Glass Manufacture

by Walter Rosenhain

Fluorine occurs in a number of glasses in the form of dissolved or suspended fluorides, principally fluoride of aluminium. The element is not essentially a colouring substance, and is only mentioned here because the fluoride named is the most frequently used means of producing “opal” glass. The fluoride is most frequently introduced into the glass mixtures as calcium fluoride, used in conjunction with felspar, or as cryolite, a natural mineral which consists of a double fluoride of sodium and aluminium.

Manganese is one of the most important colouring elements used by the glass-maker. When introduced into glass in the absence of other colouring ingredients, compounds of manganese produce a range of colours lying in the region of pinkish-purple to violet, according to the chemical nature of the glass. The exact colour produced varies according as the glass has lead, lime or barium as its base, and it also depends upon the presence of soda or potash as the alkaline constituent. The nature and intensity of the colour, however, which the addition of a given percentage of manganese will produce depends upon other factors besides the chemical composition of the bases used in the mixture. The heat and duration of the “found” and the reducing or oxidising conditions of the furnace in which it has been carried on very materially affect the result. Thus, a glass having a slight tinge of pink or purple derived from manganese can be rendered entirely colourless by the action of reducing gases or by introducing into the glass a reducing substance, such as a piece of wood. It will thus be seen that while manganese is a most useful element for the glass-maker, its employment requires much skill and care, and generally involves some troublesome manipulations before the desired result is attained.

In practice, manganese is most frequently used with other colouring ingredients for the production of what may be called “compound” colours, the function of the manganese being to provide the “warm” element, i.e., the pink or purple component, required. One of the most important uses of manganese coming under this head is its use as a “decolouriser.” By a “decolouriser” the glass-maker understands a substance which can be used to improve the colour of a glass which, from the nature of its raw materials and conditions of melting, would have a greener colour than is thought desirable for the product in question. It may be said at once that the most perfect and satisfactory method of obtaining the better colour required is to adopt the use of purer raw materials and methods of melting less liable to lead to contamination of the glass. On the other hand, this radical course is often impossible on the ground of expense, and the less satisfactory course must be adopted of covering one undesirable colour by another complementary colour which would, in itself, be equally undesirable. The rationale of this procedure depends upon the fact that a slight amount of absorption of light is not readily detected by the human eye if it be uniformly or nearly uniformly distributed over the whole range of the visible spectrum, i.e., if the colour of the resulting light is nearly neutral, while an equally slight absorption in one region of the spectrum, while actually allowing more light to pass through the glass, is at once detected by the eye owing to the colour of the transmitted light. Now it has been found that the colour produced in glass by the addition of very small proportions of manganese is approximately complementary to the greenish-blue tinge of the less pure varieties of ordinary glass; the addition of manganese in suitable proportions to such glass therefore results in the production of a glass which transmits light of approximately neutral, usually slightly yellow, colour, the increased total absorption only becoming noticeable in large pieces. This “covering” of the greenish tinge is generally most completely successful in the case of soda-flint glasses, but the method is also used to a certain extent in the case of the soda-lime glasses used for sheet and plate-glass manufacture. Manganese added to glass for this purpose is generally introduced into the mixture in the form of the powdered black oxide (manganese dioxide), which is available as a natural ore in a condition of sufficient purity. Added in this form, the manganese compound exerts a double action, the decomposition of the dioxide resulting in the liberation of oxygen within the mass of melting glass, and this oxygen itself exerts a favourable influence on the resulting colour of the glass, since it removes organic materials whose subsequent reducing action would be deleterious, and it also converts all iron compounds present into the more highly-oxidised (ferric) state in which their colouring effects are less intense. The actual colouring effect of the manganese itself is, of course, afterwards developed, and produces the effects discussed above.

The “covering” of the greenish tints due to iron and other compounds is only possible when these are present in very small proportions. When larger quantities of these substances have been introduced into the glass the addition of manganese modifies the resulting colour, but is no longer able to neutralise it. A very large range of colours can be obtained by using various proportions of iron and manganese, the best-known of these being the warm brown tint known as “hock-bottle,” while all shades between this and the bright green of iron and the purple of manganese can be obtained by suitable mixtures. What has been said above as to the sensitiveness of manganese colours applies with even greater force to these mixed tints, since here both the iron and the manganese compounds are liable to undergo changes of oxidation. Copper-manganese and chromium-manganese colours are also used, as indeed almost any number of colouring ingredients may be simultaneously introduced into a glass mixture, the resulting colour being, as a rule, purely additive.

Iron is so widely distributed among the materials of the earth’s crust that it is exceedingly difficult to exclude it entirely from any kind of glass, although the purest varieties of glass contain the merest traces of this element. Cheaper varieties of glass, however, always contain iron in measurable quantity, while the cheapest kinds of glass contain considerable proportions of this element. The colouring effects of iron have already been alluded to at various points in the earlier chapters as well as in the section on manganese just preceding. Little further remains to be said here. Just as the less highly oxidised compounds of iron—i.e., the “ferrous” compounds—always show a decided green tint, so glasses containing iron when melted under the usually prevalent reducing conditions of a glass-making furnace, show a decided green tint whose depth depends upon the amount of iron present, provided no manganese or other “decolouriser” has been introduced. “Ferrous” compounds are, however, readily converted into the more highly oxidised or “ferric” state by the action of oxidising agents, and this change can also be brought about in molten glass by the action of such substances as nitrates or other sources of oxygen. The ferric compounds, however, show characteristic yellow tints which are much less intense and vivid than the corresponding green colours of the “ferrous” series, and a similar result is brought about by the oxidation of iron compounds contained in glass; hence the “washing” or cleansing effects ascribed to oxidising agents introduced in the fusion of glass. It should, however, be borne in mind that the oxidation of other substances besides iron compounds, viz., organic matter, carbon and sulphur compounds, may, and probably does, play a most important part in this process in the case of most varieties of glass.

Nickel exerts a powerful colouring influence on glass, in accordance with the fact that most of the other compounds of this element are also deeply coloured. The exact colour produced in glass depends upon the nature of the glass and on the condition of oxidation in which the nickel is present. The colours, however, are usually of a greenish-brown tint, although brighter colours can be produced by nickel under special conditions. This element is not, however, much used as a colouring agent in practice, although it has been advocated as a “decolouriser.” The writer is not, however, aware that it has ever been successfully used for this purpose, and, in fact, the colours to which it gives rise do not appear to be even approximately complementary to the ordinary green and blue tints which “decolourisers” are intended to cover.

Cobalt is one of the most powerful colouring agents in glass, and is very largely used in the production of all varieties of blue glass. The blue colour produced by cobalt is, in fact, probably the most “certain” of the colours available to the glass-maker, this tint being least affected by all those circumstances that lead to variations in other tints. Almost the only difficulty involved in the use of cobalt is the great colouring power of this element, which requires that for most purposes only very small quantities may be added to the glass mixture. Formerly cobalt was added to glass mixtures in the form of “zaffre,” which was a very impure form of cobalt oxide. At the present time, however, the more expensive but much more satisfactory pure oxide of cobalt is in almost universal use. This substance shows a perfectly constant composition and, by means of accurate weighing, enables the glass-maker to introduce precisely the right amount of cobalt into his batch.

The range of colours which are available to the modern glass manufacturer are, as will be seen from a consideration of the list of colouring elements given above, practically unlimited, particularly as these substances can be used in almost any combination to produce mixed or intermediate tints. This practically infinite variety of possible tints, indeed, involves the principal difficulty encountered by the manufacturer of coloured glass, i.e., that of matching his tints, or of keeping the colour of any particular variety of glass so constant that pieces produced at various times can be used indiscriminately together. This ideal is, perhaps, never entirely realised, but in the case of glasses intended for special technical uses the ideal degree of constancy is very closely approached.

In addition to being called upon to produce a large variety of different tints, the glass-maker is also called upon to produce various depths of the same tint. In many cases this can be readily done by the simple means of varying the amount of colouring material added to the glass. Where the colouring effect of small quantities of these substances is not excessively powerful there is no very great difficulty in doing this, but in certain cases this mode of regulating the intensity of the colour is not available. Thus copper-ruby glass cannot readily be made of so light a tint as to appear of reasonable depth when used in sheets of the thickness of ordinary sheet-glass. As has already been indicated, the desired tint is obtained by the process of “flashing,” i.e., of placing a very thin layer of deep ruby-coloured glass upon the surface of a sheet of ordinary more or less colourless glass of the usual thickness. This is generally accomplished by having a pot of molten ruby glass available close to a pot from which colourless glass is being gathered. A small gathering of ruby glass is first taken up on the pipe, and the remaining gatherings required for the production of the sheet are taken from the pot of colourless glass. When such a composite gathering is blown into a cylinder in the manner described in the previous chapter, the ruby glass lies as a thin layer over the inner face of the cylinder, but special care and skill on the part of the gatherer and blower is required to ensure that this layer shall be evenly distributed and of the right thickness to produce just the tint of ruby required. Since the whole layer of red glass is so thin, a very slight want of uniformity in its distribution leads to wide variations of tint, and in practice these are often seen in the less successful cylinders of such glass.

The chemical composition of the ruby and the colourless glass which are to be employed for this purpose must also be properly adapted to one another in order to produce two glasses which shall have as nearly the same coefficient of thermal expansion as possible. If this requirement is not met, the resulting glass is subjected to internal strains which may lead to fracture, while, if the ruby glass has the higher coefficient of expansion, the sheet after flattening tends to draw itself up on the “flashed” side and cannot be passed out of the annealing kiln in a properly flat condition.