Fig. 7. End plates for mold. E, upper plate. E', lower plate. N N, natches.
The plaster vase is laid upon its side on a piece of soft clay and a thin bat or plaster slab is cut to fit the outline. This template should fit with reasonable accuracy but need not be absolutely exact. A pair of these will be required, one to fit each side of the form. These slabs or sheets of plaster are always useful and if a sheet of glass is kept handy any excess of plaster left from a mixing may be poured on to it. This upon setting is easily detached and will present a smooth face where it has rested on the glass. The pair of templates must include, in their outline, both the spare and the foot piece but should not extend beyond either of these. The outside diameter of the mold is now to be determined and the templates cut to this dimension so that the two together, with the vase between them, constitute a longitudinal section of the mold.
The vase must now be divided accurately into two halves by a line running from top to bottom. There are several ways of doing this. While the form is still on the bat a diameter of the bat may be drawn and a perpendicular erected from each end of this diameter. These perpendicular lines will, of course, mark the center of the vase on each side; or after the vase has been cut off another method is possible. With a pair of dividers find the center of both the top and the bottom of the vase. Mark each with a small hole or the point of a pencil. Now lay the vase on its side on the clay cradle upon a glass sheet or other level surface and raise or depress one end until the two centers are exactly the same height from the glass. Take this height in the dividers and, sliding one of the compass legs along the glass, gently scratch the plaster vase with the other or upper point. If the two centers have been accurately adjusted this scratch line will be the exact center of the form.
Some soft clay is now built up on each side of the vase and the templates are pressed down upon it, one on each side until the upper face of each corresponds with the scratched line. The vase is now seen to be buried as to one half in a plaster surface, and plaster poured on this will give a half mold. There is yet, however, nothing to confine the plaster and it would flow away as fast as poured. Two end plates are necessary and these must rise in a half circle above the bed formed by the templates. The part below may be of any shape but must be high enough to cause the diameter of the half circle to coincide with the plane of the templates. Two pieces of cardboard, wood, or rubber belt are now bound to the sides, the apertures at the top and bottom, caused by the curve of the end plates, are stopped with clay and the whole presents the appearance of a vase, only half of which is visible, lying in a shallow trough. All the fitting should be carefully done but the tying up is not yet. The whole is now taken apart and well sized. Vase, foot piece, templates and end pieces are all to be sized thoroughly in the manner described. They are then put together again and bound around with twine. It is necessary now to make provision for the proper fitting of the halves of the mold. This is done by providing knobs and hollows which fit together. These are technically known as "natches" and will be referred to as such. Take two pieces of moderately stiff clay each about the size of a cherry. Roll them into neat balls and cut them in two with a thin knife. Lay each of the halves, flat side down, upon the templates, two on each, placing them in pairs opposite to each other. Affix two or more of these on the inner face of the bottom end plate. Now mix and pour the plaster. This should be poured to the height of the top of the end plates and, after pouring, shake this well down by dipping the fingers into it, so that no bubbles may cling to the surfaces below. As soon as the plaster has become firm but while it is still soft remove the string and the side boards, pull off the pieces of clay and with a straight, thin piece of wood scrape off the surplus of plaster by following the line of the end plates and thus making a half cylinder.
As soon as the plaster has become warm the whole may be turned over and the templates and end plates removed. The four half spheres of clay will be found embedded in the face of the plaster and these, being removed, will leave four hemispherical depressions. The vase can now be gently detached from its bed and the first half of the mold is completed. A little dressing will be necessary. All overhanging edges and rough places should be finished off and the hollow natches smoothed with a piece of muslin on the end of a finger.
The second half is simple. Replace the vase in the half mold, set the foot piece in its place, replace the end plates with the diameter on the line as before but with the semi-circular edges upward, and set two or three clay natches on the bottom one. Size, bind up, pour and scrape off as before, thus completing the two halves of the mold in cylindrical form. It only now remains to make the bottom for, at present, the mold is open at both ends.
The two halves with the vase inside are bound very tightly together with twine and set on the table bottom upwards. The clay natches in the bottom are taken out and the hollows smoothed. The foot piece is taken out and the rough places dressed. The bottom end of the vase is now visible and this, together with the end of the mold, is sized. A strip of stout paper is bound around the mold, projecting about an inch above the end and plaster is poured to fill it. When this is set the paper is peeled off and the edges of the mold are dressed smooth. The bottom may now be detached by inserting a thin knife at the junction, the mold opened and the form taken out. The mold is now in three parts which may be put together at will and used for casting the vase in clay.
CHAPTER VII
Cases and Working Molds
The mold described in the previous chapter is called, technically, a "block mold" and is not, as a rule, used for making the clay ware. The reason for this is that molds will wear out more or less rapidly and to repeat the process of making new ones from the original form would be tedious and expensive.