GLASS MANUFACTURE. This trans parent and beautiful substance, though ex ceedingly brittle while cold, is rendered so flexible and tenacious by a high degree of heat that it may with the utmost facility be moulded into any form. It is so ductile while heated, that it may be spun into filaments of the greatest conceivable fineness, and these when cold are pliant and elastic in a high degree.
The time at which glass was invented is very uncertain. The ancient Egyptians were certainly acquainted with the art of glass making. The manufacture was long carried on at Alexandria, from which city the Romans were supplied with that material ; hut before the time of Pliny the manufacture had been introduced into Italy, France, and Spain. Glass utensils have been found among the ruins of Herculaneum. The application of glass to the glazing of windows is of compa ratively modern introduction. The earliest manufacture of flint glass in England was begun in 1557, and that of plate glass in 1673. The principal seat of the manufacture in Eng land is at Newcastle-upon-Tyne and the neigh bouring town of Shields ; next in importance is Stourbridge; then the works in and near Liverpool, including the Plate-glass Com pany's establishment at Ravenhead ; next follow Bristol, Warrington, Birmingham. Leeds, and London. There are several manu factories in Scotland and Ireland.
There are five distinct kinds of glass, which differ from each other in regard to some of the ingredients of which they are made, and in the processes of manufacture. These kinds are flint glass, or crystal; crown glass, or German skeet glass; broad glass, or common window glass ; bottle glass ; and plate glass.
The principal ingredients used for the pro duction of each of these kinds of glass are silo; or flint, or sea sand, and an alkali. The differences in the various kinds result from the description of alkali employed, and from the addition of certain accessory materials, usually metallic oxides. The alkali employed for making fine flint glass is purified pearl ash. Barilla, kelp, and wood ashes, com bined with many impurities, are used for ma king inferior kinds of glass : the impurities even assist towards fusing the silex. Coarse alkaline substances all contain iron in some degree, and it is to the presence of this metal that the green colour of common glass is owing.
Flint Glass, known in other countries under the name of crystal, is the most generally useful, the most brilliant, and the heaviest description of glass. The following is one
among many ratios of ingredients:— 120 parts fine clean white sand, 40 „ well purified pearlash, 35 „ litharge, or minium, 13 „ nitre ; and a small quantity of the black oxide of manganese. The litharge is employed to assist the sand to melt ; nitre is used to dissipate carbon, and manganese to dissipate colour. The ingredients are inti mately mixed together before they are put into the crucibles or pots, which are previ ously placed in the furnace. A very strong and long continued heat is necessary, not only for the perfect fusion and amalgamation of the materials, but also for the discharge of the impurities which they contain. The glass is cooled down to a pasty mass, and then wrought. There is perhaps no process of manufacture which excites so much the surprise and admi ration of a stranger as that of fashioning flint glass into all the various objects of conve nience and ornament for which it is employed. To see a substance, proverbially brittle, blown by the human breath, pulled, twisted, cut, and then joined again with the greatest faci lity, never fails to strike with astonishment those who are unaccustomed to the sight. The tools with which all these operations are performed are of the most inartificial descrip tion, and do not appear to have received any notable improvement from the earliest records of the manufacture.
Crown Glass.—This is the best description of window glass ; and one recipe for it is :— 120 parts of white sand, CO „ purified pearlash, 30 „ saltpetre, 2 „ borax, 1 „ arsenic, with the addition of a minute quantity of manganese. Crown glass is made by blowing, in the form of circular plates of 50 to CO inches diameter. A quantity of glass in the pasty state is collected upon the end of a bel low iron tube; and this glass is then con verted, by blowing throngh the tube, into a hollow globe of the requisite thickness. This globe is transferred to the end of the rod, and after several re-beatings it is twirled round by the workman somewhat in the manner that a mop is twirled•to drive off the moisture; with this twirling the softened material is conti nually driven off from the centre by the cen trifugal force, until at length the whole sub stance is converted into a flat disc of circular form, and, except at the centre, where it is attached to the rod, of a uniform thick ness.