass would be interesting. Who first drew glass into rod form, who pressed it into cupshaped pieces, and—more important still —who made the first lens, and thus made possible the discovery of spectacles, microscopes, and telescopes? says a wriaer in he Melbourne "Age.” One factor in the development of the glass industry is unique, inasmuch as an unusual technique had to lie evolved in order to fabricate the material into many of the desired shapes. As is well known, hollow vessels, such as flasks, bottles, and tubing, are “blown” by air presure, and until a few years ago, comparatively, the blowing was done by lungpower. Unless one classes the playing of, say, a saxophone as a trade, the glass industry is the one major industry employing such a source of energy. Further, a special type of manipulative skill calling for deftness of hand, eye, and even of the mouth, had to be evolved. It is not surprising that trade secrets were jealously guarded, and that the manufacture of special types of ware became highly localised.

The "blowing” of glasware depends on the fact that glass is really a fluid, although solid in the ordinary acceptance of the term. To the scientist the true solid is crystalline as regards its internal structure, i.e., there is a definite geometrical three dimensional lattice-like arrangement of its ultimate particles- The internal structure of fluids is haphazard. Unless chemical decomposition intervenes, all solids on being heated melt sharply at a definite temperature, e.g,, ice; "glasses” (the term is often used generically), however, simply become less viscous when heated, passing through a pastic stage on their way to thin fluidity. It is while in this plastic state that the glass can be bent, twisted, and blown, and the glassmaker has to adjust the composition cl his product so that it will retain a workable plasticity over that range of temperature likely to be met with during the fabrication of a given article. All amateur glass blowers know of the difficulty of manipulating glass that softens too suddenly. CONSTITUENTS. As mentioned earlier, ordinary glass is made from sand, lime, and soda, with the possible addition of certain minor constituent?, and perhaps colouring material. For centuries little alteration in composition was made, except that potash, and, in certain optical glass, lead oxide partially replaced the soda. Towards the end of the last century the makers of optical instruments began to call for- a much more extended range of glasses for lenses. The firm of Schott, of Jena, therefore set themselves the task of making, as far as possible, a range of glasses which should possess definite values for certain desired characteristics, such as dispersive power, tint, refractive index, etc., the real aim being to correlate chemical composition and optical behaviour. The essential constituent of all glasses is silica, which occurs pure in Nature in the form of quartz, and glasses are silicates, compounds of silica with the oxides of other, elements. Schott and Co. experimented with the oxides of practically all the elements, but naturally many of the glasses produced were technically useless. However, the firm established a world-wide and well-deserved reputation by bringing on to the market many new and useful glasses. These were not all optical glasses. They conferred a boon on the practical chemist by turning out a glass with a very high softening point, their so-called “hard glass”; also an apparatus glass, which resists attack by chemicals, and which shows little liability to crack when suddenly heated. This glass marks the beginning of the present-day glass casserole. Schott’s work was the virtual beginning of modern experimental work on glass technology, which has now been put on a much more scientific, basis. Special glasses for a variety of purposes are now available, e.g., glass which absorbs heat waves almost entirely, thus enabling a workman to view the interior of a furnace without the risk of harming the eyes. Collectors of glassware and housewives, no doubt, find that glass tends to become more brittle as it ages. This “devitrification” is due to the partial change of the material from the vitreous to the crystalline state, and can be explained by the slow rearrangement of the molecules of the constituents from a random to a systematic, i.e., crystalline, pattern. The glass blower not infrequently comes across cases when this readjustment occurs quite suddenly. When a piece of old glass is gently warmed it may suddenly lose its transparency, and become frosted in appearance. The slight warmth has increased, and als.o facilitated the movement's of the molecules, enabling crystallisation to take place quickly, just as if crystals had been formed by evaporation of a solution.

The material is no longer glass, it cannot be bent or blown into shape; its jelly-like structure has been destroyed. The ingredients used in the making of the glass were all crystalline substances, ami man has in the making of the glass destroyed this manifestation of Nature’s orderliness. Natural forces, however, at once set to work slowly, but surely, and at a favourable moment Nature takes her revenge by undoing the results of man’s ingenuity.