Glass, a solid, transparent, brittle substance, produced by melting together sand, flint, alkaline salts, etc. besides which, there are various saline matters employed, namely, Polverine, or ro-chetta, which is prepared from Glasswort, or Salso/a Kali, an indigenous plant; but which is chiefly imported from the Levant, where it is cut down in the summer, dried in the sun, and burned in heaps, when the ashes fall into a pit, where they concrete into a hard mass. A similar salt is obtained from the ashes (help) of the Fucus vesiculous, or Common Sea-wrack, or Sea-ware, a marine plant growing on the sea-coasts.— The sand used in the manufacture of glass is found at Lynn, in Norfolk, and Maidstone, in Kent.
These various articles are first carefully washed, and, after extracting all the impurities, they are conveyed to the furnace in pots made of tobacco-pipe clay, for the purpose of resisting the fire. Here the mixture is fused, and disposed of according to the different kinds of glass intended to be manufactured.
Round glass, such as phials, drinking-glasses, etc. are blown. When the different materials are sufficiently liquefied, the workmen dip long iron pipes into it, and blow the metal till it lengthens like a bladder. It is then rolled on a marble slab to polish it, when it is blown a second time, in order to form it into the shape of a globe. Next, it is cut off at the collet, or neck, adhering to the pipe: for this purpose, the latter is rested on an iron bar close to the neck, and a drop of water poured on it, by which means, it is cracked about a quarter of an inch, when it is slightly struck, or cut by a pair of shears, and imme-diately separated. - Now the workman dips the rod or pipe into the melting metal, whence he draws out as much as will connect the glass already made, to which he fixes the rod, opposite the opening caused by the breaking or cutting of the neck. In this position, the glass is carried to the mouth of the furnace, in order to be heated, or scalded : thus it becomes so soft, that it may be pierced, opened, and moulded at pleasure., without any apprehension of its breaking. The vessel, however, is not finished till it has again been returned to the mouth of the furnace, where, after being thoroughly heated, and, turned quickly round, it will open to any size, by means of heat and circular motion. Should any impurities remain, they are cut off with the shears, as the glass continues flexible till it becomes cool. And, if the vessel thus made require a foot or handle, or any other ornament, the operator forms them separately, and unites them by the help of hot metal, drawn from the pots with the iron-rod : - the last operation for completing the glass, is that of Annealing. - See vol. i. p. 65.
Window Table-glass, is worked nearly in the manner above described : the workman blows and manages the metal, so that it extends two or three feet in a cylindrical form. It is then carried to the fire, and the operation of blowing repeated till the metal is stretched to the dimensions required, the side to which the pipe is fixed diminishing gradually till it ends in a pyramidal form ; but, in order to bring both ends nearly to the same diameter, while the glass continues flexible, a small portion of hot metal is added to the pipe ; the whole is drawn out with a pair of iron pincers, and the same end is cut off with a little cold water as before.
The cylinder thus open at one end, is returned to the mouth of the furnace, where, it is cut by the aid of cold water, after which it is gradually heated on an earthen table, in order to unfold its length, while the workman with an iron tool alternately raises and depresses the two halves of the cylinder : by this process, the latter accommodates itself to the same fiat form in which whiich it is again heated, cooled on a copper-table, hardened 24 hours in the annealing furnace, and afterwards preserved for use.
Window-glass is divided into various sorts, the principal of which are : 1. Crown-glass, which is the clearest and most expensive. The best window-glass is made of white sand, 60lbs.; purified pearl-ashes, 30lbs.; salt-petre, 15lbs.; borax, llb.; and of half a pound of arsenic. These materials are melted in the manner before mentioned, and if the glass should assume a dusky-yellowish hue, a sufficient quantity of manganese must be added to remove that defect.
2. Newcastle-glass, which is most commonly used in England : it is of an ash-colour, and frequently speckled, streaked, and otherwise blemished. Its preparation consists of 60lbs. of white sand, 25lbs. of unpurified pearl-ashes, l0lbs. of common salt, 2lbs. of arsenic, and 2 oz. of manganese.
3. Phial-glass is an intermediate kind between flint and the common bottle or green-glass. The better sort is made of 120lbs. of white sand, 50lbs. of unpurified pearl-ashes, l0lbs. of common salt, 5lbs. of arsenic, and 5oz. of manganese. The composition for green or common phial-glass consists of 120lbs. of the cheapest white sand; 80lbs. of wood-ashes well burnt and sifted, 20lbs. of pearl-ashes, 15lbs. of common salt, and llb. of arsenic.
4. The common bottle, or green glass, is prepared from any kind of sand fused with wood-ashes, to which may be added the clinkers of forges.
Plate-glass is the last and most valuable kind, and is thus called, from its being cast in plates or large sheets: it is almost exclusively employed for mirrors or looking glasses, and for the windows of carriages. - It is composed of 60lbs. of white sand cleansed ; 25lbs. of purified pearl-ashes; 15lbs. of salt-petre, and 7lbs. of borax; and, if a yellow tinge should appear in the glass, a small quantity of manganese and arsenic are added, in equal proportions.
Plate-glass was formerly bloivn, but that method having been found very inconvenient, casting was invented; namely, the liquid metal is conveyed from the furnace to a large table, on which it is poured, and all excrescences, or bubbles, are immediately removed by a roller that is swiftly passed over it. It is then annealed in the manner already referred to.
The last process is that of grind-' ing, which is performed by certain machinery, that is not generally known. In Britain, it is practised to the greatest perfection by Bohemians.
The colouring of glass with various shades, is an art known only to a few persons, and as it is not an object of domestic economy, we shall only notice a patent granted In February 1778, to Mr. JOHN Kent Tarrant, for his invention of painting, spangling, gilding, and silvering glass. - This is effected by applying, the painting to the back of the glass, so that it may appear on the front, when finished: the colours are to be prepared with oil or varnish. Those parts which are intended to be gilt, must be previously traced on the glass, and when perfectly dry, the gold-leaf is to be applied': a similar method is to be followed for silvering. With respect to spangling, the patentee directs this process to be performed after the parts have been properly shadowed ; and, as soon as the outlines are dry, the glass ought to be varnished with a solution of gum copal, and the spangles strewed on while it is wet ; when they are perfectly dry, it is necessary to varnish them over two or three times. Glass is so remarkably elastic, that if the force with which glass balls strike each other, be computed at 16, that with which they recede, from their elasticity, will be nearly 15. Hence wehave seen glass wigs, and even glass brushes, ma-nufactured by Bohemian artists.— If glass be exposed to the influence of dew, it becomes moist, which does not happen either with silver or any other metal. And if a gob-let, or other drinking-glass, be fill-ed with water, and rubbed on the brim with a wet finger, it will im-part musical notes,higher or lower, in proportion as the glass contains more or less of that fluid : it likewise possesses considerable eletri-cal properties, and is therefore frequently employed in experiments on electricity.
Before we conclude this article, we cannot omit to take notice of the numerous accidents that frequently happen in consequence of person inadvertently, or adventurously, swallowing fragments of broken glass. In such case, the safest remedy is to administer, as speedily as possible, large draughts of olive or other demulcent oils, by which the membranes of the stomach and intestines may be lubricated, and thus the injurious effects of the glass timely counteracted. If children, or other improvident persons, have cut themselves with glass, it is of the first import ance to ascertain whether 3ny par-ticles of it have remained in the wound: these should, at all ha? zards, be immediately extracted by a skilful operator ; as, without such precaution, the most dangerous consequences are to be apprehended, in parts thus injured.
For different methods of uniting broken glass, we refer to the articles Cement, vol. i. p. 476 ; and Garlic, vol. ii. p. 366.