British Manufacturing Industries: Pottery, Glass and Silicates, Furniture and Woodwork.

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 about1 6½ inches by 5 inches, and 2/10 inch thick, was placed between two wooden frames, and a weight of over 3½ ounces (100 grammes2) was allowed to drop upon it from a height of more than 13 feet (4 mètres3) 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, about 1/10 inch 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.) and 1/6 inch (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 kilogrammes4) was placed, and upon it as a base, other weights were placed, care being taken to avoid all shock.

1º Ordinary glass, broke with a weight of about 70 lb. (35 kilos.) having resisted weights of from 30 to 50 lb.

2º Toughened glass resisted 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.) and 1/5 inch (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.

1º Ordinary glass allowed to fall flat from a height of 1-2/10 inch (0·03 m.) was broken at the first trial.

2º Toughened 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.

1º Ordinary glass raised to a height of 20 inches (0·50 m.) was broken on falling.

2º Toughened glass resisted 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.) and 1/5 inch (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.

1º Ordinary glass was broken with a fall of about 2 feet (0·60 m.).

2º Toughened glass resisted 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.) and 1/5 inch (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 glass resisted 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 glass resisted 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.) and 1/6 inch (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 the

tumbler'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 called devitrification. 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.

There is also a very ingenious way of making silicate of soda, discovered by Mr. Gossage, and performed as follows: common salt is heated to a high temperature and volatilized, and in this condition is brought into contact with steam also at a high temperature, when a double decomposition takes place. Steam is composed of oxygen and hydrogen; common salt, of sodium and chlorine. The chlorine of the common salt unites with the hydrogen of the steam, and the oxygen of the steam with the sodium, so that hydrochloric acid and oxide of sodium are formed. Now, if these two substances at this high temperature were allowed to cool together, the action would be reversed, and the re-formation of steam and chloride of sodium would be the result; but in the strong chamber lined with fire-clay, in which these vapours are brought into contact, silica is placed in the form of sand made up into masses, and when the oxide of sodium is formed, it unites with the sand to make silicate of soda, and thus is removed from the action of the hydrochloric acid, not entirely, but sufficiently to produce a large yield of silicate of soda.

The properties of silicate of soda, as applied to the arts, are somewhat different from those of silicate of potash, so that one cannot always be substituted for the other. Both these substances are, when in solution and concentrated, thick and viscid, and have the property of causing paper, wood, &c., to adhere when applied as a gum or glue, and hence have been called "mineral glue." In a dilute state they can be used for coating stone, brick, or cement, and have the power of rendering them for a time waterproof, or nearly so, and of preventing the action of atmospheric influences, which too often produce the decay of some of the softer stones used for building as well as for cement. It has already been stated, that when carbonic acid is passed through a solution of silicate of soda, silica will be precipitated. Now, inasmuch as there is carbonic acid in atmospheric air, when these solutions are applied to the surfaces of a building, they will be acted upon slowly by the acid, and silica will be precipitated in the pores of the material to which the silicates are applied. But this operation is extremely slow, and, before it can be thoroughly completed, the silicates, being soluble, will get in part dissolved out by rain and moisture, and it is therefore advisable to use with them some material which will, by a double decomposition, form a silicate insoluble in water. The silicate, however, which is formed, should have cohesion amongst its particles, so that it will not only adhere to the stone itself, but its own particles will adhere to one another when it gets dry. Various methods have been tried to cause this insoluble substance to be formed upon the surface of stones, so as to fill up its pores and to make a protecting cover for it; but most of them have signally failed, because the new silicate produced by double decomposition has not had the necessary coherence amongst its particles. If a solution of chloride of calcium be added to one of silicate of soda, a silicate of calcium will be precipitated, and it was therefore thought, that by applying to a stone successive washes of silicate of soda and chloride of calcium, an insoluble silicate of calcium would be produced in the pores and on its surface. It is true that such a silicate is precipitated, and that, if the silicate employed be in excess of the chloride of calcium, the particles will be glued together by the adhesive powers of this silicate when it dries; but then the action of moisture upon it is to cause it to run down the surface of the building, and set free the particles of silicate of calcium which it held in combination. Other processes of the same kind have been tried, and with similar results; one great difficulty in the way of the success of this method of applying silicates being that, from the peculiar colloidal or gluey nature of the silicate, it does not penetrate to any considerable depth into the stone, and, if laid on first, prevents the penetration, as far even as it has itself gone, of the solution of chloride of calcium. If the chloride of calcium be used before the silicate, it will penetrate farther than the solution of silicate is able to reach, so that it is impossible to obtain, even supposing the substance to be used in equivalent proportions, a complete decomposition of the one by the other.

The great object to be attained in the preservation of stone by any silicious process, is to use one solution possessing the substances which, when the water has evaporated, will form a perfectly coherent mass for the protection of the stone surface. The depth of penetration, if it is sufficient to protect the outside of the stone from the disintegrating action of the atmosphere, need not be carried much more than one-sixteenth of an inch below the surface, for when old stones which have long been in positions in buildings, and which have not decayed at all, are examined, it will be found that they are covered with an extremely thin film of a hard substance, not thicker than a sheet of writing paper, which has for ages protected and preserved them from decay. This film is produced by a determination from the inside to the outside of the stone of a silicious water, which existed in it in the quarry, and which, when the stone was placed in the building, gradually came to the surface, the water evaporating and leaving behind it a thin film of silica, or of a nitrate – most likely the latter.

If alumina be fused with potash, aluminate of potash, soluble in water, is made; if, however the solution is too concentrated, a certain quantity of the alumina will be precipitated; but if it be dilute, the whole of the alumina will remain in solution. When aluminate of potash of specific gravity 1·12 is mixed with a solution of silicate of potash of specific gravity 1·2, no precipitate or gelatinization will take place for some hours; the more dilute the solution, the longer will it remain without gelatinization, and of course the thinner it will be, and the greater power of penetration it will have when applied to a porous surface. When solutions of aluminate of potash and of silicate of potash of greater density are mixed together, a jelly-like substance is almost immediately formed, and sometimes even the whole mass gelatinizes. If this jelly be allowed to dry slowly, it will contract, and at last a substance will be left behind sufficiently hard to mark glass, though the time for this hardening may be from one to two years; and on examination it is found that this substance has very nearly the same chemical composition as felspar, and is perfectly insoluble in ordinary mineral acids. Now, suppose a dilute solution of this mixture to be applied to the surface of stone, the silicate and aluminate of potash will gradually harden and fill up the interstices of the stone; and as both the substances entering into combination are contained in the same solution, they will both penetrate to the same depth. Inasmuch as the artificial felspar is not acted upon by destructive agents which would disintegrate the stone, it becomes a bonding material for its loosened particles, and at the same time gives a case-hardening to the stone, which no doubt will as effectually protect it against atmospheric influences as in the case of the hardening of the natural one. We have a tolerable guarantee that this will be so, if we consider the number of enduring minerals into the composition of which silica, alumina, and potash enter, and also of the almost imperishable character of granite, which is so largely composed of felspar. Many experiments have been performed on an exhaustive scale with these materials, and in every case it has been found that they have answered the expectation of those who have thus tested them. It is, however, necessary to state, that in making these experiments, great care must be used to employ the mixed substance in solution before gelatinization has set in, for if this has occurred, even to the slightest extent, a surface coating is formed on the stone, which, not having formed a bond with it, easily rubs off.