Manufacture of Common Stone-uare by " Expression."— Drain-pipes, roofing-tiles, perforated bricks, and similar articles, are produced by mechanical pressure. The processes are as follow. The ball-clay and burnt broken ware are separately ground under pairs of iron edge-runners. A scraper follows the runners, and drags the ground clay over an iron grating, through which the fine powder falls, the coarser particles being thrown back for regrinding. From the receptacle beneath the grating, the ground ball-clay is removed, aud cariied upwards in open pockets attached to an endless band moved by machinery. As the pockets turn to descend, the powder is thrown into a hlunger, where it is iocorporated with a small quantity of water, and a measured proportion of the ground burnt clay. The mixture is removed from the blunger, and supplied to a vertical pug-mill, by the knives of which, it is compressed, and forced downwards and outwards. The stream of prepared mixture is cut into blocks, and the blocks are carried by an endless hand, fitted with shelves, to an upper floor, where they are stored for use. The expressing-machine occupies two floors, and the feeder is upon the upper floor, to which the blocks of clay are carried direct from the pug-mill. The principle of an expressing-machine in its simplest form is illustrated by Fig. 1135. The plunger and part of the cylinder in which it moves are represented. The clay can be introduced into the cylinder immedi ately below the plunger, and the door by which it is inserted can he securely closed ; as the plunger, which moves airtight in the cylinder, is caused to descend, it com pels the clay beneath it to assume the form of any resisting environment ; and, if there he an opening, to assun3e its out line, and to stream through it so that a section of any part of the stream has the same outline as that of the aperture through which it has passed. If the aperture be a simple slit, the clay issues as a ribbon. Roofing-tiles are made by cutting into lengths a ribbon produced as described, placing the separate lengths whilst still plastic upon plaster moulds of the form which the tiles are required to possess, and baking them when dry. If the aperture be annular, the clay issues from it as a continuous hollow pipe. In order to form a perfect annular opening, it is necessary to support a core in the centre of the main opening, and in such a way that the supports shall not interfere with the continuity of the resultant pipe. The core e is attached to the base of the cylinder by the supports a. The cylinder is enlarged below the attachment of the supports, in order to allow the reconsolidation of the clay after having been cut by the snpports, and before issuirig from the annular space b. As the pipe issues from b, it is cut by wire ioto any required length& Additional apparatus is necessary to form pipes with flanges or sockets attached. A movable iron mould or core of the shape of the im-ide of the socket is placed by hand in contact with the base of the core e, so that the pipe, if the pressure be continuous, must be forced over it, and be enlarged in its passage. This internal mould is supported in position by a rim attached to the base of an iron collar, formed of two jointed halves. The internal form of the collar is the same as that which the exterior of the socket is intended to receive, and ita internal ourface fonns a continuation of the outer edge of the annular aperture b. When the two halves of the collar are united. there remains aperture for the escape of the clay, and it therefore forced to adapt itself to the internal form of the seeket-mould. Cooical pin-holes are, however, provided in the collar, to permit the escape of imprisoned air, and the consequent perfect adaptation of the clay to the mould. The exudation of clay through these pin-holes marks the time when the socket has been formed, and when the collar and internal mould must be removed, in order to allow the simple pipe to follow after the socket. Wooden "forms" are uaed to rectify any inaccuracies in the shape of the sockets or barrels of the pipes. Expression may, in a similar manner, be applied to the production of a great variety of wares, as, for instance, in the manufacture of perforated and damp-course bricks.
Stone-ware jars, bottles, and jugs are fashioned on the wheel, which, in large manufactories, i8 generally driven by power. Tho more delicate specimens of decorative stone-ware, which are known as Lainbeth ware, are formed on the hand-driven wheel. For the different decorative processes applied to stone-ware, see that section.
Tall chimney-pots are at times made up of as mnny as three lengths, fashioned separately on the wheel, and built up one upon the other. V-shaped pipes aro made by the union of two sepa rately formed pipes. Siphon-pipes are made by moulding. Fig. 1136 represents the half of a plaster mould for this purpose : a are the studs or depressions by which the two halves of the mould are fastened tog, then "Bats" or thin sheets of clny are spread carefully by halal over the entire surface of the two half-moulds. The two parts of
the mould are then united, rind the division between the two halves of the pipe is carefully closed by the hisiTtion of strips of plastic clay. Largo filters are similarly fashioned ; but io this case, external decoration is produced at the same Hine as the aotual form of the ware.
possesses a dense, opaque, and generally white body, with a rough fracture. The white ness, opacity, and stability of earthen-ware are in a great measure due to the presence of a considerable proportion of calcined fliut. There are many species of earthen-wure, distinguished by their fracture, or by the tints of their body or glaze. Tbe ingredients of which earthen-ware is composed are ball-clay, kaolin, flint, nnd Cornish etone; and the glaze with which earthen-ware is generally coated is soft, oontaining plumbic oxide and borax. The blue or ball-clay and the kaolin or China clay, after their arrival at the works, are exposed to the action of the weather. It is convenient if the clay-banks can be so placed as to be above or on the same level with the sheds in which the clays are " blunged," that is broken up with water. When the clays are ripe, a certain quantity of each is moved to the blunging-shed, and subjected to the process of blunging. The clays are thrown into tanks containing pure water, and are mixed with the water, either by the mechanical action of blades attached to a horizontally-moving wheel driven by steam power (see p. 1559), or by a laborious proce:s of manual stirring with a wooden instrument resem biting a large paddle. Separate tanks are provided for blunging the ball-clay, the kaolin, and scraps and shavings of broken uoburnt ware. The density of the contents of each blunger is tested by weigh ing one pint of each in a standard pint measure. If the liquid in either case be too dense, more water is added ; if not dere-e enough, the proportim of clay is increased. The weight of the staudard mixture of ball-clay is 24 oz. a pint, that of kaolin being 26 oz. If the density be correct, each liquid mixture, or slip, is run separately by gravitation, the blungers being purposely erected on an elevation, either through a series of sieves, or into a horizontal, rotatory, cylindrical sifter. Figs. 1137, 1138 show an arrangement of sieves. Fig. 1137 is a vertical, and Fig. 1138 is a horizontal sectipn of the apparatus. A series of sieves, with " lawns" of increasing fineness (two only are shown in the figure), are placed one above another, in such a way that the material can pass from one into the next. A backward and forward motion is communicated to each sieve by a hooked rod, loosely attached to a point on the circumference of a wheel which revolves in a vertical plane. The bases of the sieves rest upon narrow slabs of plate-glass, by which means, friction is reduced.
The actual form of the rotatory sifter is octagonal rather than cylindrical, and it receives a shaking motion in addition to rotation, through a strap driven by power. The sifting medium of the sieves and of the cylindrical sifting-machines is silk lawn, brass gauze having been tried without success. It is customary for potters to contract with the makers of the silk sifting-machines to keep their sieves in working order, as, owing to the great delicacy of the material, they are constantly liable to damage. The sediment retained by the sieves or sifting-cylinders is emptied from time to time into convenient receptacles, and returned to be reblunged. After sifting, the clay slips are run separately into the mixing-tank, in which, a measure fixed to one side indicates in inches the quantity of each material received, the density of each liquid having already been determined by weight. The relative quantities of the clays, as of the other substances, vary according to the nature of the ware it is intended to produce. The mixing-tank is generally of stone, measuring about 6 ft. sq. The flints, after calcination and crushing (see p. 1561), and the Cornish stone, after crushing, are ground separately with water. In the mills used for this purpose, the necessary fric tion is obtained between two surfaces of differently-grained chert. A cast-iron bed is evenly paved with blocks of carefully selected stone, but in such a manner that the circumference of the bed shall be at a higher level than the centre; the bed is surrounded by sides of wrought-iron, rind the centre is pierced hy an opening, in which, revolves an iron vertical shaft, driven by gearing from below. To the shaft, are attached four curved projecting arms, each provided with vertical, wooden, iron tipped bars, which reach almost to the bottom of the pan, and propel heavy masses or " runners" of the chert rock.