Saggers.—Saggers are not required to withstand a very intense temperature for prolonged periods, nor the COM:1911/e action of fused material ; the economy, however, of a manufactory depends in some measure upon the possibility of using them several times, and upon their withstanding, without breaking, repeated heating, cooling, and reheating. As the Baggers in a kiln are piled in columns, the result of the breakage of one Bagger during firing may be disastrous. Considerable care is therefore expended upon the selection and preparation of fire-clays for sagger-making, but their purity is inferior to the fire-clay used for crucibles and fire-bricks. The mixture generally employed consists of inferior fire-clays, together with a proportion of the powder of broken burnt saggers.
The fire-claye, after arriving at tho works, are exposed in heaps to weather " for as long a time as possible (see Clay—Fire-clay, p. 639). When required for use, the clay and broken eaggers are coarsely ground under iron wheels working upon an iron revolving base. The mixture is then thrown through a grating into a circular underground tank containing water, and is crushed and mixed with the water by the revolution of a borizontal.bladed wheel. When the liquid mixture has been tested, and found to be sufficiently dense, it ie run through a long imperceptibly-sloping trough, in order that the coarser grit and particles of iron may be precipitated by gravitation, and be intercepted by depressions arranged in the trough at regular intervals. From the trough, the mixture passes into a steam-jacketed iron tank, in which it is reduced by evaporation to a suitable consistency for manipulation. The surplus heat from the tank is utilized in the drying-rooms.
The plastic clay is removed from the evaporating-tank, and, in order to ensure a close-grained and tenacioua mass, is eithe,r repeatedly rolled and beaten by manual labour, or passed through a mill (" pug-mill"), in which it is mixed, pressed, and kneaded by the revolution of a cylinder armed with splayed knives, and from which it is finally driven in a continuous compact stream of the exact form of the orifice from which it iasues. The stream of clay is cut into blocks, which are carried wherever they are needed for manipulation.
Baggers are manufactured in different ways, according to the different purposes for which they are intended. They may be moulded by hand on large potters' wheels, or whirling-tables, in the same manner as deep hollow ware is formed, or the basea and sides may be formed separately. The baaes are formed by beating the plastic clay into iron rings of the same shape, but of larger circumference than that of the Baggers of which they are to form part.
The sides of small saggera may be made from lengths cut from a cylinder issuing from the annular opening of an expressing-naachine (compare Stone-ware), and may be cemented by liquid elip on to bases farmed as described. The sides of large saggers are fornaed from strips of the clay
mixture which have passed under a roller-prese. The press consists of three parallel iron cylinders, supported so as to impend over a movable iron table. The blocks of clay are placed in shallow troughs of varying width, resting upon the iron table. The table can be moved backwards and forwards beneath the cylinders, and carries with it the troughs and their contents. The cylinders are caused to revolve by the resistance offered by the clay, which, at the Berne time, is evenly spre,ad and compressed. The depth of the troughs forms a gauge for the thickness of the clay. The strips of compressed clay are now removed, and wound round wooden drums, which rest upon the sagger bases, but in such a manner as to leave a margin to which the sides can be attached. The side is attached to the base by kneading with liquid slip, and the edges are united in the same manner. The staggers thus formed are dried and baked.
Fire-bricks.—The consideration of the manufacture of fire-bricks and shaped blocks for the con struction of furnaces hardly falls within the scope of this article. Suffice it to say, that the im portance of non-liability to shrinkage in ware of this description rests upon the fact that the greater part, and especially the crowns and beds, of furnaces intended to resist intense and pro longed heat, are built of green or unbakecl material. (See Glass, p. 1049 ; also Spons' Dictionary of Engineering, article Brick.) Crucibles.—Crucibles are generally formed from a mixture of almost pure fire-clay (compare analyses, p. 1559), with a greater or leas proportion of fire-clay specially burned for the purpose, or of the powder of ground broken crucibles. The burnt clay is always coarser than the raw, fire-clay. Crucibles, and especially large crucibles for melting glasa, are built up layer by layer by band (aee Glass, p. 1046). Crucibles of various sizes are made on the wheel, and by machinery. Machinery is largely used at the works of the Battersea Plumbago Crucible Co. The processes there employed are as follow. A graphite is selected which is as free from foreign matter as possible. The fire-clay and graphite are dried and ground separately ; they are then weighed and mixed in nearly equal proportions. The mixture, incorporated with a small quantity of water, la passed through a pug-mill, and the stream of compressed and plastic material, as it issues from the mill, is cut into blocks and stored for future use.