In place of the spread ing- and annealing-kiln just described, it has been pro posed to use two contiguous circular kilns with revolving beds. The split cylinder is introduced at one side of the revolving bed, and carried round to the heated side, where it is spread. It is then passed through a narrow opening on to the bed of the contiguous kiln, and carried by it gradually away from the fire.
Blowing Sheet - glass.— Mechanical appliances,cal led " iron men," are used to assist the blowers in 'making pieces of unusual size and thickness. The iron man merely carries the weight of the cylinder, during the operation of blowing, and has no blowing-apparatus iu connection with it. It is believed that some arrangement with bellows has been used.
Moulding Cylinders.—The cylinders used to be moulded in wooden blocks, and are so still to some extent ; but metal blocks of various kinds are now largely substituted.
Size of Cylinders.—Cylinders are made in a great variety of sizes, from 40 in. to 80 in. long, and from 20 in. to 50 in. wide. In Fig. 768, a, 10, c, d, e represent tho stages of the manufacture of a cylinder.
Glass Shades.—The manufacture of glass shades is almost identical with that of sheet-glass, exoept that the process is not carried so far. The materials and manipulation are identicaL In e, Fig. 768, if the elongated globe be cut from the blow-pipe at a point represented by the dotted line, a shade would be formed. The dimensions and forms of shades are regulated by the size and forms of the moulds into which the glass is blown.
Steam-gauge tube.—T his is made from the same material as plate- and sheet-glass ; for process of manufacture, compare Flint-glass.
Bohemian percentage composition of Bohemian glass is Bohemian glass is properly a mixture of the silicates of potash and lime ; it is, however, in this condition so difficult to work, from its high melting-point, that, for ordinary purposes, it has been found advisable to re place half the potash in this mixture by an equi valent of soda. Bohe mian glass is, as has already been noticed, but little affected by chemical reagents. For this reason, and on ac count of its high melt ing-point, it is chiefly valuable for the produc tion of apparatus used in chemical research. Bo hemian combustion-tube, which is the form in which Bohemian glass is probably best known, may be represented by the formula CaO, Owing to the absenoo of oxide of lead from its composition, Bo hemian glass oan be melted in open crucibles, and thus the greater part of the beat of the fur nace is available for the fusion of the glass. The methods of working the glass, whether for chemical or ornamental purposes, are similar to those employed in the manipulation of flint-glass.
The production of a ruby colour, commonly met with in Bohemian vases, is referred to under Coloured Glass.
or and Ornamental are various kinds of (compare Optical Glass), but that generally used in the manufacture of domestic and ornamental articles may be represented by the formula PbO, A glass, answering approximately to this formula, but varying slightly in every manufactory, is used in the production of tube and cane for lamp-workers, and of chemical and physical apparatus, as well as of tumblers, jugs, bottles, I:vine-glasses, ornaments, and vases. The essential qualities of a glass intended for these purposes,
are ductility, long-continued viscosity, absolute purity of colour and substance, together with brilliancy and perfect transparency. The raw materials of lead-glass are sand, carbonate of potash, red-lead, nitrate of potash, trioxide of arsenic, and binoxide of manganese. Transparency, and purity of colour and substance, depend upon the purity of the raw materials, upon their right com bination, and upon the regularity of the heat. Brilliancy is due to the powerful refraction and dispersion of light, caused by the presence of silicate of lead. Although lead-glass offers many advantages, it has the following drawbacks :—(1) it is so soft as to be easily scratched ; (2) it is liable to tarnish when exposed to the action of sulphides ; (3) it is costly, on account of the high price of lead ; (4) also on account of the difficulty of obtaining it free from blemishes.
The raw materials, after having been weighed, are placed in a large bin, and are passed three or more times through a brass-wire sieve, so that they may be thoroughly mixed. To this mixture, is added broken or ladled lead-glass, in a proportion varying with the quantity available. The broken glass must first be carefully sorted, in order to remove even the smallest fragments of coloured or chemically different glass. The raw material is now ready to be introduced into the crucibles. In order to avoid the reduction of the oxide of lead, a lead-glass crucible is domed over, and only one small opening is left, which is turned away from the fire, for the introduction and withdrawal of the glass, and for the convenience of the glass blower. The manipulation of lead-glass in large manufactories ceases, as a rule, on Friday, to be resumed on the following Monday morning. The raw materials are, therefore, generally placed in the crucibles on Friday evening, so that the glass may be ready for use when the workmen return. The length of time required for the complete fusion and purification of lead-glass may be greatly modified by the nature of the furnace. After the raw materials have been placed in the crucible, the opening or mouth is closed by means of a fire-clay slab, and the joint is cemented with moist fire-clay. An average-sized crucible holds 10-12 cwt. of glass. It is usually filled by 3-4 distinct charges, one being allowed to become partially melted before another is added. The crucibles stand around the grate of the furnace, and under a common dome ; the mouth of each protrudes through an arch in the side of the dome, and the crucibles are separated by pillars, through which, flues pass upwards into the main cone. Gas-furnaces are now being used in lead-glass manufactories, but the old-fashioned circular coal- or coke-furnace, with certain economical modifications, and sometimes fed by Frisbie's apparatus, is still most common.