Εικόνες σελίδας

cleanse or purify, admits of various significations, and evidently, from the context, means some precious stone. It is said that the Egyptians were taught the art of glass-making by Hermes. Among the Greek writers Dr. Falconer contends that Herodotus uses the word taxoc, with this signification; but this is very dubious; the historian more probably means the natural crystal, or some transparent natural stone, capable of forming the transparent case he is describing. Aristophanes, Aristotle, Alexander, Aphrodiscus, Lucretius, and St. John the divine, put it out of all doubt that glass was used in their days. Pliny relates, that it was first discovered accidentally in Syria, at the mouth of the river Belus, by certain merchants driven thither by a storm; who being obliged to dress their victuals by making a fire on the shore, where there was great plenty of the herb kali; of the ashes of that plant the salts mixed and incorporated with the sand, or stones fit for vitrification, and thus produced glass; and that, this accident being known, the people of Sidon in the neighbourhood first attempted to form it artificially, and brought glass into use. Be this as it may, the first glass-houses mentioned in history were erected in Tyre, where the only staple of the manufacture was for many ages; the sand which lay on the shore for about half a mile round the mouth of the Belus being peculiarly adapted to the making of glass; and the wide range of the Tyrian commerce af. fording an ample channel for the productions of the furnace. The first time we hear of glass made among the Romans was in the reign of Tiberius, when Pliny relates that an artist had his house demolished for making glass malleable, or rather flexible; though Petronius Arbiter and others assure us, that the emperor ordered the artist to be beheaded for his invention. It is certain that a plate of glass was found at Herculaneum, which was destroyed, A. D. 80; and that glass vessels were made at Rome under Nero. The earliest mention made of glass windows is by Lactantius, in the third century. Before the conquest of Britain by the Romans, glass-houses had been erected in this island, as well as in Gaul, Spain, and Italy. Hence, in many parts of the country, are to be found amulets of glass, having a narrow perforation and thick rim, denominated by the remaining Britons gleineu naid-reedh, or glass adders, and which were probably in former times used as amulets by the druids. See ANGUINUM Ovum. It can scarcely be doubted that the Britons were sufficiently well versed in the manufacture of glass, to form out of it more useful instruments than glass beads. History indeed assures us, that they manufactured a considerable quantity of glass vessels. These, like their amulets, were green, blue, yellow, or black, and many of them curiously streaked with other colors. The process of the manufacture was nearly the same with that of the Gauls or Spaniards. The sand of the shores, being reduced to a sufficient degree of fineness by art, was mixed with three-fourths of its weight of nitre, and both were melted

together. The metal was then poured into other vessels, where it was left to harden into a mass; and afterwards replaced in the furnace, where it became transparent in the boiling; it was now figured by blowing, or modelling in the lath, into such vessels as they wanted. According to Bede, artificers, skilled in making glass for windows, were brought over into England in 674, by abbot Benedict, and were employed in glazing the church and monastery of Weremouth. According to others, they were first brought over by Wilfrid, bishop of Worcester, about the same time. Till this time the art of making glass for windows was unknown in Britain; and sash windows did not begin; to be common before 1180. Italy had them first; next France, whence they came to England. Venice, for many years, excelled all Europe in the fineness of its glasses; in the thirteenth century, the Venetians were the only people that had the secret of making crystal looking-glasses. The glass manufacture of England was first established in 1557: the finer sort being made at Crutched Friars, London; the fine flint glass, little inferior to that of Venice, in the Savoy. This manufacture appears to have been much improved in 1633, when it was carried on with pit coal instead of wood; and a monopoly was granted to Sir Robert Mansell, who was allowed to import the fine Venetian flint glasses for drinking. But in the reign of William III. these were made at home with considerable skill. The first plate glass, for looking glasses and coach windows, was made in 1673 at Lambeth, under the patronage of the duke of Buckingham; who introduced it by the means of Venetian artists, and with amazing success. So that, within the last century and half, the French and English have not only equalled but even excelled the Venetians, and are now no longer . from abroad. The French made a considerable improvement in the art of glass, by the method of casting very large plates, till then unknown. This was imitated in Lancashire in 1773, and is now very flourishing. Of the vitrification of glass generally—Pure substances vitrify with difficulty, and the glass which proceeds from them is in general dry and very brittle. But the same substances mixed, enter more easily into fusion. Alumine and lime, although unvitrifiable separately, are easily reduced into glass when mixed together. The alkalies facilitate the fusion and vitrification of all the earthy principles. On account of this property, these salts are employed for forming the base of the composition of glass manufactured for our use. Besides the degree of fusibility which the alkalies communicate to the earthy substances, they give to the glass which proceeds from their mixture with the earths a liability which admits of its being wrought, §. extended, and even hammered while it is warm and soft. The manufactories where glass is made, are called glass-works. The compositions, the working, and the furnaces, vary in the different manufactories, according to the nature of the glass made in them: hence the various denominations of bottle-glass, flint-glass, plate-glass crystal-glass, &c. But, whatever may be the nature of the glass to be made, there are certain principles essentially dependent upon science which are applicable to all glass-works, and according to which all the operations are directed. These general principles have for their object every thing relating to the manufacture of the pots or crucibles, to the composition of the substances, to the construction of the furnace, to the management of the fire, and to the manner of working the glass. We shall glance at each of these subjects in succession. Of the manufacture of crucibles, or glass pots.—Good crucibles ensure the success of a glass-work. This truth can only be felt by those who have appreciated the loss occasioned by pots which break or melt, the loss of time, and the difficulty of replacing them. Clay forms the basis of glass-house pots. But as the qualities of clays are very variable, because they are naturally and constantly mixed in various proportions, with lime, silex, iron, and magnesia, which renders them more or less fusible, the clay must be picked before employing it. The qualities of a good clay are as follow: 1st. It must not vitrify upon an exposure of several days in the hottest place of the furnace. 2nd. It must preserve its form without sinking down, or becoming soft. 3rd. It must be wrought and moulded easily. 4th. It must undergo the action of the fire without contracting, or cracking. 5th. Good clay assumes, upon being fired, a very great hardness and compactness. When we have ascertained all these qualities in the clay, it must be still picked, in order to separate from it every thing foreign or prejudicial. To this effect it must be carefully picked, in order to take out the pyrites and all the small colored veins, which render it fusible: we may content ourselves with raking together the pieces tinged with ochre, and separating all the coloring principle from them. After having taken away every visible impurity, the clay must be diluted and soaked in water; it is afterwards passed through sieves, in order to separate the coarse, weighty, and insoluble bodies from it. Sand, quartz, or mica, do not sensibly injure the qualities of clay, particularly if they are in small quantity: but mixtures of calcareous earths, plaster, pyrites, and metallic oxides, render clay improper for glass-house pots, as it is material to give to the sides of a crucible such a thickness only as to render it capable of resisting the effects of the substance it contains, and the shocks it receives in the work. M. Loysel has suggested that we should calculate the tenacity of the clay, by forming small sticks of it in the form of parallelopipedons, which he dries at a temperature of 25° of Reaumur, and one of the extremities of which he reduces to a diameter of six lines. He fastens this extremity in a cubical cavity; and at the distance of eighteen lines he suspends, from one of these sticks, the saucer of a pair of scales, in which he places weights until they produce a fracture in the stick. He observed that good clay, employed for crucibles of three feet diameter by three feet six lines thick, did not break, ex

cept with a weight of 56 oz.; and that of a furnace of fusion, of eight feet diameter, by a weight of 24 oz. But clay, employed by itself, contracts too much, and it is mixed for forming the composition of pots with the broken pieces of crucibles, ground, and well cleared of all vitrified matter, or with clay strongly fired. Great care must be taken not to employ sand in forming pots, because the alkali employed in making the glass would act upon the sand, dissolve it, and speedily destroy the crucibles. After having prepared the clay well, it is mixed with the cement formed of ground fragments of crucibles, and a paste is made with it which has such a consistence that a leaden bullet of 4 oz. weight may sink into it completely upon falling from a height taken between sixty-six and eighty-three inches. This paste must be dressed with the greatest care in a proper place, and out of the way of all dust, and the mixture of every foreign substance. When the paste is thus prepared, either the one or the other of the two following processes may be employed for making the crucible. 1st. In some glass-houses they have a wooden mould, furnished in the inside with a strong and wellstretched cloth. Rolls of paste are applied to the interior surface of this cloth, and the frame of the crucible is successively raised, by gradually diminishing its thickness from the bottom to the upper edge. 2nd. In other glass-houses the workman has a round piece of wood, a little broader than the crucible is to be, and he raises with his hand, and without a mould, his crucible upon this kind of foundation. This last method is preferable to the former, because the workman can work his paste at all places, and he leaves no cracks nor crevices in the body of the crucible, and he can join perfectly and uniformly all the parts. This process is particularly necessary in the bottle glass-houses, because this composition corrodes the crucibles more than any other. When the pots are manufactured they are allowed to dry in the shade, at a temperature of 10° or 15° of Reaumur's thermometer. We should equally dread a too strong heat, which might crack the pot; or a too sharp cold, which might freeze it, dampness and currents of air should be also carefully avoided: the apartment which serves as the drying-place should be shut, and very little frequented. When the pots begin to be dry they are enclosed in a close place, where the heat is constantly kept up to 25° or 30° of Reaumur. From this they are brought out to be put in use. For this purpose they are exposed by degrees to a heat which produces redness, and in this state they may be placed upon their seat in the furnace. Prudence requires that they should not be charged with any composition after they are placed upon the furnace until they have undergone the strongest possible heat for twenty-four hours. Of the construction of glass-house furnaces— The paste intended for making the bricks of a glass-house furnace is prepared by mixing crude with fired clay, or rather with broken piecesofcrucibles: white infusible quartz, or a very refractory sand, are also employed instead of fired clay. In order to pound the pieces of quartz more completely, they are made red-hot, and then thrown into water. This operation, as is well-known, renders them pulverulent, without hurting their refractory quality. Bricks are sometimes used which have not been fired; these are merely dried in the air to such a degree that a leaden bullet, falling from a height of from twenty-five to forty-five feet, only sinks half its bulk into the brick. The furnace of a glass-house is always erected in the middle of very spacious premises, in order that the working and the surface of it may be easy. The draught of the furnace is effected by means of four currents of air, which enter the hall at separate apertures, and unite at right angles at the grate of the fire. The interior form of the furnace is almost always that of a square, or of a rectangular parallelogram, the broadest sides of which are occupied by the pots, which are supported and fixed on trevets or shelves. The interval between these shelves, or trevets, forms the grate upon which the combustibles are placed. The fire is fed by apertures made in the sides: the pots are charged and emptied by means of openings immediatel above them, and which exactly correspond wit them, in order that the business may be more easily conducted. The furnace is surmounted or terminated by an arch, which rests upon the two longest sides, and which is full of holes, in order to establish a proper draught, and to give a passage to the flame, which also heats other arches placed before these angles, or above the vault. Of the substances employed in the composition of glass–Silex and the alkalies form the base of glass in all countries: the other ingredients are, properly speaking, only accessary for facilitating the flux and purifying the glass, or for giving it any peculiar quality. The purest silices and alkalies form the clearest glass, and it is this composition which forms the basis of all the operations of glass-houses. But silex and alkali exist no where pure; it is only by troublesome, difficult, and expensive processes that we can bring them to this degree of purity. These substances are therefore very generally employed in the state in which nature and commerce afford them. Attention must be paid, however, among the varieties which these two substances present, to choose such as experience has shown to give constantly the production we are desirous of obtaining. In some delicate works such as the making of fine crystal or plate-glass, the alkali of commerce is purified, in order to clear it of all foreign bodies. In general, white sand is the purest, but it is also the most refractory; the colored sands fuse much more easily. Alicant soda holds the first place among the alkalies of commerce. It is therefore most employed in the delicate operations of the glass works. Sicilian ashes, the salicornia, and sea-wrack, are employed in the manufacture of all the common clear glass. Potassa and salt are also well adapted for vitrification: the latter is employed in most of the manufactories of drinking-glasses and crystal-glass, as it is called. In France the ashes of our fires, melted with sand, is the most general composition of bottle-glass. When the sand is very fusible, lixiviated ashes may be employed. I have seen, says M. Chaptal, most Vol. X.

excellent bottle-glass formed with lixiviated ashes and river-sand, mixed with equal parts of quartz and rubbish of lava. The salts contained in the alkalies enter into fusion, and swim upon the surface of the metal (as the workmen call it) in the state ef a very fluid liquid, which must be carefully taken off with a ladle or skimmer before beginning to work the glass. This precaution is only necessary when sodas are employed highly charged with marine salt. The glass-works where these kinds of soda were used, made a considerable trade of this salt, which was sold by the name of glass-house salt, when the gabelle or salt-tax rose to such an enormous height in France. Glass-house salt is also known by the name of gall of glass, or sandiver; and when the matter is not well melted, or when all the marine salt is not evaporated, it is found dispersed through the glass in small grains, which injure much the beauty and solidity of the article. When we wish to purify soda for delicate operations, it is dissolved in water, in order to separate by a previous operation every thing that may be insoluble; it is afterwards evaporated, and concentrated to 40° of Baume's arcometer, in order to precipitate the foreign salts, which crystallise; the remaining liquor is afterwards concentrated to dryness, and by this means we obtain a very pure salt of soda. We may even obtain it in crystals, by stopping the evaporation at the degree of a syrupy consistence. The proportions of the substances which form the composition of glass vary according to the nature of the sand, the purity of the alkalies, the quality of the glass, and the degree of heat in the furnace. Experience alone must prescribe and determine the most proper composition: the more fusible the sand is, the less alkali it requires; the purer the alkali, the greater is the quantity of sand which is necessary in the composition. In order to facilitate the fusion of the compounds, and to give the glass more ductility, more weight, and less hardness, oxide of lead is added to the composition, in variable proportions, according to the object in view. Minium, or red-lead, is always preferred for this purpose in the manufactories of crystal-glass. The oxide of manganese is also used, by the name of glass-maker's soap, in order to clear the glass of all coloring matter. Its effect must probably be chiefly ascribed to the facility with which it gives up its oxygen, which combines with the coloring principles, and destroys them. Too much red-lead makes the glass yellow; this defect may be corrected by applying a little oxide of cobalt, which, in its turn, will produce a blue color, if in excess. Too much manganese gives it a violet color, and forms streaks, or violet-colored ribands, in the thick parts of the glass. This fault may be corrected by throwing a combustible body into the melted mass. There are circumstances where a tried composition attains a proper degree of fusion with great difficulty; this may proceed from the draught of the furnace being interrupted, or when the fire is ill managed; in this case, borax, or arsenic, must be resorted to for restoring the fusion. The latter substance is held in the bottom of the pots, until it has evaporated in fumes; it spreads through the whole mass, agitates it, and hastens the flux of it. Arsenic serves in particular for destroying the green color of glass, besides the advantage it has of facilitating the flux. The glass is colored with the metallic oxides; cobalt makes blue; manganese, violet; glass of antimony, yellow; precipitate of Cassius, purple; chrome, green, &c. Various colors may be obtained by the mixture of these oxides; and we unay obtain all the shades we desire. Muriate of soda and sulphate of soda may be employed, and at times with advantage, in glass-making. A casting is readily obtained of very fine glass, having, when about three or four lines in thickness, a very slight green tinge. Its composition is as follows:—decrepitated muriate of soda 100 parts; slaked lime 100; sand 140; clippings of glass, of the same quality, from 50 parts to 200. Sulphate of soda likewise offers a great economy in its employment. The results are very satisfactory. The glasses made with this salt were of a very fine quality. The following is the composition:—dry sulphate of soda 100 parts; slaked lime 12; powdered charcoal 19; sand 225; broken glass from 50 to 200. These proportions give a rich colored glass, which may be employed with advantage in glass-houses, where a fine quality is sought after. The following is the second way of operating with sulphate of soda; the proportions may be as follows:—dry sulphate of soda 100 parts; slaked lime 266; sand 500; broken glass from 50 to 200. According to this process, it is obviously easy to operate in a regular manner, and to avoid expensive trials in the manufacture.—Annales de l'Industrie Nationale. According to M. Achard, equal parts of lime, magnesia, and silica, melt into a greenish colored glass, hard enough to strike fire with steel. When the magnesia exceeds either of the other two ingredients, the mixture is infusible; when the silica exceeds, the only fusible proportions were, 3 silica, 2 lime, 1 magnesia; and, when the lime is in excess, the mixture usually melts in a strong heat. With mixtures of lime, alumina, and silica, a fusible compound is usually obtained, when the lime predominates. The only refractory proportions were,

Lime . - - 2 3 Silica . - - 1. 1 Alumina - - 2 2

Excess of silica gives a glass or porcelain, but excess of alumina will not furnish a glass.

When, in mixtures of magnesia, silica, and alumina, the first is in excess, no fusion takes Fo at 150°; when the second exceeds, a porceain may be formed; and 3 parts of silica, 2 magnesia, and 1 alumina, form a glass. From Achard's experiments it would appear, that a glass may be produced by exposing to a strong heat equal parts of alumina, silica, lime, and magnesia. Other proportions gave fusible mixtures, provided the silica was in excess.

M. Westrumb is said to have found, that the salts of potassa and soda, deprived of their water of crystallisation, answer as well as the pure alkali for the manufacture of glass. In order to make an excellent glass, 24 parts of sulphate

of soda are thoroughly dried, and mixed with 8 parts of powdered charcoal, and 16 of good white sand. The mixture must be calcined in the drying oven, until the sulphate is dissipated, and is then put into the pots for fusion.—Anmales Gen. de Phys. de Brurelles, May 1820. Of the flur of the substances forming the composition of glass.—The flux of the substances embraces two principal operations; first the fritte; second the fusion. If we throw into the crucible the substance which forms the composition, without having prepared it by a previous strong calcination, the crucibles would be destroyed in a short time, in consequence of the water which would be disengaged on the first impression of the fire; the flux would be almost impossible, in consequence of the greater fusibility of the alkali, which would come to the surface; the glass would be cokered, and the paste itself would experience a swelling which would drive it over the crucible. In order to obviate all these inconveniences, the substances undergo the fritte, in all the glass-works, before put into the pots to be melted. The fritte is conducted on the substances either separately or in their state of mixture and composition. The second method is preferable, for the reasons above given. The fritte is executed in furnaces made in the glass-house; and which very often communicate with the melting furnace, from which they receive the heat by apertures made at the base of the great arch, and at the angles. These places are then called fritte arches. The substances are fritted some time, keeping them red-hot, and by this means they often receive a commencement of a pasty fusion, which unites the parts of the composition so as to form one mass. The manufacturers of bottleglass, already mentioned, give the form of bowls to their composition, in order to roast it more completely. Others throw the composition, when well mixed, upon the bottom of the arch, taking care to strew it very thinly, in order that the calcination may act equally upon all the parts. Previous to putting the composition into the melting-pots, a new o: is given to the fire, and it is stirred three or four hours before charging them. The pots are charged at two and even three times; a fresh quantity of composition is not added until the first quantity is melted. As soon as the pot is filled, the fire is carefully kept up, for a longer or shorter time, according to the fusibility of the composition and the draught of the furnace. Ten or twelve hours are sufficient to melt the whole composition; but, although it is well melted, it is not yet fit for working. It must be allowed to settle, to clear itself of the numberless bubbles which are dispersed through the paste; and this effect can only be produced by keeping the composition at a very liquid fusion for some hours. This operation is called fining. When the glass is thus fined down, or rendered fit for working with, the heat of the fire is allowed gradually to diminish by adding no more coals to it. The vitreous mass then assumes a little more consistency, which facilitates the work. Of working the glass in glass-houses.—The working of glass is very simple; but, notwithstanding this, it requires a great deal of practice, and no one can expect to become a good artist in this branch of the business, if he has not acquired the art early in life. Every thing respecting the working of the glass may be reduced to the act of blowing or running it. In blowing the glass, an iron tube about five feet long is used; with this the workman takes out of the pot the quantity of glass necessary for his operation : the air, which he exhales from his lungs through the hollow of the tube into the mass of glass he has taken up, distends it; he afterwards gives this mass, while it is distending, the form and dimensions he wishes. Compasses, scissors, and other iron tools, are employed to shape, pare, or dilate the glass. Care is taken to present it to the furnace as soon as it begins to cool; when again heated, and it begins to melt, it is withdrawn, in order to bestow additional labor upon it. The softness of glass, when it is made red-hot, forms such a contrast to its fragility when it is cold, that it would be difficult to conceive how easily it may be kneaded, soldered, and distended, if we did not see it actually done before our eyes. Much has been sqid of the malleability of glass; researches have been made in order to recover this important art, which it was thought the ancients possessed; and people have been unwilling to allow that there is no metal more ductile or more malleable than glass, when red-hot; or that this art, supposed to exist among the ancients, is practised among the moderns every day in our glasshouses. Plate-glass is formed by pouring melted glass upon a copper table, the surface of which is very flat, and by passing a level above the melted matter, in order to give the plate a uniform thickness. This operation is very similar to that by which metallic tablets are formed, by throwing melted metal upon sand. In order that the glass may be less brittle, it is necessary that it should be cooled very slowly: this last operation is called annealing. In the large manufactories of bottle-glass, the glass is annealed in furnaces made in the angles of the room where the melting furnace is: these furnaces are red-hot when the glass is deposited in them, and, as soon as they are filled with the glass articles, the apertures are closed, and the heat allowed to subside of itself. In small glass-houses, the annealing furnace is generally Placed upon the melting-furnace, or at one side of it, so as to be heated by the current of flame which escapes from the furnace; this is merely, properly speaking, the commencement of a very wide flue, and which insensibly diminishes in width the further it is removed from the fire; so that the glass, deposited at its base, gradually cools as it is drawn towards the extremity. The glass is annealed very imperfectly in this manner, because it cools too quickly. Of the combustibles employed in glass-works.Two kinds of combustibles are used in glassworks; wood and coals. The employment of the latter is very advantageous, but it colors the glass by producing a fuliginous matter which is deposited upon the melted mass, and tinges it of a yellow hue. When we wish to make a clear

or crystal glass, therefore, we must take the precaution of covering the pots, to which only one aperture must be left, corresponding with the working hole; this is called working with covered pots. When wood is employed, it must be carefully dried; the flux in this case is easier, and the work expedited. Elm, beech, and oak, are the three best woods for a melting-furnace. The resinous woods give out too much smoke. It requires an active and intelligent person to manage the fire of a glass-house; care must be taken neither to choke it with too much fuel, nor to let the heat fall off. It must be fed by renewing the fuel in small quantities at a time, and at short intervals. The weight of clear glass to that of water, is :: 23 : 10. That of argil and aikali :: 25: 10. That of lime and alkali :: 27 : 10. The metallic oxides add to its gravity. Such is very much the practice in France. We shall now detail the processes adopted in our own country, and describe the materials made use of in the several manufactures. It will have been observed that glass contains, invariably, two essential ingredients, silex, and an alkali; these are the only things necessary; these, as we have seen, were the only substances from which glass was made accidentally on the shores of the river Belus; the sand existed on the spot, and the saline substance was the substance in contact with the sand, and made use of as supports to the kettles in which the provisions were to be dressed. The fire, rendered fierce by being exposed to the open air, soon united the sand and the saline substances in fusion, and produced that glass which was the object of so fortunate and important a discovery. Though sand and a saline substance are all that are absolutely necessary in the manufacture of glass, yet several other substances are likewise made use of for particular purposes, among which may be particularly noticed, lime, in the form of chalk, that is, a combination of lime and carbonic acid, or what is chemically denominated, a carbonate of lime, borax, the oxides of lead, manganese, arsenic, and nitre. A brief account of these will be necessary to render this article complete: though they are treated of alphabetically under their respective heads. Silex may be found in almost all parts of the known world ; but of different kinds, and of various degrees of purity, and such will be selected in the manufacture as is adapted to the nature and fineness of the glass required. The siliceous material generally used in this country is sea-sand, which it is well known consists of minute rounded grains of quartz, which are sufficiently small to be used without any other preparation than that of washing. Sand, well adapted for the manufacture of glass, is found on the coast of Norfolk, near Lynn, and likewise on the western shores of the Isle of Wight. Common black gun-flints afford a very pure kind of silex, which before they are used must be heated red-hot, and instantly quenched in cold water. The heat whitens the flints, and the water splits them in every possible direction. after which they may be ground without disficulty, in milis constructed for this kind of work. This ground flint is chiefly confined o: inanu2

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