The next step is the making of a plaster bowl or dish for the purpose of drying out slip or glaze. A convenient size should be determined upon as it is best to have all the dishes the same. Upon any flat, smooth surface a mound of clay is reared which shall be the size and depth of the inside of the proposed dish. About twelve inches in diameter and three inches deep is a good size, though fourteen inches is not too large for the former dimension. This mound should be made as nearly circular as possible and the clay finished as smoothly as may be. The rubber belt is then set around the mound in the form of a hoop leaving a space of two inches between the clay mound and the rubber hoop. The rubber is fastened either by tying with string or by binding the overlapping ends with clothes pins. A roll of soft clay is laid down where the belt joins the table and pressed down outside to prevent leakage. Enough plaster to fill the space within the belt is now mixed and poured, covering the clay mound to a depth of at least one inch. When the plaster has set the rubber is detached, the whole turned over and the clay dug out. We have now a circular plaster dish three inches deep but we have only one. The trouble of rebuilding the clay is unnecessary a second time because a "case" or reverse can be made from which as many dishes as may be necessary can be formed.
Fig. 2. C, plaster dish. D, rubber belt. E, plaster case or reverse.
Fig. 3. Plaster case, with rubber belt, arranged for pouring.
The dish is carefully smoothed and trimmed. The sharp edge is removed and the inside is dressed with fine sandpaper to a perfectly smooth surface. Size is now applied to the inside and upper edge until a bright slippery surface is obtained. The rubber belt is now bound closely around the dish and plaster is poured to a depth of about one and one-half inches on the edge. This, of course, makes a depth of four and one-half inches in the center. When this new plaster has set in turn the rubber is removed and the two castings can be easily separated by inserting a knife at the junction. The knife should be gently driven in with a hammer. Obviously it is now possible to make a number of dishes from the reverse thus obtained, by simply binding the rubber belt around each time and pouring plaster as at first. The original mold having been sized is no longer absorbent but must be kept in case additional reverses are needed. The molds or dishes must be thoroughly dried out before being used.
The molding of a vase form is more elaborate but not really difficult. Even if one does not intend to produce pottery by molding there is always an advantage in having a number of simple forms upon which to make experiments.
The vase to be molded is first drawn to exact size upon paper and a plaster model is turned on a lathe. This can be done equally well on the potters' wheel and the method is as follows: A plaster bat is saturated with water and set upon the wheel so as to run true when the wheel is revolved, and is cemented to the wheel head by a little slip. A few deep scratches are made on the face of the bat and a cylinder, either of the rubber belt or of stiff paper, is rolled up and set on end in the center of the bat. The size of the cylinder should be a little larger every way than the proposed vase. Plaster is now mixed and poured to fill the cylinder. It will adhere to the bat below by reason of the scratches. When the plaster has set, the cylinder is unfastened and removed and the turning may begin. To turn plaster well involves a good deal of practice but it is better to spoil three or four plaster cores in the learning than to spend a long time on one for fear of damaging it.