The colours employed are clays coloured naturally, or by the artificial admixture of the oxides of iron, nickel, manganese, or cobalt.
The wet process consists in forming a pattern in relief in plaster, and framing and backing it in a metallic support. Into the mould thus formed 2t thin bat of white plastic clay, backed with a clay of inferior quality, is pressed, and receives from the plaster mould depressed pattern. A layer of white clay is then spread upon the common clay in order to counteract any contortion arising during firing from a disagreenient between the common clay and the white clay on the face. The tile is heated until it can readily be removed from the mould. Into the depressed pattern on the surface, coloured clay slips are instilled from suitable vessels, and when the exc,ess has been removed by scraping, the pattern is discovered.
The dry process is a further application of that already described. Upon the central block, Lt pattern is dusted with powdered clay through a thin metallic stencil-plate ; the collar is then raised, and a layer of clay is in serted, and compressed by the de scent of the plunger ; a layer of inferior clay is next added and compressed ; and finally a layer of the same clay as that used for the surface, the collar being raised to receive each fresh layer. The removal of the tile is facilitated by the depression of the collar. The same results may be obtained both for plain and inlaid tiles, by raising or depressing by a pedal a movable base working inside an immovable collar.
One of the earliest machines and moulds used for moulding articles of a small size in powdered clay is represented in Fig. 1160 : A is the wooden bench on which the whole is fixed, the bench being sustained on legs standing on the floor ; BD E, the frame, formed in one piece of cast-iron, the base B being fixed on the bench by screw-bolts, the up right standard D sustaining at its upper end the boss E, wherein the nut or box I is fixed for the reception of the vertical screw F ; the screw F works through the box a, and has a handle G g h applied on the upper end of the screw ; the handle is bent downwards at g, to bring the actual handle h to a suitable height for the person who works the machine ; by pulling the handle h towards him, the screw F is turned round in its box a, and descends. The lower end of the screw F is con nected with a square vertical slider II, which is fitted into a socket I, fixed to the upright part D of the frame, and the slider H is thus confined to move up or down with an exactly vertical motion, when it is actuated by the screw. Thus far, the machine is an ordinary serew-press, such as is commonly used for cutting and compressing metals for various purposes.
The tools with which the press is furnished consist of a hollow mould e, formed of steel, the interior cavity being the exact size of the article to be moulded. The mould is firmly fixed on the base B of the frame, so as to be exactly beneath the lower end of a piston or plug f, which is fastened to the lower end of the square slider H, and the plug f is adapted to descend into the hollow of the mould, when the elider H ia forced downwards by action of the screw F, the plug f being exactly fitted to the interior of the mould. The bottom of the mould la a movable piece n, which la exactly fitted into the interior of the monld, but which liee at relit in the bottom of the mould during the operation of moulding ; but afterward°, the movable bottom can be raieed by preesing the foot upon one end R of a pedal-lever R S, the fulorum of whieh is a centre-pin r, supported in a standard resting upon the floor ; the end S of the lever operatea on an upright rod m, which ia attached at ite upper end to the movable bottom of the mould. A email horizontal table T is fixed around the mould, and on that table, a quantity of powdered clay ie laid in a heap in readiness for filling the mould.
The operation ie very simple ; the operator holding the handle h with hie right hand, puts it back from him, so as to turn back the ecrew F, and raiae the elider H and plug f quite out of the mould e, and clear above the orifice of the mould, ae ehown in Fig. 1160. Then, with a bone spatula held in the left hand, a email quantity of the powder is moved laterally from the heap, along the eurface of the table T, towards the mould e, and gathered into the hollow of the mould with a quiet motion, so as to fill that hollow very completely ; and by scraping the epatula evenly across the top of the mould, the euperflous powder will be removed, leaving the hollow cavity of the mould exactly filled with the powder in a loose etate. The handle h being drawn forwarde, with a gentle movement of the right hand, it turne the ecrew F, Bo as to bring down the slider II and plug f into the mould upon the loose powder, and hegine to press the powder with a gentle motion, without any jerk, in order to allow the air contained in the loose powder to make ite escape ; but the pressure, after having been commenced gradually, ie continued and augmented to a great force, by pulling the handle etrongly at the last, ao aa to comprcse the earthy material upon the bottom n of the mould, into about one-third of the space it had occupied when it waa in a atate of looee powder.