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factures of the potteries, and is but seldom resorted to in glass-works. The alkali used in the manufacture of glass is either soda or potash. It is used in the state of a carbonate, though it is evident that the carbonic acid gas is driven off in the process, and the glass is a compound of silex and pure alkali, and not an alkaline carbonate. The finest flint-glass requires the best pearl-ashes, purified by solution and evaporation; but inferior glass is made with coarser substances, as barilla, where it is abundant, with common wood-ashes, and with kelp. These alkalies, it is true, are impure; but this does not prevent their dissolving the silex into a very good and perfect glass, for the very impurities, consisting of neutral salts, lime, and other earths, assist in vitrification. Glass made from these alkalies has always a greenish tinge, owing to the iron contained in them. Lime, in the form of chalk, is used only in small proportions, because if much is used, the glass becomes opaque and milky on cooling, though it was perfectly transparent when hot: hence the reason of what is called smoky glass among the glaziers. The proper proportions are, to 100 parts of silex and alkali, only 6 or 7 of quick-lime can be added. Lime, though mischievous, if used too liberally, has its particular uses when properly proportioned; for, besides affording a cheap flux, it renders the glass easier to work, and much less liable to crack by sudden and violent changes of temperature. Borax is the best flux that is known; its high price is the only objection to its more general use; this prevents it from bein used in common glasses, ". it is never omitt in the finer kinds of plate glass, and those other articles of manufacture that are required to be clear and free from specks and bubbles. Borax renders all vitrescent compounds into which it enters remarkably thin-blowing, as the phrase is, and therefore peculiarly adapted for being cast in a mould, which is the way plate-glass is manufactured. A very small quantity of borax will correct any deficient strength in the alkali. The oxides of lead, of which litharge and minium are the only ones employed in the large way, are of great importance in glass-making. Litharge, of itself, melts into a very dense, clear, yellow, transparent glass, fusible at a low degree of heat; and when melted it acts so powerfully on all kinds of earthen vessels, as to run through the common porous crucibles in a very short time, like liquor through a filter, but vitrifying and corroding the bottom of the crucible in its passage. Litharge, therefore, is not only a most powerful flux to all earthy mixtures, but imparts to glass the valuable qualities of greater density, and greater power of refracting the rays of light; of bearing any sudden changes of temperature; of greater tenacity when red-hot, and therefore easier to be worked. Most of the finer glasses contain a considerable quantity of this oxide, particularly the London flint-glass, or that species which is used for most of the purposes of the table, for lustres, and other ornamental works, which, when cut into various forms, display such beauty and brilliance, as to present a most dazzling appearance, for artificial gems and for most optical purposes. Glass,

however, in which there is much lead, has the defect of being extremely soft, so as to be readily scratched and injured by almost every hard body it rubs against. It is likewise so fusible, that thin tubes made of it will bend with ease in the flame of a candle, and will sink down into a shapeless mass, at a moderate red-heat. This quality is often very useful for chemical puroses, but in other cases it is a great defect. If ead is in excess, there is great danger that the glass will be corroded by the contact of acrid liquors. The black oxide of manganese has been long used in this manufacture: its ancient name was glass soap, which proves that it was used for the urpose of clearing the glass from any accidental oulness of color, which it might otherwise contract from the impurity of the alkali or other materials employed. The oxide of manganese is a very powerful flux for earthy matters, which is seen in the result of all attempts to reduce it to a reguline state in the usual way of combining with a saline carbonaceous flux, and heating in a naked crucible. Not a particle of the oxide is reduced in this way, but the crucible constantly runs down in a heat sufficiently intense for the reduction of the manganese, together with all its contents, into a green flag. The only way known at present of reducing this oxide, is to enclose it without any saline or earthy addition in a crucible lined with charcoal, and apply to it a very intense heat. Manganese, like lead, gives a density to glass, and has, like that metal, a tendency to settle to the bottom of the pots where it accumulates, and, being here out of the way of most of the discoloring additions, it yields a purple tinge immediately adhering to the bottom, and partly corrodes the pots, so that, when they are worn out and broken up, they are thickly incrusted with a purple vitrescent flag, easily separable by the hammer. The white oxide of arsenic is another flux used in this manufacture: this is volatile in the fire in proportion as it approaches the metallic state, and hence it is of great advantage to employ nitre to oxygenate it more highly, and to render it more fixed. Arsenic is a powerful and a cheap flux, but it must be used only in great moderation, as taking alonger time to mix intimately with glass, and, allowing it to be perfectly clear, than almost any other additions that can be employed. Glass, in which arsenic is not most intimately combined, has a milky hue, which increases by age; and when this oxide is in excess, the glass tends to deliquesce, and gradually to become soft, and at length a decomposition will take place. Drinking glasses, and others used for [... connected with our food, should not e made with this flux, as being one of the most dangerous poisons. As arsenic is entirely volatilised, when in contact with any carbonaceous matter, another use has been made of it, which is to disperse the carbon that may remain in the glass pot, owing to any defect in the calcination of the alkali, or any other more latent cause. When this happens small lumps of white arsenio are thrust to the bottom of the glass pots, and stirred in with the contents, and the fumes of the arsenic, meeting with the existing carbon diffused through the glass, unites with it, is speedily volatilised, and the glass is left entirely free both from the carbon and the arsenic that was added. Nitre is used, in glass-making, only in small quantities, and is an accessary ingredient for particular purposes. Nitre is readily decomposed, giving out a large quantity of oxygen, some nitrous gas, and azote, leaving behind its pure potash. It is of great service in destroying any carbonaceous matter in the ingredients of glass: it is also useful in fixing arsenic, and in keeping up the tinging power communicated by manganese. The same circumstance, of keeping metallic oxides up to their highest state of oxigenation, also renders this salt often useful, sometimes indeed essentially necessary, in the Poio of certain colored glasses. While glass is in fusion, the substances which enter into its composition may be considered as combined with each other, so as to form a homogeneous mass similar to water, holding in solution a variety of salts. If it be cooled down very gradually, the different tendency of the constituents to assume solid forms, at peculiar temratures, will cause them to separate successively in crystals, in the same manner as salts held in solution in water assume the form of crystals, when the liquid is slowly evaporated. But, if , the glass be rapidly cooled down to the point of congelation, the constituents have not time to separate in succession, and the glass. remains the same homogeneous compound as while in a state of fusion; just as would happen to a saline solution if o exposed to a degree of cold sufficient to congeal it completely. Hence it should seem that the vitreous quality depends entirely upon the fusibility of the mixture, and the suddenness with which it is cooled down to the point of congelation. The solid substance is precisely the same, as to its chemical composition, as if it were still in a state of fusion; the sudden abstraction of heat having been the means of fixing the constituents before they had time to assume a new arrangement. All fusible mixtures, as we have seen, of the earths with fixed alkalies, &c., may be made at pleasure to assume the form of glass, or the appearance which characterises stone or porcelain, according to the rate of cooling; and glass may be deprived of its vitreous form merely by fusing it, and cooling it down with sufficient slowness to enable the constituents to separate in succession. Experiments have been made on this subject by Reaumur and Lewis, who have both pointed out the method of converting different kinds of glass into an opaque, white, hard, refractory substance like porcelain. Lewis, however, demonstrated, by a variety of experiments, that it is not every kind of glass that can be converted into porcelain. He succeeded only with those that were composed of a variety of constituents, because such glasses alone contain ingredients that become solid in succession. Green glass, which is apt to acquire a crystallised form, succeeded the best with him, and he found that the temperature, which was peculiarly adapted to the change, is that in which the glass is softened without being melted. It was the curious ex it is brittle; but when at red-heat, it is one of the most ductile bodies known, and may be drawn into threads nearly invisible to the naked eye. It is almost perfectly elastic, and of course is one of the most sonorous of bodies. Few chemical agents have any action upon it; but fluoric acid dissolves it with great rapidity. Although glass is chiefly made of sand, flints, fixed alkalies and metallic oxides, yet there are various other substances which frequently enter into the composition, and which should therefore not be wholly omitted in the description. Polverine or Rochetta is one that is procured from the Levant, and is prepared from a plant called kali, which is cut down in the summer, dried in the sun, and burnt in heaps either on the open ground or on iron grates; the ashes falling into a pit grow into a hard mass, and are fit for use when purified. Kelp which grows upon our coasts, and the ashes of the fucus vesiculosus furnish a similar salt: to these we may add the barilla of Spain. To prepare ashes for making glass-Take what quantity and what sort of wood-ashes you will, except those of oak; have a tub ready with a spigot and faucet towards the bottom, and in this tub put a layer of straw, and fling your ashes on it; then pour water upon them and let the ashes soak thoroughly until the water stands above them: let it thus continue over night, then draw out the faucet and receive the lie in another tub, put under the first for this purpose: if the lie looks troubled, pour it again on the ashes, and let it settle until it is clear and of an amber color. This clarified lie put by, and pour fresh water on the ashes; let this also stand over night; then draw it off, and you will have a weak lie, which, instead of water, pour upon fresh ashes; the remaining ashes are of use in the manuring of land. After you have made a sufficient quantity of lie, pour it into an iron caldron, bricked up like a brewing or washing copper, but let it not be filled above three parts full. On the top of the brickwork place a little barrel with lie; towards the bottom of which bore a hole, and put a small faucet in, to let the lie run gently into the caldron, in a stream about the roundness of a straw; but this you must manage according to the quantity of lie, for you ought to mind how much the lie evaporates, and make the lie in the barrel run proportionally to supply that diminution. Care must be taken that the lie do not run over in the first boiling; but, if you find it will, put some cold lie to it, and slacken the fire, and let all the lie boil gently to a dry salt: when this salt is cold, break it and put it into the calcar, and raise your fire by degrees until the salt is red-hot, yet so as not to melt it. If you think it calcined enough, take out a piece and let it cool, then break it in two, and if it is thoroughly white, it is done enough; but if there remains a blackness in the middle, it must be put in the calcar again, until it comes out completely white. If you will have it still finer, you must dissolve it again, filtrate it, boil it, and calcine it as before: the oftener this is repeated the more will the salt be cleared from the earthy particles, and it may be made as clear as crystal and as white as snow. Of this may

periment of Sir James Hall on basalt and greenstone that first led to an explanation upon what the vitreous state of substances depends. He found that glass, consisting of various earthy bodies, loses its vitreous state, and assumes that of a stone, if more than a minute or two elapses while it is cooling down from the complete fusion to the point at which it congeals. There are, it is well known, different kinds of glass in common use in this country, adapted to various purposes. The finest is plate-glass, of which looking-glasses are manufactured: flintglass, or, as it is frequently denominated, crystal, is not much behind the plate-glass in the excellence of its qualities. These are both perfectly transparent and colorless, heavy, and very brilliant. They are composed of fixed alkali, pure silex, calcined flints, and litharge. The proportions, as far as can be obtained, will be given hereafter. Flint-glass contains also much oxide of lead; though it is solid, it does not appear to be absolutely impervious to gaseous bodies, at least when heated nearly to the melting point. Dr. Lewis surrounded a piece of it with charcoal powder, and kept it some time in a heat not quite sufficient to melt it. The lead was revived in drops through the whole substance of the glass. Dr. Priestley ascertained, that glass tubes filled with hydrogen gas, and heated, became quite black, from the revival of the lead. When alkaline hydrosulphurets are kept in glass phials, the inside is coated with a black rust, which is, in fact, the lead separated by the sulphur from the lass. g Crown-glass is made without lead; it is, therefore, much lighter than flint-glass. It consists chiefly of fixed alkali, fused with siliceous sand. Bottle-glass is the coarsest and cheapest kind, and in this but little fixed alkali enters into the comosition. It consists of an alkaline earth, comined with alumine and silica. In this country it is composed of sand and the refuse of the soapboiler, which is the lime employed in rendering his alkali caustic, and of the earthy matters with which that alkali was contaminated. Some of this kind of glass was analysed by M. Vauquelin, and was found to be composed of

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be made the finest glass possible. According to Dr. Merret, the best ashes in England are burnt from thistles and hop-stalks, after the hops are gathered: and among trees the mulberry is reckoned to afford the best salt. The most thorny and prickly plants are observed to yield better and more salt than others; also herbs that are bitter, as hops, wormwood, &c. Tobacco stalks, when burnt, produce likewise plenty of salt: and it is observed that fern ashes yield more salt than any other ashes. Dr. Thomson, to whose admirable work on chemistry we have been indebted for part of this article, says the fullest account of glass-making is to be found in a treatise by Neri, an Italian. Dr. Merret, an Englishman, translated it into Latin, and enriched it with notes. Kunkel translated this Latin edition into German, with additions, which were the result of his own numerous experiments on glass-making. Kunkel's work was translated into French in 1752. An elaborate account of glass-making has been published in the Arts et Metiers; and since that a small volume on glass-making has been written in French by Loysell. To make the glass frit.—Take white silver sand, wash it, and separate all the impurities from it, and let it dry, or rather calcine it. Of this take sixty pounds, and of prepared ashes thirty pounds, mix them well together, then set them in the melting furnace; the longer it is melting, the clearer will the glass be made. If it stands for two days and two nights, it will be fit to work with, or to tinge with what color you please. Before you work it, add forty pounds of lead and half a pound of manganese to it. Or take ashes, prepared as above, six pounds, of prepared silver sand 160 pounds, arsenic four pounds, white lead two pounds, clear dry nitre ten pounds, borax two pounds; mix all

‘well together, and proceed as has been directed,

and you will have a beautiful crystal. Of glass-blowing.—Glass-blowing is the art of forming vessels of glass. This term, however, is exclusively applied to those vessels which are blown by the mouth. The operation is exceedingly simple: the workman has a tube of iron, the end of which he dips into a pot of melted glass, and thus gathers a small quantity of glass on the end of it; he then applies the other end of the tube to his mouth, and blows air through it: this air enters into the body of the fluid glass, and expands it out into a hollow globe, similar to the soap-bladders blown from a tobacco-pipe. Various methods are used to bring these hollow globes into forms of the different utensils in common domestic use. The first and greatest of the glass-blower's implements is the furnace, which consists of two large domes set one over the other; the lower one stands over a long grating (on a level with the ground), on which the fuel is placed; beneath the grate is the ashpit, and a large arch leading to it conveys air to the furnace. In the sides of the lower dome, as many holes or mouths are made as there are workmen to make use of the furnace, and before each mouth a pot of melted glass is placed. The pots are very large, like crucibles, and will hold from 300 to 400 cwt of liquid glass: they are supported upon three small piers of brickwork, resting on the floor of the furnace. The form reverberates the flame from the roof down upon the pots, and they are placed at some distance within the furnace, that the flame may get between the wall and the pots. The upper dome is built upon the other, and its floor made flat by filling up, round the roof of the lower dome, with brick-work; there is a small chimney that opens from the top of the lower dome into the middle of the floor of the upper one, which conveys the smoke away from it, and a flue from the upper dome leads it completely from the furnace. The upper dome is used for annealing the glass, and is exactly similar to a large oven; it has three mouths, and in different parts a small flight of steps leads up to each. A green-glass furnace is square; and at each angle it has an arch for annealing or cooling glasses or bottles. The metal is wrought on two opposite sides, and on the other two they have their colors, into which are made linnet holes for the fire to come from the furnace to bake the frit, and to discharge the smoke. Fires are made in the arches to anneal the work, so that the whole process is done in one furnace. These furnaces must not be of brick, but hard sandy stones. In France they build the outside of brick; and the inner part, to bear the fire, is made of a sort of fuller's earth or tobacco-pipe clay, of which they also make their melting pots. In Britain the pots are usually made of Stourbridge clay. It is observed, that the roughest work in this art is the changing the pots when they are worn out or cracked. In this case the great working hole must be uncovered; the faulty pot must be taken out with iron hooks and forks, and a new one must be speedily put in its place through the flames (for glass-furnaces are always kept burning) by the hands only. In doing this the man guards himself with a garment made of skins, in the shape of a pantaloon, that covers him all but his eyes, and is thoroughly wetted all over: his eyes are defended by proper shaped glass of a green color. We now come to describe the smaller implements, which are as follows:–1. A bench, or stool, with two arms at its ends, which are a little inclined to the horizon. 2. A pair of shears, or rather pliers, formed of one piece of steel: they have no sharp edges and spring open of themselves if permitted. 3. A pair of compasses to measure the work, and ascertain when it is brought to the proper size. 4. A pair of common shears for cutting soft glass. 5. A blowingpipe, which is a wrought-iron tube, three or four feet long, covered with twine at the end by which it is held. We may now explain the use of these tools in the manufacture of some vessel, as a lamp, &c. The operation is conducted by three workmen. The first takes the blowing-pipe, and after heating it to a red-heat, at the mouth of the furnace, dips it into the pot of melted glass, at the same time turning it round that it may take up the glass, which has then much the consistence of turpentine; in the quantity of metal he is guided by experience, and must proportion it to the size of the vessel to be blown; he then brings it from the furnace to the stool, and rolls

the lump of glass upon it to bring it to a round form, after which he blows through the pipe, resting the glass upon an iron plate bif the stool and rolling it backwards and forwards. The blowing makes the glass hollow, and he has several methods of bringing it to a proper shape to be worked ; by simply blowing, it would assume a figure nearly globular; if he wants it any bigger in the equatorial diameter, he lays the pipe on a hook driven into the side of the stool, and turns it round very quickly; the centrifugal force soon enlarges it in the equator. If, on the other hand, he wishes to lengthen its polar diameter he holds the pipe perpendicular, the glass hanging downwards, its weight lengthening it; and to shorten the polar diameter he holds the pipe upright, the glass at the top; by blowing through the pipe the capacity is increased, and the thickness of the glass of the vessel diminished. We now suppose that, by a very dexterous application of the above methods, the workman has brought it to a proper shape; he now carries it to the mouth of the furnace, and holds, it in to get a fresh heat (for by this time it is become too stiff to work easily), taking care to turn it round slowly, that it may not alter its figure. The vessel in this stage is delivered to the second, or principal workman, the other two being only assistants; he is seated upon the stool, and lays the blowing-pipe with the glass at its end across its arm, and with his left hand rolls the pipe along the arms, turning the glass and pipe round at the same time; in his right

hand he holds the pliers, whose blades are rub

bed over with a small piece of bees'-wax, and as the glass turns round he presses the blade of the shears against it, following it with the shears as it rolls, at the end or side as occasion requires, until he has brought it to the proper size, which he determines by the compasses, though not materially altering its figure, the first workman kneeling on the ground, and blowing with his mouth at the end of the pipe when directed by his principal. The third workman now produces a small rod, which is dipped into the melting-pot to take up a small piece of metal to serve as cement; the end of this rod he applies to the centre of the glass just opposite the blowing pipe, the principal workman directing it by holding its end between his pliers; the rod by the small piece of glass on its end immediately sticks to the glass vessel, and the third workman draws it away, both workmen turning their rods round, but in contrary directions; this operation forms a short tube on the end. The principal workman then takes the short tube between the blades of a pair of pliers exactly like the others, but which are not covered with bees'-wax, the cold of these pliers instantly cracks the glass all round, and a very slight jerk struck upon the rod breaks it off. A hole is now made in the end of the glass, which is enlarged by the pliers while the glass is turned, until the neck is brought to the proper size and length to fit the brass cup as before described, and the inferior half of the lamp is brought to its shape and size in the same manner. In order to form the upper half, the third workman has in the mean time been preparing a round lump of glass on the end of one of the rods, which he applies hot to the end of the neck, it being guided [. the principal workman, and it immediately holds tight; he then breaks off the other neck by the cold pliers, and thus separates it from the blowing-pipe. The glass is now heated a third time, and brought from the furnace to the principal workman, who enlarges the small orifice at the end by turning it round, and holding the pliers against it until he enlarges it to the right shape: it is now finished, and the third workman takes it to a stool strewed over with small coals; he rests the rod upon the edge of the stool, and with a file files the joint at the bottom neck: it soon breaks off and the lamp falls upon the coals, the distance being so very small as to be in no danger of breaking; a boy now puts the end of a long stick into the open mouth of the glass, and thus carries it to the annealing oven, where it remains some hours; when taken out it must be cooled gradually, and is fit for sale. About forty-six years ago a Bohemian manufacturer first attempted to incrust in glass small figures of a grayish clay. The experiments which he made were in but few instances successful, in consequence of the clay not being adapted to adhere properly to the glass. It was, however, from the Bohemian that the idea was caught by some French manufacturers, who, after having expended a considerable sum in the attempt, at length succeeded in incrusting several medallions of Buonaparte, which were sold at an enormous price. From the extreme difficulty of making these medallions, and their almost invariably breaking while under the operation of cutting, very few were finished, and the manufacture was on the point of being abandoned, when it was fortunately taken up by a French gentleman, who, with a perseverance not less honorable to himself than in its results advantageous to the arts, prosecuted a series of experiments, by which, in a few years, he brought the invention to a state of great improvement. The French have never succeeded, however, in introducing it into articles of any size, such as decanters, jugs, or plates; but have contented themselves with ornamenting smelling-bottles and small trinkets: nor had the invention been applied to heraldry, or any other useful purpose, antecedently to the recent improvements upon the art of this country. England has always been famed for bringing to perfection, and directing to a useful application, the crude inventions of other countries. A patent has recently been taken out by Mr. Pellat of St. Paul's Church-yard for ornamental incrustations, called crystallo ceramie, which bids fair to form an era in the art of glass-making. By the improved process ornaments of any description, arms, cyphers, portraits, and landscapes, of any variety of color, may be introduced into the glass, so as to become perfectly imperishable. The substance of which they are composed is less fusible than glass, incapable of generating air, and at the same time susceptible of contraction or expansion, as, in the course of manufacture, the glass becomes hot or cold. It may previously be formed into any device, or figure, by either moulding or modelling; and

may be painted with metallic colors, which are fixed by exposure to a melting heat. The ornaments are introduced into the body of the glass while hot, by which means the air is effectually excluded, the composition being actually incorporated with the glass. In this way every description of ornamental glass ware may be decorated with embossed white or colored arms or crests. Specimens of these incrustations have been exhibited, not only in decanters and wineglasses, but in lamps, girandoles, chimney-ornaments, plates, and smelling-bottles. Busts and statues on a small scale, caryatides to support lamps or clocks, masks, after the antique, have been introduced with admirable effect. The composition used in the patent incrustation, is of a silvery appearance, which has a superb effect when introduced into richly cut glass. Miniatures, however, may be enamelled upon it, without the colors losing any of their brilliancy; and thus, instead of being painted on the surface of the crystal, may be embodied in it. A most important advantage to be derived from this elegant invention respects the preservation of inscriptions. Casts of medals and coins present no equal security for perpetuating them. The inscription, when once incrusted in a solid block of crystal, like the fly in amber, will effectually resist for ages the destructive action of the atmosphere. Of making plate-glass–The materials of the finest plate-glass are white sand, soda, and lime, to which are added manganese and zaffre, or any other oxide of cobalt for particular coloring purposes. The sand is of the finest and whitest kind, and is previously passed through a wire sieve of moderate closeness into water, where it is well stirred and washed till all dirt and impurity are got rid of. The sharpest grained sand is preferred, and indeed it is found that the grains of moderate size melt with the alkali sooner than either the very fine dust or the larger fragments. The alkali used is always soda, and there seems good reason to prefer this to potash, as glasses made with soda are found to be softer and to flow thinner when hot, and yet to be equally durable when cold. Besides, the neutral salts with the basis of soda, which constitute the glass-gall in this instance, such as the muriate and sulphate of soda, appear to be dissipated more readily by the fire than the corresponding salts of potash. Lime is of considerable use, and adds much to the fusibility of the other materials, supplying in this respect the use of litharge in the flint glass. Too much lime, however, impairs the color and solidity of the glass. The coloring, or rather discoloring, substances used are azure, or cobalt blue, and manganese. The latter is here in the state in which its effect is that of giving a slight red tinge, which mixes with the blue of the cobalt, and the natural yellow of the other materials; and if properly proportioned they neutralize each other so that scarcely any tint remains. Besides these ingredients there is always a great quantity of fragments of glass arising from what is spilt in the casting and the ends cut off in shaping the plates, which are made friable by quenching in water when hot, and used in this state with the fresh materials. Of the above materials the

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