652 GLASS 



under the name of British or German plate. 3. Crown-glass comes next, or window- 

 glass, formed in largo circular plates or discs. This glass is peculiar to Great Britain. 

 4. Flint-glass, crystal-glass, or glass of lead. 5. Plate oijinc mirror-glass. 



THE POTS. The materials of every kind of glass are vitrified in pots made of a 

 pure refractory clay; the best kind of which is a species of shale or fire-clay dug 

 out of the coal-measures near Stourbridgo. It contains hardly any lime or iron, and 

 consists of silica and alumina in nearly equal proportions. The masses are carefully 

 picked, brushed, and ground under edge iron wheels of considerable weight, and 

 sifted through sieves having 20 meshes in the square inch. This powder is moistened 

 with water (best hot), and kneaded by the feet or a loam-mill into an uniform smooth 

 paste. A largo body of this dough should be made up at a time, and laid by in a damp 

 collar to ripen. Previously to working it into shapes, it should bo mixed with about 

 a fourth of its weight of cement of old pots, ground to powder. This mixture is 

 sufficiently plastic, and being leys contractile by heat, forms more solid and durable 

 vessels. Glass-house pots have the figure of a truncated cone, with the narrow end 

 undermost ; those for bottle- and window-glass being open at top, about 30 inches 

 diameter at bottom, 40 inches at the mouth, and 40 inches deep ; but the flint-glass 

 pots are covered in at top with a dome-cap, having a mouth at the side, by which the 

 materials are introduced, and the glass is extracted. Bottle and crown-house pots are 

 from 3 to 4 inches thick ; those for flint-houses are an inch thinner, and of propor- 

 tionally smaller capacity. See CLAY. 



The well-mixed and kneaded dough is first worked upon a board into a cake for 

 the bottom ; over this the sides are raised, by laying on its edges rolls of clay above 

 each other with much manual labour, and careful condensation. The clay is made 

 into lumps, is equalised, and slapped much in the same way as for making pottery. 

 The pots thus fashioned must be dried very prudently, first in the atmospheric tem- 

 perature, and finally in a stove floor, which usually borrows its heat directly from the 

 glass-house. Before setting the pots in the furnace, they are annealed during 4 or 5 

 days, at a red heat in a small reverberatory vault made on purpose. When com- 

 pletely annealed, they are transferred with the utmost expedition into their seat in the 

 fire, by means of powerful tongs supported on the axle of an iron-wheel carriage 

 frame, and terminating in a long lever for raising them and swinging them round. 

 The pot-setting is a desperate service, and when unskilfully conducted without duo 

 mechanical aids, is the forlorn hope of the glass-founder. 



The glass-houses are usually built in the form of a cone from 60 to 100 feet high, 

 and from 50 to 80 feet in diameter at the base. The furnace is constructed in the 

 centre of the area, above an arched or groined gallery which extends across the whole 

 space, and terminates without the walls, in large folding doors. This cavern must 

 bo sufficiently high to allow labourers to wheel out the cinders in their barrows. 

 The middle of the vaulted top is left open in the building, and is covered over with 

 the grate-bars of the furnace. 



1. Bottle-glass. The bottle-house and its furnace resembles nearly^. 1092. The 

 furnace is usually an oblong square chamber, built of largo fire-bricks, and arched 

 over with fire-stone, a siliceous grit of excellent quality extracted from the coal- 

 measures of Newcastle. This furnace stands in the middle of the area ; and has its 

 base divided into three compartments. The central space is occupied with the grate- 

 bars: and on either side is the platform or fire-brick siege (seat), raised about 12 

 inches above the level of the ribs upon which the pots rest. Each siege is about 3 

 feet broad. 



In the sides of the furnace semi-circular holes of about a foot diameter are left, 

 opposite to, and a little above the top of each pot, called working-holes, by which 

 the workmen shovel in the materials, and take out the plastic gas. At each angle 

 of the furnace there is likewise a hole of about the same size which communicates 

 with the calcining furnace of a cylindrical form, dome-shaped at top. The llunm 

 that escapes from the founding or pot-furnace is thus economically brought to rover- 

 berate on the raw materials of the bottle-glass, so as to dissipate their carbonaceous or 

 volatile impurities, and convert them into a frit. A bottle-house has generally ci^ht 

 other furnaces or fire arches ; of which six are used for annealing the bottles after they 

 are blown, and two for annealing the pots, before setting them in the furnace. 



Generally, for common bottles, the common river-sand and soap-boilers' waste are 

 used. About 3 parts of waste, consisting of the insoluble residuum of kelp mixed with 

 lime, and a little saline substance, are employed for 1 part of sand. This waste is first 

 of all calcined in two of the fire arches or roverberatories reserved for that purpose, 

 called the coarse arches, where it is kept at a red heat, with occasional stirring ; from 

 24 to 30 hours, being the period of a journey, orjoumec, in which the materials could 

 be melted and worked into bottles. The roasted soap-wasto is then withdrawn under 

 the name of ashes, from its arch, coarsely ground, and mixed with its proper proportion 



