Materials for making of GLASS. The materials whereof glass is made, we have already mentioned to be salt and sand, or stones. The salt here used, is procured from a sort of ashes, brought from the Levant, called pulverine, or rochetta; which ashes are those of a sort of water-plant, called kali, cut down in summer, dried in the sun, and burnt in heaps, either on the ground, or on iron grates; the ashes falling into a pit, grow into a hard mass, or stone, fit for use.
To extract the salt, these ashes, or pulverine, are powdered and sifted, then put into boiling water, and there kept till one third of the water be consumed; the whole
whole being stirred up, from time to time, that the ashes may incorporate with the fluid, and all its salts be extracted: then the vessel is filled up with new water, and boiled over again, till one half be consumed; what remains is a sort of lee, strongly impregnated with salt. This lee, boiled over again in fresh copper vessels, thickens in about twenty-four hours, and shoots its salt; which is to be ladled out, as its shoots, into earthen pans, and thence into wooden pans to drain and dry. This done, it is grossly pounded, and thus put in a sort of oven, called calcar, to dry. It may be added, that there are other plants, besides kali, which yield a salt fit for glass: such are the alga, or sea weed, the common way-thistle, bramble, hops, wormwood, wood, tobacco, fern, and the whole leguminous tribe, as peas, beans, &c.
The sand or stone, called by the artists Tarso, is the second ingredient in glass, and that which gives it the body and firmness. These stones, Agricola observes, must be such as will fuse; and of these such as are white and transparent are best; so that crystal challenges the precedence of all others.
At Venice they chiefly use a sort of pebble, found in the river Tesino, resembling white marble, and called cuogolo. Indeed Ant. Neri assures us, that all stones which will strike fire with steel, are fit to vitrify: but Dr Merret shews, that there are some exceptions from this rule. Flints are admirable; and when calcined, powdered, and searched, make a pure white crystalline metal: but the expence of preparing them makes the masters of our glass-houses sparing of their use. Where proper stones cannot be so conveniently had, sand is used; which should be white, and small, and well washed, before it be applied: such is usually found in the mouths and sides of rivers. Our glass-houses are furnished with a fine sand for crystal, from Maidstone; the same with that used for sand boxes, and in scouring; and with a coarser for green glass from Woolwich. For crystal glass, to 200 lb of tarso, pounded fine, they put 130 lb of salt of pulverine; mix them together, and put them into the calcar, a sort of reverberatory furnace, being first well heated. Here they remain baking, frying, and calcining, for five hours, during which the workman keeps mixing them with a rake, to make them incorporate: when taken out, the mixture is called frit, or bollito.
It may be further observed, that glass might be made by immediately melting the materials without thus calcining, and making them frit: but the operation would be much more tedious.
A glass much harder than any prepared in the common way may be made by means of borax, in the following manner. Take four ounces of borax, and an ounce of fine white sand, reduced to powder, and melt them together in a large close crucible set in a wind furnace, keeping 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 hard, pure glass, capable of cutting common glass almost like a diamond. This experiment duly varied, says Dr Shaw, may lead
to some considerable improvements in the art of glass, enamels, and artificial gems. It shews us an expedient method of making glass without the use of fixed salts, which has generally been thought an essential ingredient in glass, and which is the ingredient that gives common glass its softness; and it is not yet known, whether calcined crystal, or other substances being added to this salt, instead of sand, it might not make a glass approaching to the nature of a diamond.