In various chapters throughout the foregoing portions of this book we have had occasion to refer to the colour of glass and the causes affecting it, but these references have chiefly been made from the point of view of the production of glasses as nearly colourless as possible under the circumstances. While it is obvious that for the great majority of the purposes for which it is used the absence of all visible coloration is desirable or even essential in the glass employed, there are numerous other uses where a definite coloration is required. Thus we have, as industrial and technical uses of coloured glass, the employment of ruby, green and purple glasses for signalling purposes, as in the signal lamps of our railways, the red tail-lights of motor-cars, or even the red or green sectors of certain harbour lights and lighthouses; again, coloured glasses, ruby, green, and yellow, are extensively employed in connection with photography. Rather less exacting in their demands upon the correctness of the colour employed are the architectural and ornamental uses to which coloured glass is so extensively put in both public and domestic buildings, while, finally, coloured glass is largely the foundation upon which the stained-glass worker builds up his artistic achievements; in another direction, coloured glass is also utilised in the production of ornamental articles and of some table-ware. While it must be admitted that in a great many cases the colour-resources of the glass maker are hopelessly misapplied, yet in really artistic hands few other materials are capable of yielding results of equal beauty.

By the “colour” of a glass is generally understood the tint or colour which is observed when it is viewed, in comparatively thin slices, by transmitted light; the actual colour is thus a property, not so much of the kind or variety of glass as of each individual piece, since thick pieces out of the same melting will show a different tint from that seen in thinner pieces. As we have already pointed out, such glasses as sheet or plate, which appear practically colourless when viewed in the ordinary way, show a very decided green colour when viewed through a considerable thickness. In the same way, a very thin layer of the glass known as “flashing ruby” shows a brilliant red tint, but a thickness of one-sixteenth of an inch is sufficient to render the glass practically opaque, giving it a black appearance by both transmitted and reflected light. Again, cobalt blue glass, when examined with a spectroscope in thin layers, is found to transmit a notable proportion of red rays, but thicker pieces entirely suppress these rays. These phenomena will be readily understood when we recollect that colour in a transparent medium arises from the fact that the medium has different absorbing powers for light of different colours. All transparent substances, and certainly glass, are only partially transparent: all light waves passing through such a substance are gradually absorbed, and the extent to which they are absorbed differs according to the length of these waves. It always happens that for some special wave-lengths the substance has the power of absorbing the energy of the entering waves and converting it into heat-vibrations of its own molecules or atoms. In the most transparent and colourless glasses this process, so far as the waves of ordinary light are concerned, only goes on to a negligibly slight extent; if, however, we extend our view beyond the range of ordinary visible light, and consider the region of shorter waves that lies in the spectrum beyond the violet, we find that ordinary colourless glass becomes strongly absorbent; thus to waves of about half the length of those which produce upon our eyes the impression of yellow light, ordinary glass is as opaque as is a piece of metal to white light. In this wider sense, then, we may fairly say that all glasses are coloured—i.e., all have a power of selective absorption; but in the case of those which are nearly colourless in the ordinary sense, this absorption takes place only for waves which are either decidedly shorter or decidedly longer than those to which our eyes are sensitive. Those glasses which appear coloured in the ordinary sense, on the other hand, owe this property to the fact that the power of absorption for light-waves extends into the region of the visible spectrum; thus a blue or violet glass is practically opaque to red rays, while a red glass is opaque to blue, green or violet rays. This statement may be verified in a striking manner by holding over one another a piece of deep blue or green glass and a similar piece of ruby glass—the combination will be found to be very nearly opaque even when each glass by itself is practically transparent. The question which now naturally presents itself to us is, what is the essential difference between, for instance, a piece of red glass and a piece of “white” glass that confers upon the former the power of absorbing blue light? A perfectly complete and satisfactory answer to this question is not, in the writer’s opinion, available in the present state of our knowledge, but to a certain extent the difference between the two kinds of glass can be explained. The difference is produced, in the first instance by introducing into the colourless glass some additional chemical element or elements, the substances in question being generally known as “colouring oxides,” although they are by no means always introduced in the form of oxides, and are frequently present in the glass in entirely different forms. To a certain extent the colour of the glass may be ascribed to a definite “colouring” property of the chemical elements concerned; thus most of the chemical compounds of such elements as nickel, cobalt, iron, manganese and copper are more or less deeply coloured substances, and it would seem as if the atoms or “ions” of these elements had the specific power of absorbing certain varieties of light-waves while not materially affecting others. But this specific “colouring” property is not so easily explained when we recollect that the colours of iron compounds, for example, may be green or red according to the state of combination in which that element is present, and that iron has also the power of imparting either a green or a yellow colour to glass according to circumstances. The detailed discussion of these questions, however, lies outside our present scope, and we must confine ourselves to the broad statement that colouring substance in glass may be roughly divided into two kinds or groups; the first and probably the largest group are those bodies which occur in glass in true solution, the element itself being present in the combined state as a silicate or other such compound (borate, phosphate, etc.) which is soluble in the glass. In this class, the colouring effect upon the glass is specifically that of the element introduced, and is brought about in the same way as the colouring of water when a coloured salt—such as copper sulphate—is dissolved in it. The second class of colouring substances, however, behave in a different manner; they are probably present in the glass in a state of extremely fine division, and held not in true solution, but really in a sort of mechanical suspension that approximates to the condition of what is known as a “colloidal solution.” The point which is known beyond doubt, thanks to the researches of Siedentopf and Szigmondi on ultra-microscopical particles, is that in certain coloured glasses, of which ruby glass is the best example, the colouring substance, be it gold or cuprous oxide, is present in the form of minute but by no means atomic or molecular particles suspended in the glass. The presence of these particles has been made optically evident, although it can hardly be said that they have been rendered visible, and it is at all events probable that these suspended particles act each as a whole in absorbing the light-waves characteristic of the colour which they produce in glass. This being the case, it is easy to understand how readily the colour of such glasses is altered or spoilt by manipulations which involve heating and cooling at different rates—too rapid a rate of cooling producing a different grouping of the minute particles, altering their size or shape, or even obliterating them entirely by allowing the element in question to go into or to remain in solution in the glass.

While it would be entirely foreign to the purpose of this volume to give in this place a series of recipes for the production of various kinds of coloured glass, it will be desirable to state in general terms the colours or range of colours which can be produced in various kinds of glass by the introduction of those chemical elements which are ordinarily used in this way. In general terms it may be said that the lighter elements do not as a rule tend to the production of coloured glasses, while the heavier elements, so far as they can be retained in the glass in either solution or suspension, tend to produce an intense colouring effect. The element lead appears to form a striking exception to this rule, but this is due to the fact that while the silicates of most of the other heavy elements are more or less unstable, the silicate of lead is very stable, and can only be decomposed by the action of reducing agents. When lead silicates are decomposed in this way, however, the resulting glass immediately receives an exceedingly deep colour, being turned a deep opaque black, although in very thin layers the colour is decidedly brown. On the other hand, glasses very rich in lead are always decidedly yellow in colour, and it has been shown that this coloration is due to the natural colour of lead silicates and not to the presence of impurities. What has just been said of lead applies, with only very slight modification, also to the rare metal thallium and its compounds, which have been introduced into glass for special purposes. Leaving these two exceptional bodies on one side, we now pass to a consideration of the elements in the order of their chemical grouping. The rare elements will not be considered except in certain cases where their presence in traces is liable to affect results attained in practice.

The Alkali Metals, sodium, potassium, lithium, etc., and their compounds, have no specific colouring effect, although the presence of soda or of potash in a glass affects the colours produced by such substances as manganese, nickel, selenium, etc.

Copper, as would be anticipated from the deep colour of most of its compounds, produces powerful colouring effects on glass. Cupric silicates produce intense green, to greenish-blue tints. Copper, either as metal or oxide, added to glass in the ordinary way, always produces the green colour; but when the full oxidation of the copper is prevented by the presence of a reducing body, and the glass is cooled slowly, or is exposed to repeated heating followed by slow cooling, an intense ruby coloration is produced. In practice this colour is produced by introducing tin as well as copper into the mixture, and so regulating the conditions of melting as to favour reduction rather than oxidation of the copper. Under these circumstances the copper is left in the glass in a finely divided and evenly suspended state; if exactly the right state of division and suspension is arrived at, a beautiful red tint is the result, although the coloration of the glass is so intense that it can only be employed in very thin sheets, being “flashed” upon the surface of colourless glass to give it the necessary strength and thickness for practical use. It is further very easy to slightly alter the arrangement of the copper in the glass, with the result of producing an opaque, streaky substance resembling sealing-wax in colour and appearance, this product being, of course, useless from the glass-maker’s point of view. Finally, by exceedingly slow cooling, and under other favouring conditions which are not really understood, the particles of suspended colouring-material—be it metallic copper or cuprous oxide—grow in size and attain visible dimensions, appearing as minute shimmering flakes, thus producing the beautiful substance known as “aventurine.”

Silver is never introduced into glass mixtures, the reason being that it is so readily reduced to the metallic state from all its compounds that it cannot be retained in the glass except in a finely-divided form, causing the glass to assume a black, metallic appearance resembling the stains produced by the reduction of lead in flint glasses. On the other hand, silver yields a beautiful yellow colour when applied to glass as a surface stain, and it is widely used for that purpose.

Gold is introduced into glass for the production of brilliant ruby tints; its behaviour is very similar to that of copper, except that the noble metal has a great tendency to return to the metallic state without the aid of reducing agents. No addition of tin is therefore required, but the rate of cooling, etc., must be properly regulated, since rapidly cooled glass containing gold shows no special colour, the rich ruby tint being only developed when the glass is re-heated and cooled slowly. The colouring effect of gold is undoubtedly more regular and uniform than that of copper, and it is accordingly possible to obtain much lighter shades of red with the aid of the noble metal. “Gold ruby” can therefore be obtained of a tint light enough to be used in sheets of ordinary thickness, and the process of “flashing” is not essential. The elements of the second group, such as magnesium, calcium, strontium, barium, zinc and cadmium, exert no strong specific colouring action on glass, with perhaps the exception of cadmium, and that element only does so to any considerable extent in combination with sulphur, sulphide of cadmium having the power of producing rich yellow colours in glass. The sulphur compounds of barium also readily produce deep green and yellow colours, and the formation of these tints is, indeed, very difficult to avoid in the case of glasses containing much barium. A colouring effect has sometimes been ascribed to zinc, but this is not in accordance with facts.

Of the elements of the third group, only boron and aluminium are ever found in glass in any notable quantity. Boron is present in the form of boric acid or borates, and as such produces no colouring effect, nor does there seem to be any tendency for the separation of free boron. The compounds of aluminium also possess no colouring effect, although certain compounds of this element are utilised for imparting a white opacity to glass for certain purposes—such glass being known as “opal.”

The elements of the fourth group are of greater importance in connection with glass. Carbon is capable of exerting powerful colouring effects when introduced into glass. These effects are of two kinds, viz., indirect in consequence of the reducing action of carbon on other substances present, and direct from the presence of finely-divided carbon or carbides in the glass. The latter are similar in kind to those produced by the presence of other finely-divided elementary bodies (copper, gold, lead, etc.) except that the lightness of the carbon particles tends to the production of yellow and brown colours rather than of red and black, while the chemical nature of carbon renders the glass in which it is suspended indifferent to rapid cooling, so far as the carbon tint is concerned. The indirect effects of carbon, in reducing other substances that may be present in the glass, become evident with much smaller proportions of carbon than are required to produce visible direct effects. As we have seen above, carbon, in the form of coke, charcoal or anthracite coal, is regularly introduced, as a reducing medium, into glass mixtures containing sulphate of soda. If even a slight excess of carbon be used for this purpose, the formation of sulphides and poly-sulphides of sodium and of calcium results, and these bodies, like all sulphides, impart a greenish-yellow tint to the glass, at the same time bringing other undesirable results in their train.

Silicon, in the form of silicic acid and its compounds, is a fundamental constituent of all varieties of glass, and in this form is in no sense a colouring substance; on the other hand, there is no doubt that under some conditions silicon may be reduced to the metallic state at temperatures which normally occur in glass-furnaces, and it is practically certain, that if present in glass in this condition, silicon would colour the glass. It is just possible that some of the colouring effects produced in ordinary glass by powerful reducing agents, such as carbon, either in the solid form or as a constituent of furnace gases, may be due to the reduction of silicon in the glass.

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.

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.

Although most usually applied to copper-ruby glass, the flashing process is often used with other colours also. Coloured glass of this kind is at once recognised when looked at through the edges. Thus examined the glass simply shows the greenish tint of ordinary sheet-glass which constitutes practically the entire thickness of the sheet. In the same way, if such “flashed” glass be cut or etched in such a way that the layer of coloured glass is removed in places, the resulting pattern appears in white on the coloured ground—a feature which is utilised for certain decorative purposes. The flashing process just described, it should be noted, is applicable to any form of glass-ware which is blown from a gathering, and the coloured layer can be applied either upon the inside or outside of any object thus produced.

In addition to the palette of colours which the glass-maker is able to supply, the artist in stained glass has a further range of colours at his disposal in the form of stains and transparent colours which can be applied to the surface of glass and developed and rendered more or less permanent by being properly “fired.” The colours produced in this way are also, in one sense, coloured glasses, or rather glazes, whose raw materials are put upon the glass by the brush of the painter, and only subsequently caused to combine and melt by suitable heating. The degree of heat applicable under these circumstances is, however, very limited by the necessity of avoiding any great softening of the substratum of glass, while many of the colours themselves are composed of materials which could not resist very high temperatures. The fluxes used in the composition of these colours must for this reason be of a very fusible kind, with the inevitable result of a greatly reduced chemical stability as compared with the glass itself.

The whole subject of painting on glass, even from the purely technical as apart from the Æsthetic point of view, is a very wide one, and lies outside the scope of the present volume. Only one further technical point in connection with glass-painting and stained glass work will therefore be touched upon here. This is an example of the fact that the more technically “perfect” modern product is not always preferable for special purposes which have been well served by older and far less “perfect” products. The production of technically excellent coloured glass in modern times was, somewhat surprisingly at first, accompanied by a very marked decline in the artistic beauty of stained glass windows produced with this modern material; the ancient art of stained glass was, therefore, for a time regarded as a “lost art,” and glass-makers were blamed for being unable to produce the brilliant and beautiful tints which had been formerly available. More careful study, however, revealed the fact that while the actual colour of modern glass was at least as brilliant and varied as that of ancient glass, the difference lay in the fact that the modern glass was practically entirely free from such imperfections as air-bubbles, striÆ, and other defects which improved appliances and methods had enabled the glass-maker to eliminate from his products. Finding the beauty of his wares greatly improved by this increased purity of the glass in the case of window glass and table ware, it was natural for the glass-maker to endeavour to produce the same “improvement” in the coloured glasses intended for artistic purposes and, indeed, it is more than likely that the stained-glass workers themselves pressed this line of improvement upon him by a demand for “better” glass. It turned out, however, on close examination, that this very perfection of modern glass rendered it less adapted for these artistic purposes. A perfect piece of glass, having smooth surfaces and no internal regularities, allows the rays of light falling upon it to pass through undeflected in direction, and merely changed in colour, according to the tint of the glass in question. On looking at the glass, external objects can be quite clearly seen, and much of the interest and mystery of the glass itself is lost. On the other hand, when falling upon a piece of glass having an irregular surface, and containing all manner of irregularities such as striÆ, air-bells, and even pieces of enclosed solid matter, the light is scattered, refracted, and deflected into all manner of directions until it almost appears to emanate from the body of the glass itself, which thus appears almost to shine with an internal light of its own; the eye can hardly perceive the presence of external objects, and the whole window appears as a brilliant self-luminous object.

Once their attention had been drawn to these facts, modern glass-makers endeavoured, and with much success, to reproduce the desirable qualities of the ancient glass, while still availing themselves of modern methods to produce more stable glasses and a wider range of colours. The irregular surface of the old glass is imitated by using rolled or “muffed” instead of ordinary blown glass, while the internal texture is rendered non-homogeneous by the deliberate introduction of solid and gaseous impurities and by manipulations so arranged as to leave the glass in layers of different density, which appear in the finished glass as “striÆ.” As a consequence, it is probably not too much to claim that the modern workers in coloured glass have materials at their disposal which are at least as suitable for the purpose as those that were available in the best days of the ancient art.

Some reference has already been made to the technical uses of coloured glass, but one or two further points in that connection remain to be discussed. For such technical purposes as railway and marine signals, the consensus of practical experience has decided in favour of certain colours of glass, such as red and green of particular tints. On the other hand, for various purposes in connection with photography, the glass-maker does not appear to have been able to meet the new requirements, with the result that flimsy and otherwise unsatisfactory screens made of gelatine or celluloid stained with organic dyes are employed in place of coloured glass in such cases, for example, as the covering of lamps for use in photographers’ “dark” rooms, and for the light-filters used for orthochromatic and tri-chromatic photography. In all these cases it is necessary to use a transparent coloured medium which transmits only light of a certain very definite range of wave-lengths, and there is no doubt that for the glass-maker, who is confined to the use of a number of elementary bodies for his colouring media, it is by no means easy to comply with these requirements of exact transmission and absorption. On the other hand, the field of available coloured glasses has not been fully explored from this point of view, the only extensive work on the subject having been done in connection with the Jena firm of Schott, who have put upon the market a series of coloured glasses of accurately-known absorbing power. There is, however, little doubt that a much greater extension of this field is possible, and that it will be opened up by a glass-maker who undertakes the exhaustive study of coloured glasses from this point of view, although it must be admitted that there is considerable doubt whether the results obtainable by the aid of aniline and other dyes as applied to gelatine can ever be equalled by coloured glasses.