POTTERY, Manufacture of. Clay in it self is not sufficient to make good pottery. The materials used for mixing with it are flint, Cornish stone and feldspar. The first process is to reduce all these materials to a common fineness by means of grinding and levigation. These in their proper proportions are mixed with water, the resultant liquid mass being known as slip. If plastic clay is required the slip is pumped into a clay press. The press consists of a number of what are termed cham bers lined with very heavy cloth into which the slip is forced, pressure is brought to bear on them and the water expelled, leaving the clay mass inside the cloth or '
i Throwing is perhaps the most fascinating oper ation in all the industrial arts. Taking a ball of clay the thrower throws it on a revolving hori zontal wheel and grasps it on the side with hands held rigid, which forces the clay to the exact centre. He then with an upward pressure raises it into a cone and when this has been repeated once or twice inserts his thumb in the top to make the aperture, and according to the shape required so guides and gently forces the clay to the proper contour entirely by the use of his fingers. Watching a skilled workman, the clay appears as if imbued with life, obedient to his slightest wish or thought. There is, how ever, a good deal of nonsense talked about the expression of the thrower's individuality, and whilst this may be true of the few craftsmen who have mastered the process, it is ridiculous as applied to commercial wares, for when the individual work of the thrower is finished it is passed on to the turner, who places it on a horizontal lathe and turns out all the marks of the potter's thumb and fingers until it as sumes the exact size and shape required, no better and no worse than if it had come out of the mold. The process is very little employed to-day. For the presser much preliminary work must be done. An artist must design a shape, a model of it must be made and from this model must be made the molds, so constructed that the different parts of them lift cleanly away from the clay inside. Taking these molds, the parts held firmly together with a strap, the presser lines them with bats of clay, which he presses firmly down and then sponges the inside to make it smooth. Evaporation causes the clay to harden and the particles to shrink more closely together, and the piece can then be re moved from the mold. If it requires a handle
it is passed to the handler who presses the handles from molds and then with a little water or slip adjusts them to their proper place. Plates, saucers, etc., are made by ma chinery, but the principal is the same, the molds being fixed on a revolving disc, which greatly facilitates the work. The ware is now in the state, and when thoroughly dry is ready for the first firing, after undergoing which it is called biscuit ware, the first form of pot tery. If instead of the plastic clay slip is used it is poured into a plaster mold, which being made somewhat softer absorbs some of the water, the clay particles floating to the side. The mold is kept full of slip until the requisite thickness is attained, the surplus slip is emptied out and the mold put away until the piece frees itself by shrinkage. All mold-made pieces re quire fettling, that is, the mold marks are re moved with a knife or sponge. Biscuit ware can be decorated either by painting with a brush or by printing. The former enables the artist to express himselfjust as on canvass, except that his colors will be absolutely per manent. For printing, the design is engraved on a copper plate; an impression on thin paper is obtained from this and transferred to the ware, the paper being afterward washed off. In both cases the colors are mixed with oil, and to remove this the ware is fired in the uhardening ono kiln before the glaze is applied. Biscuit ware may also be dipped in a plain or colored glaze, which simply requires the %lost' fire to complete. The glazing is done by im mersion in a tub of glaze, the dipper holding the piece with the least possible contact with his finger and quickly and dexterously plunges it in the glaze, which covers it all over with a thin film. It is now ready for its final firing. In the biscuit kiln it is immaterial whether one piece touches another, but this would be fatal to glazed pieces, which are separated by means of separators called stilts, spurs, pins, etc., ac cording to how they are used and which are easily removed when the firing is finished. A biscuit kiln will hold from 1,500 to 2,000 dozens of ware and takes from 45 to 50 hours to cook A glost kiln does not take so long, the clay ware requiring a greater heat than the glaze. If the biscuit ware has been glazed and fired it can be decorated in a number of ways, either by painting with the brush, by applying decalcomanies or by printing. The colors used in this case are heavily fluxed and only require a fire of about 8 to 10 hours. The average Centigrade heat for biscuit ware is 1,270°, for the glaze 1,210° and for decorations on the glaze 840°. The heat conditions in the kiln were formerly determined by the color and the withdrawal of certain trials, but small triangular cones made to melt from the lowest to the highest heat are placed in the kiln within view of the fireman and when the heat they represent is reached they bend and melt, show ing that the firing is finished.