Glass Manufacture

by Walter Rosenhain

Tin by itself does not appear to have any colouring effect upon glass, except that its oxide, in a finely suspended state, produces opalescence and, in large quantities, white opacity. Tin, however, is used in conjunction with copper in the production of copper-ruby, to which reference has already been made.

Lead and Thallium have already been dealt with, and it only remains to add that their presence in the glass, although not in itself producing any intense colouring action, increases the colouring effects of other substances. This is probably merely a particular case of the fact that dense glasses, of high refractive index, are more sensitive to colouring agencies than the lighter glasses of low refractive index; this applies to barium as well as to lead and thallium glasses.

Phosphorus occurs in some few glasses in the form of phosphoric acid, and this substance, as such, has no colouring effect. Calcium phosphate, however, is sometimes added to glasses for the purpose of producing opalescence. Its action in this respect is probably similar to that of tin oxide and aluminium fluoride, these substances all remaining undissolved in the glass in the form of minute particles in a finely divided and suspended state.

Arsenic does not exert a colouring effect on glass, and owing to its volatile nature it can only be retained in glass in small quantities and under special conditions. A “decolourising” action is sometimes ascribed to arsenic, but if this action really exists it can only be ascribed to the fact that arsenic compounds are capable of acting as carriers of oxygen, and their presence thus tends to facilitate the oxidation of impurities contained in the glass. A further reference to this subject will be found below in reference to the compounds of manganese.

Antimony, although frequently added to special glass mixtures, does not appear to produce any very powerful effects, except possibly in the direction of producing white opacity if present in large proportions. The sulphide of antimony, however, exerts a colouring influence, although its volatile and unstable character renders the effects uncertain.

Vanadium, owing to its rarity, is probably never added to glass mixtures for colouring purposes, although it is capable of producing vivid yellow and greenish tints when present even in minute proportions. On the other hand, vanadium occurs in small proportions in a number of fire-clays, including some of those of the Stourbridge district, and glass melted in pots containing this element is liable to have its colour spoilt by taking up the vanadium from the clay.

Sulphur is an element whose presence in various forms is liable to affect the colour of glass in a variety of ways. The colouring effects of sodium-, calcium-, cadmium-, and antimony-sulphides have already been referred to. Sulphur probably never exists in glass in the uncombined state at all, but sulphur and its oxides, which are often contained in furnace gases, sometimes exert a very marked action upon hot glass. The presence of sulphur gases in the atmospheres of blowing-holes and annealing kilns is liable to produce in the glass a peculiar yellowish milkiness which penetrates for a considerable depth into the mass of the glass and cannot be removed by subsequent treatment. Glass vessels, particularly if made of glass produced from raw materials among which salt-cake has figured, are also affected by contact with fused sulphur or its vapour, the effect being a gradual disintegration of the glass. The precise mechanism of these actions is not known at present, but they probably consist in the formation of sulphur compounds within the glass, possibly giving rise to an evolution of minute bubbles of gas.

Selenium, which is chemically so closely related to sulphur, is a relatively rare element, which is, however, finding some use in glass-manufacture as a colouring and a decolouring agent. The introduction of selenium or of its compounds under suitable conditions into a glass mixture produces or tends to produce a peculiar yellowish-pink coloration, the intensity of the colour produced being dependent upon the chemical nature of the glass as a whole and, of course, upon the amount of selenium left in the glass at the end of the melting process, this latter in turn depending upon the duration and temperature of the process in question. The pink colour of selenium glass is best developed in those containing barium as a base, but it is also developed in lead glasses, while soda-lime glasses do not show the colour so well. As a “decolouriser” the action of selenium is entirely that of producing a complementary colour which is intended to “cover” the green or blue tint of the glass; where the depth of the tint to be “covered” is small, selenium can be used very successfully in this way, although it is a relatively costly substance for such a purpose. No oxidising or “cleansing” action can be ascribed to selenium or its compounds.

Chromium is one of the most intensely active colouring substances that are available for the glass-maker, and it is accordingly used very extensively. It has the advantage of relative cheapness, and can be conveniently obtained and introduced into glass in the form of pure compounds whose colouring effect can be accurately anticipated; the colours produced by the aid of chromium have the further advantage of being very constant in character, being little affected by oxidising or reducing conditions, and only very slightly by the length or temperature of the melting process. The rate of cooling, in fact, appears to be the only factor that materially affects the colours produced by compounds of chromium. The colours produced by chromium alone are various depths of a bright green, the depth varying, of course, with the proportion of chromium that is present in the glass and with the purity of the glass itself. Very frequently, chromium is used in conjunction with either iron or copper to produce various tints of “cold blue” and “celadon green” respectively. This element is most usually introduced into the glass mixture in the form of potassium bichromate; although other compounds might be employed, this substance presents several advantages to the glass maker. In the first place, since the colouring effect of chromium is very intense, it must be used in very small quantities, and if chromic oxide itself were used, the weighing would have to be carried out with extreme care; potassium bichromate, however, contains a much smaller proportion of the effective colouring substance, so that much larger weights can be employed, and the accuracy of weighing required is proportionately reduced. A further consideration arises from the fact that chromic oxide is itself an extremely refractory body, and is therefore comparatively difficult to incorporate with glass, while its presence tends to make the glass itself more viscid and refractory; the simultaneous introduction of the alkali, as provided by the use of the bichromate, is thus an advantage in restoring the fluidity and softness of the glass when finished, while also facilitating the solution of the chromium in the glass during the fusion process; this process of solution, however, takes some time, chromium glasses being liable to appear patchy if insufficient time is given to the “founding.”

Uranium is one of the rarer and more costly elements, but is nevertheless used in glass-making for special purposes on account of the very beautiful fluorescent yellow colour which it imparts when added in small proportions. This yellow is quite characteristic and unmistakable, so that none of the other varieties of yellow glass can ever be used as a substitute for uranium glass, but the great cost of the latter prevents its extended use. Uranium is usually introduced into glass mixtures in the form of a chemical compound, such as uranyl-acetate or uranyl-nitrate, both these substances being obtainable in the form of small, intensely bright yellow crystals.