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The Salt, however, it is to be obferv'd, Jhould not be that extracted from Wolverine, or the Afhes of the Syrian Kali; but that from Saritlia, or the Arties of a Plant of that Name, of the Genus of Kalies, but growing about Alioant in Spain. — 'Tis very rare that we can have the Barillia pure; the Spaniards, in burning the Herb, make a Practice of mixing another Herb along with it, which alters its Quality; or of adding Sand to it, to increafe the Weight; Which is eafiiy difcover'd, if -the Addition be only made after the boiling of the Arties, but next to impoflible, if made in the boiling. It is from this Adulteration that thofe Threads and other Defecls in 'Plate Glafs arife. — To prepare the Salt, they clean it well of all foreign Matters; pound or grind it with a kind of Mill, and fiftjt pretty fine.
As to the Sand, 'tis to be lifted, and wafh'd, till fuch time as the Water come off very clear; and when it is well dried again, they mix it wirh the Salt, parting the Mixture thro' another Sieve. This done, they lay them in the an- nealing Furnace for about two Hours; in which time the Matter becomes very light, and white : In which State they are called Frit, or Fritta, and are to be laid up in a dry, clean Place, to give them time to incorporate, for at leaft a Tear. See Frit.
When they would employ this Frit, they lay it for fome Hours in the Furnace, adding to fome, the Fragments, or Shards of old, and ill made Glaffes; taking care firft to cal- cine the Shards, by heating them red hot in the Furnace, and thus carting them into cold Water. To the Mixture mufl likewife be added Manganefe, to promote theFufion, and Purification.
The Matter thus far prepared, is equally fit for Plate- Glafs to be form'd either by Blowing, or Catting. —
Method of Mo-wing Looiung-Glass-Pdates.
The Work-houfes, Furnaces, &c. ufed in the making of this kind of 'Plate Glafs, are the fame as thofe in the fol- lowing Article, to which the Reader is refer'd.
The Melting-pots, wherein the Materials to be blown, are fufed, are 58 Inches in Diameter, and 3 5 high. After thofe Materials are vitrified by the Heat of the^Fire; and the Glajs is fufficiently refined; the Mafler Workman dips in his Blowing-iron, once, and again, till he has got Matter enough thereon.
This done, he mounts on a kind of Block, or Stool, five !Foot high, to be more at liberty to ballance it, as it lengthens in the blowing. If the Work be too heavy for the Work- man to fuftain on his Blow»g-iron, two or more Attendants aflift him, by holding Pieces of Wood under the Glafs, in proportion as it ftretches, for fear it fhould fall off the Iron by its own Weight.
When, after feveral repeated Heatings, and Blowings, the Glafs is at length brought to the Compafs, proper for its Thicknefs, and the Quantity of Metal taken out; they cut it off with Forces, at the Extremity oppofite to the Iron, in order to point it with the 'Pointil.
The Pointil is a long, firm Piece oflron, having a Piece going a-crofs one of its Ends, in manner of a T: To point the Glafs, they plunge the Head of the T into the Melting- pot; and with the Liquid Glafs flicking thereto, theyfaften it to the Extremity of the Glafs before cut off. When it is fufficiently faften'd, they feparate the other Extremity of the Glafs from the Blowing-iron; and infiead thereof make ufe of the Pointil to carry it to the Furnaces appointed for that end; where, by feveral repeated Heatings, they con- tinue to enlarge it, till it be equally thick in every Part.
This done, they cut it open with the Forces; not only on the Side by which it fluck to the Blowing-iron, but likewife the whole Length of the Cylinder: After which, giving it a fufficient Heating, it is in a condition to be entirely open'd, extended, and flatten'd. The manner of doing which is much the fame as for Table Glafs y and need not be here re- peated.
Laftly, the Glafs being fufficiently flatted, is laid to an- neal, for ten, or fifteen Days, according to the Size and Thicknefs.
It may be obferv'd, that Looking Glaffes thus blown, fhould never be above 45, or atmoft 50 Inches long, and of a Breadth proportionable. Thofe exceeding thefe Dimenfions, as we frequently find among the Venice Glaffes, cannot have the Thicknefs fufficient to bear the Grinding; and befide are fubjeft to warp, which prevents them from regularly reflect- ing Objects.
Method of cajiing t or running large Looking-Glass- Plates.
This Art is of French Invention; and not above fifty Years old. 'Tis owing to the Sieur Abraham c The r vart t who firft propoled it to the Court of France, in 1688.
It is performed much like the cafting of Sheet Lead, among $e Plumbers: By means hereof we are not only enabled to
make Glaffes of more than double the Dimenfions of any made the Venetian way of blowing; but alfb to cart all kinds of Borders, Mouldings, $$c:
The Furnaces for melting the Materials of this Manufa- cture, are of enormous Size; and thofe for annealing the Glaffes when form'd, arc much more fo. Round a Melting- Furnace, there are at leaft twenty four Annealing Furnaces; each from 20 to 25 Foot long: They are called Carqnaffes; Each CarquafTe has two Tiffarts, or Apertures, to put in Wood, and two Chimnies. Add, that befide the Annealing Furnaces, &c. there are others for the making of Frit, and calcining old Pieces of Glafs.
All thefe Furnaces are coverM over with a large Shed; under which are likewife Forges, and Work-houfes for Smiths, Carpenters, £f?c. continually employ'd in repairing, and keeping up the Machines, Furnaces, gfe. As alfo Lodges, and Apartments, for thefe, and the other Workmen em- ploy'd about the Glafs; and in keeping up a perpetual Fire in the great Furnace: So that the Glafs-houfe, as that in the Caftle of St. Gooen, in the Foreft of Fere, in the Sbiffo- nois, appears more like a little City, than a Manufaftory.
The Infideof the Furnaces are form'd of a fort of Earth, proper to fuftain the Aftion of the Fire; and the fame Earth ferves for Melting-pots, Cifterns, &r. The Furnaces feldom laft above three Years; after wl ich they are to be rebuilt, from Bottom to Top : And to keep rhem good even for that Time, the Infide mufl: be refitted every fix Months. The Melting-pots are as big as Hogfheads, and contain above two thoufand Weight of Metal. The Cifterns are much fmaller, and ferve for the Conveyance of the liquid Glafs, which is drawn out of the Pots, to the Carting Tables.
When the Furnace is in a condition 10 receive the Pots, and Citterns, they heat it red hot : This done, they fill the Pots with Materials; which is done at three times, to fa* cilitate the Fufion. When the Matter is fufficiently vitrified, refined, and fettled, which ufually happens in twenty four Hours; they fill the Cifterns, which are in the fine Fur- nace, and which are left there about fix Hours ionger, till fuch time as they appear all white, through the exceflive Heat.
To get the Cifterns with the Metal out of the Furnace; they make ufe of a large Iron Chain, which opens and Suits with Hooks, and Eyes. From the Middle hereof, on e^ch Side, arife two mafrtve iron Pins, whereby, with the Afli- ftance of Pullies, the Cifterns are rais'd on a kind of Car- riage of a proper Height; and thus conduced to the Tabic where the Gla's is to be run. Here, flpping off the Bot- tom of theCiftern, there rufhes out a Torrent of Matter, all in Fire, wherewith the Table, prepared for that purpofe, is prefently cover'd.
The Table, whereon the Glafs is to be run, is of Pot Metal, about nine Foot long, and broad in proportion. It is fup- ported on a wooden Frame, with Truckles, for the Con- venience of removing from one CarquafTe, or Annealing Furnace to another, in proportion as they are fill'd. —
To form the Thicknefs of a Glafs, there are two iron Ru- lers, or Rims, placed a-round the Edge of the Table; and on thefe, reft the two Extremes of a kind of Roller, which ferves to drive the liquid Matter before it to the End of the Table, or Mould. The iron Rulers, being moveable, and capable of being fet clofer, or further a-part at pleafure, determine the Width of the Glaffes, and retain the Matter, that it does not run off at the Edges.
As foon as the Matter is arrived at the End of the Table, and the Glafs is come to a Confiftence, which is in about a Minute; they fhove it off into the Annealing Furnace, where it flides with eafe enough, by reafon of the Sand itrew'd thereon.
What is moft furprifing throughout the whole Opera- tion, is the Qulcknefs, and Adrefs, wherewith fuch maffy Cifterns, fill'd with fo flaming a Matter, are taken out of the Furnace, convey 'd to the Table, pour'd therein, the Glafs fpread, &c. The whole is inconceivable to fuch as have not been Eye witnefles of that furprizing Manufacture.
As faft as the Cifterns are emptied, they carry them back to the Furnace, and take frefh ones, which they empty as before. This they continue to do, fblong as there arc any full Cifterns; laying as many Plates in each Carquaffeasic will hold, and flopping them up as foon as they are full; to let them anneal, and cool again, which requires at leaft ten Days.
The firft Running being difpatch'd, they prepare another, by filling the Cifterns a-new, from the Matter in the Pots: And after the fecond, a third; and even a fourth time, till the Melting-pots are quite empty.
The Cifterns, at each Running, fhould remain at leaft fix Hours in the Furnace, to whiten; and when the firft Annealing Furnace is full, the Carting-table is to be carried to another. It need not here be obferv'd, that the Car- quaffes, or Annealing Furnaces mufl firft have been heated to the Degree proper for them. When the Pots are emptied, they take them out, as well as the Cifterns, to fcrape oft
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