Flint Glass, although called by this name, is not made from flint, but from the best sand, of pure and dazzling whiteness, obtained from Alum Bay, in the Isle of Wight, and from Fontainebleau, in France. The cost per ton is from 1l. to 1l. 15s., whereas the price of the sand used for making plate glass is about one-eighth of that amount. The alkali employed is generally extremely good carbonate of potash, whereas soda is used in the manufacture of the other kinds of glass which have been described. The addition of a small quantity of black oxide of manganese is sometimes necessary to correct the slight tint imparted by iron, which seems to be always present in minute quantities, even in the purest samples of sand. Oxide of lead in the form of red lead, in this sort of glass, takes the place of lime. The advantages derived from using the oxide are, that it makes the mixture more fusible, and also imparts that particular brilliancy and lustre so peculiarly characteristic of well-made flint glass. In different works, various mixtures are made for the composition of the glass; but to give an idea of the proportions in which the materials are mixed, it will be well to quote the statement of M. Payen, who says that of the finest crystal flint glass, the following is the composition: sand, 3; red lead 2 to 2¼; carbonate of potash, 1½ to 1-2/3. A little nitre or saltpetre is used as an oxidizing agent. The glass-pots employed in this branch of the manufacture are covered, so that the flames of the furnace do not come in contact with the materials, the object in thus isolating them from direct contact with the flame being to prevent the entrance of impurities, by which the colour might be injured. On account of the pots being covered, the materials take a much longer time to get hot, and require quite double the time in founding that sheet or plate glass does; the presence of oxide of lead materially assisting the rapidity of the fusion. When flint glass is ready for working, the time required to work off a pot of it is much longer than that which is required for a pot of crown or sheet; and it is a matter of considerable importance, that the furnace-man should so manage his fires as to keep the glass in a proper working condition, that is, he should not let it get too cold (therefore too solid) nor too fluid. Flint glass is worked off by the blower into wine-glasses, tumblers, decanters, and other suitable vessels. Let us take a wine-glass as an illustration of the method of working. A small quantity of glass is gathered on the blowpipe, which is much smaller than that used in making sheet, and is blown into a bulb, which may be slightly elongated or globular, the forms being given to it by the motion which the workman imparts to his blowpipe while he is blowing, or after he has blown, into the mass. In the case of a wine-glass, an assistant boy gathers a small quantity of glass on the end of a small pointel, or solid iron rod. This is placed on the side of the globe opposite that which is in connection with the blowpipe, which is then detached by touching the glass nearest it with a piece of iron, wetted with cold water: this causes a crack, and a gentle tap causes separation. The workman then moulds the opening made by detaching the blowpipe, in order to do which, he has to apply the glass often to the mouth of the furnace, to soften it. He then opens out the globe into the shape of a cup with a pair of small iron tongs, with legs uniform in shape, slightly tapering and smooth, and he uses a peculiar kind of scissors for trimming the edges. The other parts of the glass are moulded with the tongs, accuracy of size being obtained by means of measuring compasses and a scale. The workman sits during this operation in a seat with arms, laying the pontee on them, and turning it, so as to make it move backwards and forwards with his left hand, while with the tongs in his right he gives the glass the desired form.
Before passing on to a description of the manufacture and composition of coloured glasses, it is necessary that I should make a few remarks on the difficulties under which our English glass makers labour, owing to not paying sufficient attention to the scientific treatment of their mixtures. It has already been stated that glass is composed of a mixture of silicates, which are definite chemical compounds. Some are much more dense than others, and are therefore liable to sink, so that the glass taken from one part of the pot will be very different in composition from that taken from another part; besides this, it is found on examination, that other portions of the materials employed are present in such proportions, that they cannot possibly exist in the form of true silicates. M. Dumas, the distinguished French chemist, asserts, and with truth, that glass ought to be a true chemical compound. This, however, does not seem to be the opinion here; and sufficient attention is not paid by English manufacturers to mixing their materials, so as to form definite silicates, the result being that glass is produced with a striated effect. This is easy to be seen in the common kinds, as in bottle glass; but owing to the more careful and prolonged fusion of the finer varieties, such as plate glass, this defect is to a considerable extent remedied, though not altogether overcome. In the French manufacture of plate glass, more attention has been paid to the chemical composition of the various silicates which enter into it. At St. Gobain, a plate glass, is produced which, on analysis, is found to contain definite silicates, and without any excess of material which does not enter into chemical combination; and the consequence is, that this glass is more perfect and homogeneous than that made in this country. No doubt this superior quality is owing to the fact, that the famous chemist, Gay-Lussac, devoted much of his time to assisting in the manufacture carried on at these works. We cannot over-estimate the importance of a scientific superintendence, not only of glass-works, but of all other manufactures in which chemical reactions take place; for although experience may lead a cautious observer to produce substances of nearly correct composition, yet the assistance of a scientific observer is of the greatest importance, because, what under other circumstances must be simply empirical, is under his guidance carried on according to definite and fixed laws.
Mention has already been made of how, in the case of mixing carbonates of soda and potash, the one assists the fusibility of the other, and this is more particularly true in the mixture of silicates in the composition of the ordinary glass. Silicates of soda and potash are separately much more infusible than a mixture of the two, and the addition of other silicates to them renders them more fusible still; silicate of lead, as has already been mentioned, causing the glass into whose composition it enters to fuse at a much lower temperature than it would do if that silicate were absent. Again, if the silicate of lead be present in too large proportions, and if great care be not taken in the manufacture of lead glass, the silicate of lead, from its greater density, will sink lower among the molten silicates, and will therefore cause a larger proportion of lead to be in the glass at the bottom of the pot than there is at the top. We often notice in tumblers and decanters of the cheaper kind, that there are very distinct striæ running through the whole substance in some particular portion of the glass. Now this is owing to the greater density of the lead silicate, which sinks lower down in the collected mass of glass, and therefore imparts to it this peculiar effect. When a pot of flint glass is worked off, that which remains at the bottom usually contains more lead than that which is worked off in the earlier part of the day.
Coloured Glasses.—It has been before shown that silica unites with metallic oxides; in fact, glass is nothing but a compound brought about by the union. With certain metallic oxides, silica forms coloured silicates or glasses; and these, when fused with colourless glasses, impart to them the colour of the silicate. Oxide of iron colours glass either green or yellow, according to the nature of the oxide; the silicate of the protoxide of iron being green, and that of the peroxide, yellow of a slightly brownish tint. Copper forms two oxides, the suboxide and the protoxide; the suboxide colours glass red, while the protoxide renders it green. Black oxide of manganese colours glass purple; but if large quantities be used, it makes it perfectly black. Sesquioxide of chromium imparts a beautiful green colour to glass, while oxide of uranium produces an opalescent effect of yellow with a tinge of green. This latter, by the way, has the power of reducing the ultra-violet rays of the spectrum to luminous rays, and, when held in the rays of a spectrum obtained by the electric light, produces an extremely beautiful effect, which is called fluorescence. A small quantity of the oxide of gold tints glass pink, but the colour becomes extremely rich and ruby-like, when a larger quantity of the oxide is employed. Oxide of cobalt in very small quantities yields, with silicic acid, an intensely blue silicate. This substance, carefully prepared in a special manner and ground to a fine powder, forms the well known water-colour pigment called smalt. Oxide of silver stains glass from a delicate lemon tint to a deep orange, in proportion to the quantity of the oxide employed.
With the exception of the last-named colouring material, the above mentioned are mixed together with the substances which form the glass, and are melted in the usual way in glass-pots, except that they are treated with considerably more care, in order that their tints may be true. Oxide of silver, however, is never mixed with the materials of which the glass is made, but is applied to the surface in the following manner: a solution of nitrate of silver mixed with some substance, such, for instance, as chalk, may be painted upon the parts of the glass which it is desired to stain, and these are heated to a dull red heat, in what is called a "muffle." Wherever the oxide of silver, which is reduced from the nitrate by heat, comes in contact with the glass, the latter is stained more or less intensely, according to the quantity of silver present. Pure metallic silver may be melted with metallic antimony, and the mass ground to a fine powder in water. This powder, after being mixed with some Venetian red and gum water, is applied to the surface of the glass, which is, when dry, heated to a dull red heat in a muffle, producing the yellow stain, which can be seen after the Venetian red and the excess of silver have been scraped off. The reason why silver, or oxide of silver, is not mixed with the glass materials and fused with them, is because it does not readily unite with oxygen, and, when it has done so, it loses its oxygen again at a high temperature, and becomes reduced to the metallic state; and inasmuch as metals have no effect whatever in staining silicates, glass made in this way would not have the yellow colour which it has, when the silver is heated upon its surface to a much lower temperature in a muffle; for the temperature to which the constituents of the glass must be heated, so as to cause them to burn it in, would be so high, that the oxide of silver first formed at a lower temperature would be reduced to the reguline or metallic state. Gold also, like silver, does not unite with oxygen readily, or remain in union with it at high temperature; therefore great care is required in the preparation of glass to be coloured by oxide of gold; the form in which it is used being generally that of the purple of Cassius, made by precipitating a salt of tin with a salt of gold. This substance is mixed with the glass to be coloured, and heated in a suitable glass-pot. Portions of it are gathered and allowed to cool, these being generally of a yellowish, brownish, and sometimes reddish tint, though they have not in any case the same beautiful red colour which they produce when applied, as will be immediately described, to the surface of white glass. A certain quantity of white glass is gathered from the glass-pot in the soft state with one of these pieces of gold glass; the whole mass is heated until both become soft, and is then blown and formed into sheet, which, on examination, will be found to consist mainly of white glass, with its surface thinly covered with the glass stained with oxide of gold, while the beautiful ruby colour, which the gold imparts to the glass, appears pure and distinct. If such glass as this be heated to too high a temperature, as when it is used in the manufacture of stained glass windows, the ruby colour is in part, and sometimes altogether, destroyed, for the oxide of gold loses its oxygen, and metallic gold is left behind, which does not yield a colour to the silicate. I have in my possession a piece of French glass of a pale sapphire tint, which, when heated in the oxidizing flame of the blowpipe, assumes a brilliant and intense ruby colour, showing that in the first condition, the gold is not in a state of oxidation sufficient to impart colour to the glass.
When the suboxide of copper is mixed and fused with the glass which it is intended to colour, the result is an opaque substance, almost like red bottle-sealing-wax, which is treated in a manner exactly similar to the gold glass; viz. it is coated with white glass, and blown and shaped into sheets, which owe their intense ruby colour to a thin film of the coloured glass closely adhering to the mass of the white upon which it is placed. Glass made in this way is called "coated," and sometimes "flashed" glass, and is extremely useful for ornamental purposes, for by the action upon the coloured surface of hydrofluoric acid, the ruby coating can be eaten away, and the white glass beneath left entire. If the backgrounds of the patterns be painted upon the ruby side with a material like Brunswick black, which is able to resist the action of hydrofluoric acid, and if the plate of glass, on its ruby side, be exposed to the action of the vapour of this acid, or to the action of the acid in solution in water, in a short space of time the pattern will be eaten away; and if the Brunswick black coating be removed with turpentine, a sheet of ruby glass will be obtained with a white pattern etched upon it.
Owing to the powerful colouring properties which oxide of cobalt exerts, a very deep-coloured blue glass can be made, which can be treated like the red copper glass, and may be made to coat and cover in the same way the surface of plates of white glass. Purple glass, coloured with oxide of manganese, and green glass are also sometimes used as coating materials for white glass, but other colours are never employed in this way.
It is manifest that if different metallic oxides be used with the same glass, mixed tints will be produced, so that by mingling small quantities of oxide of cobalt and protoxide of copper, a blue glass having a greenish hue may be obtained. The revival of glass painting has caused manufacturers to turn their attention to these mixtures, in order to produce tints resembling those of ancient stained glass. Messrs. Powell and Son, of Whitefriars, were the first to perform experiments on these mixtures, and after much laborious attention and patience their efforts have been crowned with great success, for they have been enabled to produce glass as beautiful in tint and in texture as the best specimens of ancient manufacture. Their example has been followed by others, such as Messrs. Hartley of Sunderland, and Messrs. Chance and Co. of Birmingham.
While treating of the effect produced by different metallic oxides upon colour, it may be well to mention that the opaque glasses used for such purposes, as the enamelling of watch-faces, are made by mixing with the materials a certain quantity of arsenious acid (or white arsenic), in much larger quantities than when it is employed simply to correct the tint imparted to glass by the iron impurities in the sand. Oxide of tin also renders glass white and opaque, and a certain quantity of bone ash will produce a similar effect, though not in so satisfactory a manner.
Glass painting first became general in this country at the time when the Early English style of architecture prevailed, and some of the best specimens were executed during that period. By the best specimens is not meant, that the figures painted upon those windows were artistically as correct as similar works of a later date, but that they were designed and executed in accordance with those principles, which should always govern the adaptation of a substance like glass to ornamental purposes. The earlier mediæval artists depended for effect more upon the boldness of their outline, than upon the intensity of their shading or the delicacy of their manipulation. The form of a thirteenth-century figure is merely indicated by a few bold and well drawn outlines, the features being formed by lines, the pupils of the eyes by simple well-shaped masses of opaque pigment; and such a treatment as this was quite sufficient to convey what was, to the observer, more or less a symbolical, than a truthful representation of the Scripture history which they were intended to illustrate. These artists remembered that windows are openings in a building, through which light has to pass, and they did not, therefore, like many of the later imitators, render them opaque by masses of intense shadow, which perfectly obscure the colour of the glass upon which the picture is painted, and render the passage of light through it simply impossible. The thirteenth-century glass painters, too, in the treatment of their shadows, bore this great principle in mind, and instead of daubing and stippling them on, usually indicated them with a thin wash of enamel colour, intensified in parts by lines crossing one another, and therefore called cross-hatching, through the interstices of which the light, although subdued, was able, in a measure, to pass.