It should be here mentioned, that the side of the plate which touches the table is always rough, and has no polish, while that over which the roller is passed is slightly undulating, and has a bright polish similar to that of a sheet of blown glass, and which is technically known as "fire" polish. The machine to which the upper plate is attached is so arranged that, when set in motion, it causes it to move in just the same direction that a plate would do if moved by the human arm; this is therefore called an elbow motion. Boys stand by the sides of the two plates, and throw fine sand and water on the lower one, so that the opposed surfaces mutually grind one another, and when this process is completed on one side, they are reversed, and the same operation is performed on the other side. The plates have now the appearance of ground glass, and the surfaces are further ground by fine emery powder, which causes them to be much smoother and more ready for the final polishing. Formerly this was entirely done by hand, women generally being the operators, and oxide of iron, called crocus, mixed with water, the material employed for polishing. Now, however, a more rapid and perfect method is adopted by the use of machinery. A table is prepared which moves from side to side, giving to the plate a lateral motion; and above is a beam, in which holes are drilled at intervals, through which short iron rods, nearly an inch in diameter, pass. On these are padded iron buffers, covered on their under surface with leather; while, pressing down these rods, and therefore the buffers, are springs, which act with considerable force, but which are able to yield to pressure caused by any inequality over which the buffers may pass. The glass plate is fixed upon this table, and its upper surface is exposed to the action of the buffers, while oxide of iron, in a very fine state of division and mixed with water, is allowed to come upon its surface. The glass travelling from side to side is rubbed by the buffers in a lateral direction, and has also a longitudinal motion, so that every portion of it is rubbed equally. If any inequalities occur on the glass, the springs which press down the buffers give way and allow them to rise over it, and this process is continued for some time, until at last the plate receives the polish so characteristic of plate glass. It is then removed from the table and examined by skilled persons, and whatever defects can be removed by hand, are remedied.
Another kind of plate glass, called "patent rolled plate," is made by ladling out from a pot molten glass in the proper state of consistence. The ladle is brought over a small glass table, and a similar operation is performed to that already described. This patent rolled plate is sometimes made with grooves on one of its surfaces, or with patterns in imitation of diamond quarry glazing, and, in fact, with any designs, according to the taste of the manufacturer. These designs are all engraved upon the table, and communicate their patterns to the soft glass; but the smooth surface of such glass which comes in contact with the roller is slightly undulating, though polished. This method of glass making was invented and patented by Mr. Hartley, the noted manufacturer, of Sunderland.
A lighter kind of plate glass, which is principally used for glazing the better class of pictures and engravings, and called "patent" plate, is simply sheet glass polished after the manner of plate glass. Crown glass, which only admits of being cut into small squares, is also used for picture glazing, but is more carefully prepared, and is called by the name of "flatted crown."
Looking Glasses.—Plate glass is employed for making looking glasses, and two processes are now in use for silvering them, the first of which consists in applying a sheet of tinfoil saturated with quicksilver to one side of the glass. The operation is conducted as follows: on a perfectly smooth table a sheet of stout tinfoil is laid, and on it is poured quicksilver, which is distributed evenly over the surface with a hare's foot. When the whole sheet is amalgamated with the quicksilver, more of that substance is poured over it, until it flows quite freely. The glass plate to be silvered, having been made perfectly clean, is floated upon the surface of the quicksilver, an operation requiring care, and is then covered all over with weights, by which means the excess of quicksilver is pressed out, and the glass comes in contact with the amalgamated sheet of tinfoil, to which it adheres entirely. This ancient method of silvering glass has some advantages over the one next to be described. The colour of the plate is, according to artistic taste, better, and with care the plate will not lose its brilliancy for years. I have in my possession some old glasses, the silvering of which is very beautiful, except where it has suffered from mechanical injuries. Silver can be precipitated from a solution of nitrate of silver in several ways, and in some of these specimens was like a bright film. If a crystal of nitrate of silver be put into a test-tube with some bitartrate of lime, and the mixture be rendered ammoniacal and gently warmed (it being kept in motion during the experiment), its sides will be covered with a very brilliant deposit of metallic silver. Oil of cloves and grape sugar have also the power of reducing metallic silver from ammoniacal solutions of the nitrate, when gently warmed; but the mixtures must not be made too hot. In silvering plates of glass, they are first well cleaned, then placed in a perfectly level position, and the silvering liquid is poured over the surface, the room in which the operation is performed being kept sufficiently warm to assist the deposition. When enough silver has been deposited on the glass, the liquid is poured off and the plate dried, while the silver film is protected by being coated with a suitable hard varnish. The composition of the mixtures used by different persons is generally kept secret, though the chemical principle of the reduction of the silver salt is the same. Glasses silvered by this process sometimes lose their brilliancy, by becoming covered on their silvered side with small spots. It is however stated that this results either from a bad system of deposition, or from the film of silver not being sufficiently thick and solid.
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 paintingfirst 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.
But as the object of this article is not to discuss the merits of the various styles of glass painting, however much I might desire to enlarge upon it, I pass on to a description of the methods employed in the manufacture of stained glass windows. In the first place, after a design has been drawn, in which the effect of the window as a whole can be carefully considered, cartoons of the figures and ornament are made of the exact size of the intended painting. And here it should be noted, that all the lines should be extremely clear, precise, and well drawn, because it is from these that the workman, who is not usually himself an artist, has to convey on the glass the feeling of the artist. The cartoon, when completed, is laid down in pieces for convenience-sake on a table, and fastened with small nails. The glass-cutter then selects the various coloured glasses which are required to be inserted in their proper places, so as to carry out the design of the artist. For instance, a piece of white or yellow-tinted glass is cut to the shape of the face. If the figure be a small one, the hair also is included in this; and probably in the figure of a saint, the nimbus which surrounds the head may be included; while in larger figures, particularly in the earliest styles, the face was of glass of one tint, the hair of another, and the nimbus of one or more tints, different from either of these. Sometimes, in the later styles, the hair, after the face was painted and burnt in, was stained with the silver stain already described, so that when the glass was cleaned, it was of a yellow colour. However, not to enlarge more upon these points, which really belong more to the artistic than to the industrial part of window painting, let us proceed to the consideration of manipulative details. The outlines of the figures and ornament are painted with a substance called "tracing brown," made by mixing with a flux some oxide of iron, heating them together in a crucible and grinding the product to a fine powder, which is mixed with certain vehicles adapted to the particular use to which it is to be applied. Different fluxes are employed by different glass painters; some contain borax, because such fluxes fuse more easily, and therefore cause the glass which is painted to be exposed for a less time, and to a lower temperature, than when less fusible fluxes are used.
It is always satisfactory to an author, to feel that his articles have been of some use to those whom he hoped to benefit. Since this article was written a letter appeared in one of the architectural journals, complaining that the glass furnished by manufacturers to glass painters was of inferior composition to that which was used by the manufacturers of ancient stained glass windows. In fact, it was asserted that modern glass was not made with due care, and that to this was owing the unfortunate disappearance of some of the painting and tracing of modern stained glass windows; but that this is not the case, is manifest to all who understand the manufacture of glass. The real reason why the colouring matter with which glass painters outline and shade their designs, has in many instances gradually come off from the surface of the glass, is, because the fluxes used for making it adhere to the glass are of such a composition, that they themselves have by the action of time become disintegrated.
Some time ago, a person engaged in the manufacture of the enamel plates used for railway lamps, on which are written the names of the stations, called upon me, and told me, that the enamel which he employed had become dark, spotty, and in many cases had peeled off from the glass. The reason of this is identical with that which occurs in stained glass windows, viz. that the fluxes that he used were not suitable for the purpose, considering that they had to withstand the action of the weather. From an analysis made of these fluxes (not of those last alluded to, but of those which have been employed in stained glass windows), it appears that large quantities of borax have been introduced; and, wherever this is the case, no reliance whatever can be placed on the permanency of pictures painted with such fluxes. I have appended a few receipts for fluxes, which can be used with safety by any glass painter who will take the trouble to try them. But I must strongly advise that all those who are connected with the making of fluxes in any glass painting establishment, should master sufficient chemical knowledge to enable them to ascertain the behaviour of the materials, with respect to one another, as well as of the nature of the glass upon which they are employed; for very much indeed depends upon a correct knowledge of the character of the glass as to whether it be hard or soft, what it contains, and of the temperature at which the glass becomes sufficiently soft to form a firm and enduring union with the colours fluxed upon it.
Receipts for Fluxes.
The use of very soft fluxes is attended with this inconvenience, that the boracic acid contained in them is generally acted upon by moisture and becomes hydrated, and in this condition often causes the painting to peel away. Harder fluxes, although they have the disadvantage of necessitating the glass to be submitted to a much higher temperature for a longer time in the kiln or muffle, are the best, and, with judicious management, can be used without any injurious consequences to the work on which they are employed. Lead fluxes, containing oxide of lead, are sufficiently fusible for all ordinary purposes, and are not liable to the same objection as fluxes containing borax. Suppose, then, it is desired to paint the outlines of a face, the glass is cut to the shape of the face in the cartoon; it is then laid upon it, and the painter, seeing the lines through the glass, is able to trace them with his brown paint upon its surface. He generally uses gum water as his vehicle, and puts on the shading also with the same mixture, though sometimes it is found necessary to use a substance which is not affected by moisture, as for instance, tar-oil. It is impossible, in the short space of this article, to indicate those occasions on which one should be used in place of the other; a knowledge of this can only be obtained by consulting authorities in which details are more minutely given, or by watching the operations of the glass painter in his workshop. When the face is finished, it is removed, and another portion of the figure, say a piece of the drapery, is proceeded with in exactly the same way; and so, by a repetition of this process in all parts of the figure, it is completed, and looks very much like a puzzle, the parts being put together on the cartoon before the work is finished, in order to see that the whole is harmoniously treated. In shading the face, hands, and those parts of the drapery which require it, a glass easel is used, on which the figure is put together, and the parts made to adhere by wax, so that the artist is able, while painting, to form an idea by transmitted light of the effect which will be produced when the window is finished. The ornament is painted in a similar manner, but usually not with the same care in the details of its execution.
When all the glass is painted, it is fired in a muffle, upon the proper construction of which a great deal depends. It is usually made of iron, and should not be more than 15 inches from its bottom to the top, though its width may vary. It is never well to have muffles for firing glass for painted windows larger than about 2 feet wide, by 2 feet 6 inches deep. The top of the muffle is usually slightly arched from side to side, and it is placed in the furnace on a tolerably thick stone floor, so that the bottom may not get too hot. The fire, which is lighted below, is allowed to play up its sides and over its top, the flue being so built as to draw the flames in that direction, for a top heat is the best heat for firing glass regularly. The muffle is arranged with ridges in its sides, passing from front to back parallel to one another on one side, and exactly opposite to corresponding ridges parallel to one another on the opposite side. These metal ridges are intended to receive iron plates, and there is generally about an inch or rather less between the top of one plate and the bottom of another, when the muffle is perfectly filled. The plates are covered over with perfectly dry powdered chalk or whiting, and the pieces of glass are laid upon them with their painted sides uppermost. When the plates are charged, they are put into a muffle with an iron door, in the centre of which is a hole, and a conical tube with the base attached round it. It is larger than the opening at the other end, which projects some 6 or 7 inches from the surface of the muffle-door at right angles to it. A second door is then placed at a short distance from the first, the tube passing through a hole made for the purpose in it. The orifice is usually stopped by a piece of fire-clay, which can be removed at pleasure. The use of the tube is, to enable the manager of the kiln to look into the muffle, from time to time, to see that the glass does not get too much heated. When the firing is completed, the fire is raked out and the muffle is allowed to cool very slowly, and by this process the glass becomes annealed.
When it is desired to apply to any portion of white glass some yellow silver stain, this can be done either in the first firing, by floating it on to the places to be stained, and allowing it to run in a sort of stream from the brush, so that it will evenly cover the surface and cause the heavier portions of the stain, namely, the mixed metallic silver and antimony, to sink regularly to the bottom, and come fairly in contact with the glass. Not very long ago, it was mentioned to me by a glass painter of note, that the workmen much prefer using the old stain made with silver and antimony, to that which is produced by using nitrate of silver. This really is a mistake on their part, for, when properly managed (and the knowledge of how to manage this stain can be acquired with very little trouble), the nitrate of silver stain is by far the best, and produces much better tints, with less chance of what the men call sulphuring when the glass is fired. This sulphuring is simply the result of opacity, obtained by heating the glass to too high a temperature. If the staining is to be performed in the same firing as that by which the painting is to be fixed, it is quite clear that the outlines of the part to be stained must be painted in, with tar-oil, or with some such substance which is not affected by the moisture of the stain. However, in general, the staining operation is performed after the first firing, that is to say, those pieces of glass to which the silver is to be applied are stained in the method above described after the first firing, and are then fired again, because the heat required to produce a good stain from silver is of a somewhat different character from that which is required simply to fuse the flux that binds the pigment to the glass. A longer and less intense heat, technically called a "soaking," is the best for producing an even and pure yellow tint. If the temperature be allowed to rise too high, the oxide of silver, which alone can stain the glass, gets reduced wholly or in part, and when this happens to only a slight extent, it destroys the transparency of the stain; and when it happens to a great extent, it destroys its colour altogether, making the glass opaque.
It is a matter of astonishment to me that glass painters do not use a ruby stain, which, with a little practice, can be managed quite as successfully as the yellow silver one. It is true that it would be impossible to fire the ruby and the silver stains together, and it would not be at all convenient to fire the ruby stain at the first firing of the painted glass. The method of staining ruby is as follows: grind up carefully some black oxide of copper, mix it with water (or with a small quantity of gum added), float it on the parts to be coloured, place it in a kiln and heat it. Black oxide of copper, when mixed with glass and melted in a glass-pot, makes the glass green; suboxide of copper, which contains less oxygen than the black oxide, when treated in the same way, makes it red. Now, if it can be reduced to the lower oxide of copper, while the black oxide of copper on the surface of the glass is heated, it will then colour the glass red. The best way of reducing the black oxide, is to connect the muffle with a gas-supply pipe, and allow coal gas to pass during the whole time that the heating process goes on. The action of the gas, which contains hydrogen and carbon, is to take away oxygen from the black oxide of copper, when it is at a high temperature; and, as soon as sufficient is taken away by the hydrogen to reduce the black oxide to the state of suboxide, it stains the glass red. It does not matter if the reducing action be continued longer, so that the oxide of copper be reduced to the metallic state; for at that temperature, the stain produced by the red oxide of copper is not removed by the continued action of hydrogen gas. The employment of this process would certainly enable artists who paint in the later styles of glass painting, to very much enrich their draperies, and to produce, more easily, effects which now can only be obtained by a complicated system of lead-work.
When the pieces of glass which have been fired are perfectly cold, the next process is to unite them altogether by peculiarly shaped strips of lead, which are of various kinds, according to the character of the subject required. The lead has a thick part or core, and at right angles to the top and bottom of this are thin plates called the "leaves." The core is milled with little ridges running at right angles to them, so as to enable the workman to bend the lead about with facility. The edges of the piece of glass to be leaded are placed between the leaves and resting upon the core, and the lead is thus arranged all round the glass, and is then laid in its proper situation upon another cartoon, prepared from the one from which the figure was painted, and indicating simply, by lines, where the lead-work is to come. The first piece is fixed by means of nails temporarily placed through the lead. Those pieces which touch it in the design are put in their proper positions, so that the edge touching the next piece will be underneath the opposite leaves to those which confine the first. This operation is repeated, till all the parts of the design are surrounded by lead, and by it united to one another; the joints being secured by solder, generally applied by gas. Nothing now remains but to fill in the interstices between the lead and the glass, so as to make the window firm, solid, and water-tight; and this is done by rubbing into them with a scrubbing brush a cement, usually made of white lead, oil, and plaster of Paris. This composition varies in different stained glass works, nor is it material, provided that the substance hardens, does not crack, and is waterproof.
From this description it will be seen, that the various colours in the different parts of the window are put in as pieces, and that no colours, properly so called, are applied by the brush to the surface. There are, however, certain tints of the "tracing brown," which can be obtained by the addition of black oxide of manganese, or by a different method of preparation of the oxide of iron, to give it its body. Sulphate of iron, when heated, loses its sulphuric acid, and the oxide, which was, as sulphate, in the state of protoxide, becomes, by heating, the red or peroxide of iron; its tint, when made in this way, being generally lighter than the tint of that form of oxide which is employed as ordinary tracing brown. It is sometimes called flesh tint, though this is decidedly an objectionable name for it.
It has been suggested to me, that I should give some receipts for the manufacture of the enamel colours used in mediæval glass painting; I have therefore added a few which are easily prepared. Others of a more complicated nature had much better be obtained from the makers of the enamel used in porcelain painting. And here again, let me remark, that in ordering fluxes from these manufacturers, it should be stated especially that a flux is required which does not contain borax, nor should the painters in any establishment be allowed to use these softer fluxes, which they are almost certain to do, unless forbidden; for though they are easier to work with, they will infallibly lead to calamitous results.
The use of enamels—that is, substances which impart various colours to the glass, when placed on its surface by their fusion—is not admissible in windows which pretend to belong to any of the earlier styles of glass painting; though enamel painting is used for the decoration of houses, and sometimes, as I consider very improperly, for the decoration of church windows. One sheet of glass, colourless and transparent, or it may have its surface ground, is usually employed. A subject is painted on it with enamel colours, much as subjects are painted upon porcelain. When the work is completed, the glass plate is fired, and thus the colours become semi-transparent, and perfectly adherent to the plate; but they are not clear and bright, and transparent, as are the colours of glass which is coloured in the pot, and therefore have not the same brilliancy, nor do they allow of the same bold and effective treatment.
It is much to be desired that amateurs who can draw, and who have a feeling for this particular style of art, should devote a portion of their time to its execution. They will find it to be extremely agreeable and pleasant, and the few difficulties which they meet with in their first attempts will be readily overcome by perseverance, or by applying for assistance and advice to gentlemen engaged in the pursuit of this interesting profession.
Moulded and Cut Glass.—Flint glass is now very commonly blown in moulds, and this art has been brought to such perfection that moulded decanters and tumblers have an appearance very similar to that of cut glass. The moulds are always made of metal, and so constructed, that they open out into two or more pieces, which are generally hinged to the bottom of the mould. The workman places it on the ground, and fixes it by standing on projections from its side. He then gathers a suitable quantity of glass on the end of his blowpipe, which he places in the mould, and the side of the glass touching it will thus have impressed upon it whatever form is engraved on it. After the glass has become hard, the mould is opened, and the glass vessel is removed and annealed.
When it is desired to cut a design on the outside of a tumbler or wine-glass, the vessel is, in the first instance, blown of a thicker substance than if it is to be left uncut. The necessary shapes, which are usually in facets, are cut upon it by the action of sand and water, a lathe of a very simple construction being used to give a rotary motion to cutting discs, made of stone and kept continually moist by water dripping on them, so that when the glass is pressed against them, the required portion of its surface is worn away. The usual diameter of these stones is about 10 inches. After the rougher stone has been used, a finer kind of sandstone disc is employed, or a disc of slate, upon which sand and water are allowed to drop, and the already roughly cut surface is, by their action, partly polished. Copper discs with flattened circumference are used for polishing the glass, and for this purpose, emery mixed with oil, is applied to the edges of their circumference.
Ground Glassis made by rubbing the surface of glass with sand and water, just as in the first operation of plate glass polishing. But a very ingenious method is now generally adopted for grinding glass, by placing it in a cradle, so that it can swing from side to side; sand and water are placed upon the glass, and it grinds itself, so to speak, by this operation.
Annealing and Devitrification.—As the word "annealing" has been often used in this article, it will be well to explain what is its action. If a piece of molten glass be dropped into water, it will assume an oblong shape, the lower end of which will be round, while the other will taper off into a fine point. These drops, which have received the name of Prince Rupert's drops, look like pieces of ordinary glass, and if the small end of one of them be broken off, a sort of explosion takes place, and the whole mass flies into a thousand minute pieces, some of which will be found to have been driven to a considerable distance. Here then it appears, that when the skin, which is perfect and entire in the Rupert drop, is broken, the bond which held together the constituent particles is broken also, and so they are acted on by a repellent force, and fly away from one another. If hot water be poured into a thick common tumbler, it very generally cracks it: but if the tumbler be thin and of better manufacture, it will bear almost boiling water without cracking. In the first case it has been badly annealed; and besides this, glass being a bad conductor of heat, from its thickness, the heat imparted by the hot water expands the inner surface, while the outer coating, not being warmed, does not expand, and, retaining its original form, is burst. If, however, a tumbler be thick and properly annealed, there is not so much danger of its breaking, when a portion of it is exposed to a considerable rise of temperature. In the case of the Rupert drops, they are not annealed at all, and so there is no cohesive bond between the particles, such as there would be if they were properly annealed, that is, if, instead of being cooled suddenly from the molten state, they were allowed to cool in a heated chamber very slowly. After glass has been heated, the particles of which it is composed take a long time to rearrange themselves, so that in the manufacture of thermometers, it is necessary, after sealing up the bulb and tube which contain the mercury, to allow them to remain for a long time; otherwise the pressure of the air on the outside of the bulb, not being supported by any air on the inside, causes the particles of glass to become more compact, and thus renders the capacity of the thermometer bulb and tube smaller than it was, when the thermometer was first sealed. It seems that the process of annealing glass gives time for the particles to arrange themselves in such a way, that when the glass is cold, it will not be so liable to fracture from sudden changes of temperature.
Considerable curiosity has been excited of late by a new invention, which has resulted from the investigations of a Frenchman. We have been told that tumblers and wine-glasses, and other glass utensils, could be so treated that they would never break; and experiments performed upon many samples of these glasses led one to suppose, that the object had been attained. There is no doubt whatever, that some who have had experience of what is termed toughened glass know, that in many cases very uncertain results are obtained in the resisting power of the glass to the action of a violent blow. Before, however, entering into some researches which I have made on the subject, it will be well to state what is the nature of the change which the toughening process produces in the glass, and this seems to be a fit place for this consideration, as the method of making, and the behaviour, of Prince Rupert's drops, have just been discussed.
The physical properties of these Rupert's drops have been examined with great care by M. Victor de Luynes, and the results of his experiments have been communicated to theSociété de Secours des Amis des Sciences. For the purposes of this article, many of his experiments have been repeated, confirming in general his observations, and others have also been instituted. The toughness and hardness of these drops are remarkable; the thick pear-shaped portion will bear a sharp stroke with a hammer without breaking; nor can it be scratched with a diamond. To break the tapering thread or tail, as it may be conveniently called, requires considerable force. To find out what weight was required to do this, a series of experiments was performed, the results of which are given in the table following. The tail of a drop was placed over a small hole bored in the top of a table; a hook was then adjusted round a part of the tail which measured 19 on a Birmingham wire gauge; below the table and attached to this hook, a scale-pan was hung. This pan was then carefully loaded, all shock being avoided, until the thread was ruptured and the weight required to effect this was then noted:
It will be observed that the drops made from green bottle glass withstood a greater strain than those made from crown glass; the latter, in fact, did not break throughout their mass, but left a portion of the bulb unbroken, showing some fault in the tempering. It was with difficulty that the workmen could be induced to make drops out of this kind of glass, as they knew by experience that they usually failed to break perfectly, and they stated that it was quite impossible to make them with lead glass. To ascertain what force was required to fracture a thread of like dimensions that had not been tempered, one of the drops was heated to redness, and annealed by allowing it to cool very gradually. When subjected to the same trial, it was fractured by a weight of 12 ozs., and the drop did not break into small fragments, but behaved exactly like ordinary glass, thus showing that the glass had beenuntempered by the heating process. A piece of glass rod, drawn out into a thread in a gas flame, when subjected to the same conditions, bore a strain of 10 oz. A sewing-needle of the same thickness was broken by a weight of 3 lb. 14 oz., thus showing that the tail of the Rupert's drop was very much manner as to allow the tail to dip into hydrofluoric acid, it is found, that when the surface or skin is eaten away to a certain depth,Example of glassbroken. In whatever way fractured, the particles, when examined by the microscope, show a crystalline structure, and do not at all resemble pieces of ordinary glass; when rubbed between the palms of the hands, they do not cut, nor scratch, nor penetrate the cuticle. If a drop be enclosed in plaster of Paris so as to leave a portion of the tail exposed, it may then be broken and all the particles will remainin situ. On removing the plaster, it will be found that the drop has been broken up into thousands of minute needle-shaped particles arranged in cones, the apices being in the direction of the tail. It would appear then from these experiments, and from observations with polarized light, that the glass in the interior of a Rupert's drop exists under enormous tension, and that it is only prevented from bursting into fragments by the outer skin; on its being broken in any part, the bond which holds together the constituent particles is broken also, and so, being acted upon by a repellent force, they fly away from one another. There is another kind of toy resembling in some respects the Rupert's drop, known as the Bologna bottle or philosopher's flask. It has the form of a soda-water bottle with the neck cut off, the bottom being rounded off and very much thicker than the walls. These flasks are sometimes formed accidentally in glass-works by the workman, who, in order to examine the quality of the glass, takes out a portion from the pot on the end of his blowpipe, and blows a small quantity of air into the mass, manipulating it in the usual manner. Whilst still at a very high temperature, it is detached from the blowpipe, and is probably allowed to fall on the ground in a place where there is a current of cold air, the exterior thus becoming suddenly chilled. When cold, these flasks will bear very rough handling, and will withstand the blow of a hammer on the outside, it being almost impossible to break them by striking the bottom; the interior will also bear the blow of a leaden bullet falling into it from a considerable height, but if a few grains of sand be allowed to fall into it, or if the inside skin be slightly scratched, the mass splits into fragments in the same manner as a Rupert's drop. The examination of these curious phenomena leads us to the subject of "toughened glass," as it has been termed. The invention of rendering articles of glass less fragile, which has given rise to so much public attention during the last year, is due to M. Alfred de la Bastie, a French engineer. His process consists in heating the glass to be toughened to a temperature close upon its softening point, and then plunging it into a bath of oil, or into a mixture of oleaginous substances kept at a much lower temperature. When this operation is successfully performed, the glass acquires properties very similar to those of Rupert's drops; it becomes much less fragile than ordinary glass, but when sufficient force is employed to fracture it, the whole flies into small pieces. It cannot be cut with a diamond, but is immediately disintegrated when the outer skin is scratched to a certain depth.
It is to be observed, however, that in particular cases it is possible both to saw and pierce the toughened glass. M. de Luynes reports, that when a square of St. Gobain plate glass that had been submitted to the process of tempering was examined by polarized light, it showed the appearance of a black cross, the arms of which were parallel to the sides of the square. The glass was sawed in two, along the line of the stem of the cross, without causing fracture. On examining the divided glass with polarized light, black bands and fringes of colour were observed, which, by their position, proved that the molecular condition of the glass had changed; on placing one half of the divided glass on the other half, the fringes and black bands disappeared—on folding one half on to the other, the black bands presented the appearance that would have been produced by glass of double the thickness. These facts show, that the molecular forces on the glass were arranged symmetrically in reference to the line of parting: and we may conclude that toughened glass being in a state of tension, similar to that of the Rupert drop, may be divided or pierced, provided that the molecules of the pieces produced are able to rearrange themselves into a stable equilibrium. Polarized light shows the directions on which the division can be made with safety.
M. de Luynes, in his communication referred to above, gives an account of some experiments performed on plates of glass of the same quality, tempered by this process, and untempered; one or two examples will suffice. A tempered plate measuring about[1]6½ inches by 5 inches, and2/10inch thick, was placed between two wooden frames, and a weight of over 3½ ounces (100 grammes[2]) was allowed to drop upon it from a height of more than 13 feet (4 mètres[3]) without breaking it. It only broke, when double the weight was employed from the same height. A piece of ordinary glass under the same conditions broke, with the weight of 3½ oz. dropped upon it from a height 16 inches (0·40 mètre). Plates of toughened glass were allowed to fall on the floor from a height, or were thrown to a distance, without breaking. A rectangular piece of ordinary window glass, about1/10inch in thickness, was bent into the form of a bridge, and then subjected to the tempering process; placed upon the ground; it bore the weight of a man easily without breaking. A commission, instituted by the French naval authorities, to inquire into this process of M. de la Bastie, has reported at some length on the subject. The following series of experiments were tried with a view of ascertaining the comparative power of resistance of tempered and ordinary glass. The plates experimented upon were placed loosely in wooden frames constructed for the purpose.
Rectangular plates about 21 inches(0·525 m.)by 10 inches(0·248 m.)and1/6inch(0·004 m.)thick.
The frame with the glass inserted was laid on the ground, and in the middle of the plate a weight of more than 10 lbs. (5 kilogrammes[4]) was placed, and upon it as a base, other weights were placed, care being taken to avoid all shock.
1ºOrdinary glass, broke with a weight of about 70 lb. (35 kilos.) having resisted weights of from 30 to 50 lb.
2ºToughened glassresisted fracture until a weight of more than 510 lb. (255 kilos.) had been added, and then was not broken. The experiment was not carried to its limit for want of weights.
Rectangular plates, about 13 inches(0·325 m.)by 10 inches(0·248 m.)and1/5inch(0·005 m.)thick.
These plates were allowed to fall flat on to a floor of wood or thrown to a distance and allowed to fall.
1ºOrdinary glassallowed to fall flat from a height of 1-2/10inch (0·03 m.) was broken at the first trial.
2ºToughened glass.Thrown to a height 6 feet 6 inches (2 mètres) and to a distance of 13 feet (4 mètres) was also broken at the first trial. The piece, however, which had sustained the weight of 510 lb. did not break till the fourth trial.
Rectangular plates, about 10 inches(0·245 m.)by 6 inches(0·157 m.)and ¼ inch(0·007 m.)thick.
These plates were subjected to the same kind of tests as the foregoing. After raising them to a given height they were allowed to fall flat upon a wooden floor.
1ºOrdinary glassraised to a height of 20 inches (0·50 m.) was broken on falling.
2ºToughened glassresisted successive falls of from 20 inches (0·50 m.), 32 inches (0·80 m.), 5 feet (1·50 m.), and 5 feet 7 inches (1·70 m.), but was broken when dropped from a height of 6 feet 6 inches (2·0 m.).
Rectangular plates about 10 inches(0·245 m.)by 6 inches(0·157 m.)and1/5inch(0·006 m.)thick.
Placed in the frames, they were held in position in the rabbets by laths nailed to the sides so as to prevent any play. The frames were raised to different heights and allowed to fall in such a manner as to cause as much vibration as possible.
1ºOrdinary glasswas broken with a fall of about 2 feet (0·60 m.).
2ºToughened glassresisted falls from heights of 3 feet 3 inches (1 mètre), 6 feet 6 inches (2 mètres), 8 feet (2·50 m.), 9 feet 9 inches (3 mètres), and 14 feet 6 inches (4·50 m.). It was only broken by a fall of 19 feet 6 inches (6 mètres).
Rectangular plates 6 inches(0·158 m.)by 4¾ inches(0·120 m.)and1/5inch(0·006 m.)thick.
These plates were placed in the frame on the ground, as has been previously explained. Known weights falling from known heights were made to strike the plates exactly in the centre. The weights consisted of bronze spheres, one weighing 3½ oz. (100 grammes) and another of twice that weight.
1st.Ordinary glassresisted the weight of 3½ oz., falling from heights of 8 inches (0·20 m.), 12 inches (0·30 m.), 16 inches (0·40 m.), but was broken by a fall of 20 inches (0·50 m.).
2nd.Toughened glassresisted the blow of the 3½ oz. weight falling from heights of 20 inches (0·50 m.), 40 inches (1 mètre), 60 inches (1·50 m.), and 6 feet 6 inches (2 mètres). The 7 oz. weight (200 grammes) being substituted, the plate was broken by it, falling from a height of 60 inches (1·50 m.).
Rectangular plates, 6 inches(0·158 m.)by 4¾ inches(0·120 m.)and1/6inch(0·004 m.)thick.
The same conditions were maintained as in the previous trial.
1st.Ordinary glass.The 3½ oz. weight was allowed to fall from heights of 1 foot (0·30), and 16 inches (0·40 m). It was broken by the second blow.
2nd.Toughened glass.This resisted the 7 oz. weight falling from heights of 2 feet 4 inches (0·70 m.), and 2 feet 8 inches (0·80 m.), but broke when the weight fell from 39 inches (1 mètre).
It appears then from these experiments, that toughened glass will resist a blow five times as great as ordinary glass, and will bear seven times as great a weight.
I have now detailed most of the useful experiments which have been made by competent observers upon toughened glass, as well as some which have been conducted in my own laboratory. The result of my own personal investigations I will now lay before the reader. I was consulted some time ago by a gentleman interested in the introduction of toughened glass into this country, as to whether this kind would become untoughened in time. I feel no hesitation in stating that when the process has been perfectly done, the glass will remain in the same state for any length of time, provided it be not treated in any way which is calculated to rupture the external hard bond that holds together the inner particles of the glass. I feel quite sure, that no fear of this kind need interfere with the benefits, whatever they may be, which are to be derived from submitting glass articles to the toughening process.
A tumbler which had been toughened in Monsieur de la Bastie's works, was, in my presence, thrown upon the ground, yet it did not break. It was a large soda water glass. I kept it for some time, and after considering the matter carefully, I felt, that if it were thrown down in such a way that the whole of its side, from base to rim, came in contact with the ground at once, and it then stood this test, it would prove that the whole of the glass was in the condition of the Rupert's Drops, and would therefore bear the concussion without fracture. I held the glass and let it fall, so that it actually reached the hard floor on its side. It immediately broke all to pieces. Now on the first occasion when this glass was thrown down, it was tossed somewhat upwards into the air, and the bottom being heavier reached the ground first, and it did not break. I have also seen in glass-houses, where the tempering process is carried on, tumblers thrown down in a similar manner, and I noticed, that whenever they fell upon their bottoms, they were uninjured, as also in cases where they fell upon their rims in such a manner, that the curve of the rim acted as an arch, as in the old trick of turning a wine-glass off the table so as not to break; but in other cases where the tumblers fell flat upon their sides, fracture followed. I carefully gathered together the pieces of the large tumbler which I broke myself in this manner, and examined them, and found that the solid bottom was broken in the same manner as the Prince Rupert's drops break, viz., into a large number of small pieces, having in all respects similar properties. The glass for an inch or two above the bottom broke into small pieces, but larger than those into which the bottom itself broke, and the upper portion of the tumbler was fractured just as an ordinary tumbler would be. On careful examination, microscopic and otherwise, the small pieces were found to have the character of Prince Rupert's, whereas the larger from the upper part of the glass had none of these characteristics in the slightest degree.
These observations led me to perform an experiment. A toughened tumbler was filled with plaster of Paris, which was allowed to set. Its outside was then encased in plaster of Paris, and when the whole was hardened, a pair of pincers were applied to a portion of theA tumbler filled with plaster of Paristumbler's rim, and with a violent wrench the tumbler was broken. A rather smart shock was communicated to the arm of the operator, very much resembling, as he said, the shock of an electrifying machine. On removing the plaster of Paris, it was found that the whole of the tumbler was fractured, and, as will be seen by the accompanying illustration, in a manner similar to that which has already been described.
From this and other similar experiments, I was led to the conclusion that none of the toughened articles which have cavities in them, have thoroughly undergone the toughening process.
Having been requested to attend a series of experiments performed by a glass manufacturer in London, which consisted in the manufacture of a number of toughened glass tumblers, I noticed certain facts which led me to form conclusions as to how it was that the tumblers, the fracture of which I already explained, break in this peculiar manner. I will first describe the way in which these tumblers were made and toughened. By the side of the glass blower there stood a metal vessel, about three feet six inches high, and, perhaps, from two to two feet six inches in diameter. This was filled with melted fat or oil of some kind at a temperature of about 80° Fahr. Inside this vessel, which was open at the top, there was a wire cage, with a trap door at the bottom about one foot in diameter, and of about the same depth. The glass blower, after finishing his tumbler on the pontil, held the pontil in a horizontal position over this metal vessel, struck it a smart tap, and the glass tumbled off into the wire cage. The glass was at a very high temperature. In almost every instance the glass fell into the melted fat, as a glass thrown in a similar manner will fall into water. It sank gradually bottom downwards, and the liquid guggled into it as it sank. Here, then, it is clear that every portion of the hot tumbler did not come in contact with the oil at the same moment, in fact there was an appreciable lapse of time before the tumbler disappeared beneath the surface of the liquid. Now there must be a limit as to the temperature of the article to be tempered and of the liquid by which it is to be tempered, that is to say, if at a certain temperature glass can be tempered by being plunged into the liquid of a certain temperature, if these temperatures are varied similar results will not follow. The upper portions of the glass coming in contact with the tempering liquid at a lower temperature, as they must have done, were not properly tempered, and this I have clearly proved by the facts I have already stated. From these remarks it seems tolerably clear that, until some method is devised of bringing all the parts of the heated glass in contact with the cooling liquid simultaneously, the tempering of the article cannot be perfect throughout its whole surface. As I desire, and very sincerely, that these processes should be brought to perfection so as to render them useful, I willingly give this result of somewhat lengthened investigations to those whom it may commercially concern, and I hope that they will find, on investigating the matter, that my observations have been tolerably correct, and that they will be able to devise a method which will remedy in many cases manifest imperfections of their present system. All the accidents which have happened to tempered glass, which have been recorded in the newspapers, can be accounted for on the principle which I have just endeavoured to explain, for there must be instability, where the bonding material of the internal particles of the glass is in different states of hardness; so that there is no difficulty in conceiving how a gas globe could break apparently spontaneously, for a portion of it which was not fairly toughened might be exposed to a somewhat sudden rise of temperature, produced, it may be, from a draught blowing the flame upon that particular spot. Articles such as saucers, made of glass, which, being flat, or nearly so, can be plunged into the tempering liquid with great rapidity, are usually tempered all over, and these, when toughened, can be thrown about and allowed to fall on hard floors with impunity, thus proving the facts which I have endeavoured to establish. I hope to be able to continue my investigations, and should they be worth anything, will give the results of them to the public. Before quitting this subject, I shall make a few remarks upon the process for toughening glass, which is said to have been purchased by the Prussian Government.
This process is described as consisting in the application of superheated steam to the glass, brought up to a temperature near to its melting point. Having facilities for making experiments of this kind, I have had them tried with great care, but in no case have I met with a satisfactory result. This probably is owing to the fact, that I did not comply strictly with the condition of the experiments performed by the German chemist who is said to have made the invention, nor do I see from analogy how this process is likely to effect a change in the glass similar to that arising from M. de la Bastie's dipping process.
If glass, instead of being taken from the annealing kiln at the proper time, be left exposed in the hot part of it, at a temperature just below that at which it softens, it will be found to become gradually opaque on its surface. Some experiments were performed many years ago by Réaumur, who exposed pieces of glass, packed in plaster of Paris, to a red heat, which became gradually opaque, and lost altogether the character of glass, the texture of their material becoming crystalline, and also effected by sudden changes of temperature. Glass treated in this way was called Réaumur's porcelain. All glasses do not undergo this change with equal rapidity, and some do not experience it at all; but the commoner kinds, such as bottle glass, are the best to experiment upon, for the more alumina that it contains—and it is known that bottle glass contains a considerable quantity—the more readily does it undergo this change, which is calleddevitrification. In what it consists, is not at present well understood, but it offers a field for investigation, which may produce results of very considerable benefit to manufacturers of glass.
Soluble Silicates.—An article on glass in a modern scientific work like the present would not be complete without a notice of the manufacture of soluble glass and the uses to which it has been and may be applied. It has already been mentioned that when silica or sand is fused with an excess of alkali, the resulting glass is soluble in water.
Soluble glass is made on a large scale in three different ways. First of all, if flints, that is, black flints, which are found in chalk, be heated to a white heat, they lose their black colour and their hardness, and are easily crushed to small pieces; and if flint in this condition be placed in a wire cage and put into a jacketed iron digester, that is, an iron digester which has an inner and an outer skin, with a free space between the two, so that steam may be forced into it from a boiler under pressure; and if the digester be screwed down tightly with an iron cover, and steam then be allowed to pass into the space between the two, the temperature can be raised at pleasure, according to the pressure under which the steam is introduced. If the valve of the boiler be loaded with a 60-lb. weight, the temperature of the water warmed by the steam will rise considerably higher than that of ordinary boiling water; and if this water be saturated with caustic soda, it will dissolve the flints slowly, forming silicate of soda, that is to say, the silicic acid of the flint will unite directly with the soda of the solution, and silicate of soda will thus be obtained. For certain applications, the silicate so formed is not sufficiently pure, because the soda used often contains a certain amount of sulphate, which will remain with it in the solution of silicate that is drawn off from the digester. This sulphate is very objectionable for certain applications of silicates, because it crystallizes out, and so destroys the substance, which the silicate is intended to preserve.
Another and a much better method is to heat together the silica in the form of sand with alkali, either potash or soda, in a reverberatory furnace, and as the glass becomes formed, to rake it out into water, and then gradually to dissolve it by boiling in suitable vessels. Here the sulphate, if it existed in the alkali, is decomposed by the silicic acid, and the sulphuric acid passes off through the flues of the reverberatory furnace.