nealing or cooling glaffes. The metal is wrought on two oppofite fides, and on the other two they have their colours, 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, fo that the whole procefs is done in one furnace. Thefe furnaces muft not be of brick, but of hard fandy ftones. In France, they build the outfide of brick; and the inner, to bear the fire, is made of a fort of fullers earth, or tobacco-pipe clay, of which they also make the melting pots. In Britain the pots are made of Sturbridge clay. Mr Blancourt obferves, that the worst and rougheft work in this art is the changing the pots when they are worn out or cracked. In this cafe, the great working hole muft be uncovered; the faulty pot muft be taken out with iron hooks and forks, and a new one must be speedily put in its place, through the flames, by the hands only. From this work, the man guards himself with a garment made of fkins, in the shape of a pantaloon, that covers him all but his eyes, and is made as wet as poffible: the eyes are defended with a proper fort of glass. SECT. V. Of the INSTRUMENTS used in GLASS MAKING. tity of baked clay that ought to be mixed with THE manufactured glass now in use may be diided into 3 general kinds; 1. white tranfparent glafs, 2. coloured glafs, and 3. common green or bottle glass. flint glafs, and the German crystal glass, which are Of the first kind there is a great variety; as the applied to the fame ufes: the glafs for plates, for THE inftruments used in glass making, are, 1. A mirrors, or looking glaffes; the glafs for windows blowing pipe, made of iron, about 24 feet long, and other lights; and the glafs for phials and fmall with a wooden handle. 2. An iron rod to take veflels. And thefe again differ in the fubftances up the glafs after it is blown, and to cut off the employed as fluxes in forming them, as well as in former. 3. Sciffars to cut the glafs when it comes the coarfenefs or fineness of fuch as are used for off from the first hollow iron. 4. Shears to cut their body. The flint and crystal, mirror and best and fhape great glaffes, &c. 5. An iron ladle, window glafs, not only require fuch purity in the with the end of the handle cafed with wood, to fluxes, as may render it practicable to free the take the metal out of the refining pot, to put it glass perfectly from all colour: but, for the fame into the workmen's pots. 6. A fmall iron ladle reafon, either the white Lynn fand, calcired flints, cafed in the fame manner, to fkim the alkaline or white pebbles, should be used. The others do falt that fwims at top. 7. Shovels, one like a peel, not demand the fame nicety in the choice of to take up the great glaffes; another like a fire- the materials; though the fecond kind of window fhovel, to feed the furnace with coals. 8. A hook-glafs, and the best kind of phial, will not be fo ed iron fork, to ftir the matter in the pots. 9. An clear as they ought, if either too brown fand, or iron rake for the fame purpose, and to ftir the frit. impure falts, be fuffered to enter into their com 10. An iron fork, to change or pull the pots out pofition. of the furnace. 11. And laftly the GLASS POTS, Or veffels in which the glafs is melted. Thofe for the white glafs works are made of a tobacco-pipe clay, brought from the Inle of Wight, which is firft well washed, then calcined, and afterwards ground to a fine powder in a mill; which being mixt with water, is then trod with the bare feet till it is of a proper confiftence to mould with the hands into the proper fhape of the veffels. When thefe are thus made, they are afterwards annealed over the furnace. Thofe for the green glafs work are made of the nonfuch, and another fort of clay from Staffordshire; they 10ake thefe fo large as to hold 3 or 4 cwt. of metal. They have also a small fort called piling pots, which they fet upon the larger, and which contain a finer and more nice metal fit for the niceft works. The clay that is ufed for this purpofe should be of the pureft and most refractory kind, and well cleanfed from all fandy, ferruginous, and pyritous inatters; and to this it will be proper to add ground crucibles, white fand, calcined flints duly levigated, or a certain proportion of the fame clay baked, and pounded not very finely. The quan Of coloured glafs there is also a great variety of forts, differing in their colour or other proper ties according to the occafions for which they are wanted. Thefe differences depend on the preparation and management of the artifts by whom they are manufactured. See SECT. XIV. Glafs is alfo diftinguished into 3 principal kinds, by the form or manner of working it ; viz. 1. Round glass, as that of our bottles, vessels, phials, drinking glaffes, &c. See SECT. X. 2. Table or window glass, of which there are divers kinds: as crown glafs, jealous glafs, &c. See SECT. XI. And, ;. Plate glass, or mirror glass. See SECT. XII. SECT. VII. Of the COMPOSITIONS for making BOTTLE and PHIAL GLASS. I. THE Common BOTTLE OF GREEN GLASS is formed of fand of any kind, fluxed by the afhes of burnt wood, or of any parts of vegetables; to which may be added the feoria or clinkers of for ges. When the fofteft fand is ufed, 200 lb. of wood afhes will fuffice for 100 lb. of fand, which are to be ground and mixed together. The com pofition with the clinkers confifts of 170 lb. of wood afhes, 100 lb of fand, and so of clinkers, which are to be ground and mixed together. If the clinkers cannot be ground, they must be broken into small pieces, and mixed with the other matter without grinding. II. PHIAL GLASS is a kind betwixt the flint glass and the bottle glafs. The best kind may be prepared with 120 lb. of white fand, 50 lb. of unpurified pearl afhes, to lb. of common falt, s lb. of arfenic, and 5 oz. of magnefia. The compolition for green or common phial glafs confifts of 120 lb. of the cheapest white fand, 80 lb. of woodafhes well burnt and fifted, 20 lb. of pearl afhes, 15 lb. of common falt, and r lb. of arfenic. SECT. VIII. Of the COMPOSITIONS for making WHITE GLASS and CRYSTAL, TAKE of the whitest tarfò, pounded fmall, and fearced as fine as flour, 200 lb. of the falt of polverine 130 lb. mix them, and put them into the calcar, firft heating it. For an hour keep a moderate fire, and keep ftirring the materials with a proper rake, that they may incorporate and calcine together; then increate the fire for 5 hours; after which take out the matter; which being now fufficiently calcined, is called FRIT, or BOLLITO. See these articles. From the calcar put the frit in a dry place, and cover it up from the duft for 3 or 4 months. To make the white glass or crystal, take of the crystal frit, fet it in pots in the furnace, adding to it a due quantity of manganefe: when the two are fused, caft the flour into fair water, to clear it of the falt called fandiver; which would otherwife make the crystal obfcure and cloudy. This lotion must be repeated again and again, as often as needful till the crystal be fully purged; or the fcum may be taken off by proper ladles. Then fet it to boil 4, 5, or 6 days; which done, fee whether it have manganefe enough; and if it be yet greenish, add more by little and little at a time, taking care not to overdose it, because the manganele inclines it to a blackish hue. Then let the metal clarify, till it becomes of a clear and shining colour; which done, it is fit to be blown or formed into veffels. FLINT GLASS, as it is called by us, is of the fame general kind with that which in other places is called CRYSTAL glafs. It has this name from being originally made with calcined flints, before the ufe of the white fand was understood; and retains the name, though no flints are now ufed in the compofition of it. This flint glafs differs from the other, in having lead for its flux, and white fand for its body; whereas the fluxes used for the cryftal glafs are falts or arfenic, and the body confifts of calcined flints or white river pebbles, tarfo, or fuch ftones. To the white fand and lead a proper proportion of nitre is added, and a small quantity of magnefia, or manganefe. In fome works they ufe a proportional quantity of arfenic to aid the fluxing ingredients. The most perfect kind of flint glafs may be made, by fufing with a very strong fire 120 lb. of the white fand, 50 lb. of red lead, 40 lb. of the beft pearl athes, 20 lb. of nitre, and 5 oz. of magRefia. Another compofition of flint glafs, which is faid to come nearer to the kind now made, is the following: 120 lb. of fand, 54 lb. of the best pearl ashes, 36 lb. of red-lead, 12 lb. of nitre, and 6 oz. of magnefia. To either of these a pound or two of arfenic may be added, to increase the flux of the compofition. A cheaper compofition may be made with 120. lb. of white fand, 35 lb. of the best pearl-athes, 40 lb. of red-lead, 13 lb. of nitre, 6 lb. of arfenic, and 4 oz. of magnelia; of inftead of the arfenic may be fubftituted rs lb. of common falt; but this will be more brittle. The cheapest compofition for the worst kind of flint glass confifts of 120 lb. of white fand, 36 lb. of red lead, 20 lb. of the beft pearl-afhes, to lb. of nitre, 15 lb. of common tal is made of 120 lb. of calcined flints or white falt, and 6 lb. of arfenic. The beft German cryffand, 70 lb. of the beft pearl-afhes, to lb. of faltpetre, lb. of arfenic, and s oz. of magnefia. And a cheaper compofition is formed of 120 lb. of calcined flints or white fand, 46 lb. of pearlafhes, 7 lb. of nitre, 6 lb. of arfenic, and 5 oz. of magnefia. A glass much harder than any prepared in the common way, may be made by means of borax, thus: Take 4 oz. of borax, and 1 of fine fand; reduce both to a fubtile powder, and melt them together in a large close crucible set in a wind furnace, keeping up a strong fire for half an hour; then take out the crucible, and when cold break it, and there will be found at the bottom a pure hard glafs, capable of cutting common glafs like a diamond. "This experiment, duly varied, fays Dr Shaw, may lead to several use. ful improvements in making glass enamels, and factitious gems; and fhows an expeditious method of making glafs, without any fixed alkali, which has been generally thought an effential ingredient in glafs; and perhaps calcined crystal, or other fubftances, added to this falt instead of fand might make a glass approaching to the nature of diamond. a SECT. IX. Of the COMPOSITIONS for making PLATE or MIRROR GLASS. THE materials of which this glass is made are much the fame as those of other works of glafs, viz. an alkaline falt and fand. The falt, however, fhould not be that extracted from the ashes of the Syrian kali, but that from BARILLA, growing about Alicant in Spain. It is very rare that we have the barilla pure: the Spaniards in burning the herb mix another herb with it, which alters its quality; or add fand to it to increase the weight, which is eafily discovered, if the addition be only made after the boiling of the afhes, but next to impoffible if made in the boiling. From this adulteration threads and other defects in plate glafs arife, To prepare the falt, clean it well of all foreign matters; pound or grind it with a kind of mill, and finally fift it pretty fine. Pearl ashes properly purified, will furnish the alkaline salt requifite for this purpose; but it will be neceffary to add borax or common falt, to facilitate the fufion, and prevent the glass from ftiffening in that degree of heat in which it is to be wrought into plates. To purify the pearl-afhes, diffolve them in fou times their weight of boiling water, in a pot of 0002 caft caft iron, kept clean from ruft. Let the folution be removed into a clean tub, and remain there 24 hours or longer. Having decanted the clear part of the fluid from the fediment, put it again in the iron pot, and evaporate the water till the falts are left perfectly dry. Preferve them in ftone jars, well fecured from air and moisture. Pearl alhes may alfo be purified in the highest degree, fo as to be proper for the manufacture of the moft tranfparent glafs, by pulverizing 3 lb. of the best kind with 6 oz. of faltpetre in a glafs or marble mortar, till they are well mixed; and then put ting part of the mixture into a large crucible, and expofing it in a furnace to a ftrong heat. When this is red-hot, throw in the reft gradual ly; and when the whole is red-hot, pour it out on a moistened ftone or marble, and put it into an earthen or clean iron pot, with 10 pints of water, heat it over the fire till the falts be en tirely melted; let it then ftand to cool, and filter it through paper, in a pewter cullender. When filtered, put the fluid again into the pot, and eva; porate the falt to drynefs, which will then be as white as fnow. As to the fand, fift and wash it till the water come off very clear; and when it is well dried, mix it with the falt, paffing the mixture through another fieve. This done, lay them in the an nealing furnace for about two hours; in which time the matter becomes very light and white; In this ftate they are called FRIT, and are laid up in a dry clean place, for at least a year, to give them time to incorporate. When they employ this frit, they lay it for fome hours in the furnace, adding to fome the fragments of old and ill made glaffes; taking care first to calcine them by heating them red-hot in the furnace, and thus cafting them into cold water. To the mixture muft likewife be added manganefe, to promote the fufion and purification. The beft compofition for looking-glafs plates confifts of 60 lb. of white fand cleanfed, 25 lb. of purified pearl-afhes, 15 lb. of faltpetre, and lb. of borax. If a yellow tinge fhould affect the glafs, a small proportion of magnesia, mixed with an equal quantity of arfenic, fhould be added. An ounce of the magnèfia may be first tried; and if this proves infufficient, the quantity fhould be in creafed. A cheaper compofition confifts of 60 lb, of white fand, 20 lb. of pearl-ashes, 10 lb. of common falt, 7 lb. of nitre, 2 lb. of arfenic, and 1 lb. of borax, 1 4 furnace is heated, and the pots of frit are at this fet in the furnace; two other fmaller holes, called boccarellas, ferve to lade or take out the melted metal, at the end of an iron, to work the glass. At the other holes they put in pots of fufible ingredients, to be prepared, and at laft emptied into the lading pot. There are 6 pots in each furnace, all made of tobacco pipe clay, proper to fuftain not only the heat of the fire, but also the effect of the polverine, which penetrates every thing elfe. There are only two of these pots used for working: the rest serve to prepare the matter for them. The fire is made and kept up with dry hard wood, caft in without intermiffion at fix apertures. When the matter in the pots is fufficiently vitrified, the workman proceeds to blow or fashion it. For this purpose he dips his blowing pipe into the melting pot; and by turning it about, the metal fticks to the iron more firmly thau turpentine. This he repeats 4 times, at each time rolling the end of his inftrument with the hot metal thereon, on a piece of plate iron; over which is a vellel of water which helps to cool, and fo to confolidate and to difpofe that matter, to bind more firmly with what is to be next taken out of the melting pot. But after he has dipt a 4th time, and perceives there is metal enough on the pipe, be claps his mouth immediately to the other end of it, and blows gently through the iron tube till the metal lengthens like a bladder about a foot. Then he rolls it on a marble ftone a little while to polith it; and blows a second time, by which be brings it to the shape of a globe of about 18 or 20 inches diameter. Every time he blows into the pipe, he removes it quickly to his check; otherwife he would be in danger, by often blowing, of drawing the flame into his mouth: and this globe may be flattened by returning it to the fire; and brought into any form by ftamp irons, which are always ready.. When the glafs is thus blown, it is cut off at the collet or neck; which is the narrow part that ftuck to the iron. The method is this; the pipe is refted on an iron bar, close by the collet; then a drop of cold water being laid on the collet, it will crack about a quarter of an inch, which, with a flight blow or cut of the fhears, will immediately feparate the collet. The operator then dips the iron rod into the melting pot, by which he extracts as much metal as ferves to attract the glafs he has made, to which he fixes this rod at the botThe matter of which the glaffes are made, attom of his work, oppofite to the opening made the famous manufacture of Sr GOBIN in France, is a compofition of folder and of a very white fand, carefully cleaned of all heterogeneous bodies, afterwards washed feveral times, and dried fo as to be pulverized in a mill, confifting of many pestles, which are moved by horfes. When this is done, the fand is fifted through filk freves and dried. The matter thus far prepared is equally fit for plate-glafs, to be formed either by blowing or by cafting. See SECT. XII. SECT. X. METHOD of WORKING OF BLOWING ROUND GLASS. The working furnace bas fix boccas or aper. tures; at one of these called the great bocca, the by the breaking of the collet. In this position the glass is carried to the great bocca or mouth of the oven, to be heated and fcalded: by which means it is again put into fuch a foft ftate, that, by the help of an iron inftru ment, it can be pierced, opened, and widened, without breaking. But the veffel is not finished till it is returned to the great bocca; where being again heated thoroughly, and turned quickly about with a circular motion, it will open to any fize, by remain, they are cut off with the fhears; for till means of the heat and motion. If any fuperfluities the glass is cool, it remains in a foft flexible state, It is therefore taken from the bocca, and carried to an earthen bench covered with brands, or coals extinguished, |