20. TWO-PIECE MOLD

Materials — plaster of Paris; clay slip; small amount of clay

Tools —cardboard, or cardboard box; gummed brown paper tape 1" or 2" wide; funnel; melon scoop; enamel pan; spoon; string; knife; vaseline, grease, or soap; brush

Whenever several copies of the same object are desired, it is practical to cast them in a mold. This is also the case when a piece must be hollow, as salt and pepper shakers, or banks. The mold is a construction of plaster (or other material) which receives the impression of the form to be cast, and by means of which identical copies may be reproduced. Casting can be a very complex and difficult process, often requiring a great number of pieces carefully fitted together, es­pecially when intricately modeled sculpture is cast, but for the beginner, a very simple ceramic casting process, requiring only two pieces, is outlined.

It is suggested that the beginner model a piece especially for casting so that he will have no difficulty in making the mold. Try a simple animal form such as a rabbit, pig, elephant, or cat. One that can be conceived as a simple mass with the minimum of modeling and projections will serve best (see Project V). The principal rule in making a piece to be cast is that it have no undercuts or perforations. An undercut is a hollow or a projection which catches the plaster and will not allow the mold to slip off without injuring the cast piece or the mold itself.

Choose an animal or figure whose mass is solid-no projecting arms or legs—and model, finishing the forms and surface with great care. The mold is to be complete in two pieces, which means that you may plan it in one of three ways: top and bottom (Sketch 1), for example, rabbit that is basically flat and low; front and back (Sketch 2), for example, a figure or animal sitting up­right; two sides (Sketch 3), for example, a hippo, bird, or crouching figure. The primary concern is to have something essentially solid on which the distinguishing forms are actually modeled in relief. Be sure that the bottom edges are clean-cut, that the bottom itself is level and even; do not hollow the figure.

Figure 1—Draw halfway line and place coil under line, press firmly onto the object.

Start the mold when the clay is firm, not dry. A top and bottom mold made around a rabbit form is described. Proceed in the following steps, com­pare description with photographs of mold for the rabbit to clarify process.

1. Draw lightly on the surface with a pointed tool the exact halfway division of the form, the point at which the form either goes up and over, or down and under (Figure 1). In general pre­pare to cast most intricate side first.
2. Roll a neat, even coil about ¼" in diameter. If possible use a clay of contrasting color so that the meeting line of object and wall is clear and sharp. Do not use sculpture clay. Place this coil on the under side of the halfway line and press firmly onto the object. Trim so that the clay meets the halfway line at a perfect right angle (Figure 1). This first coil must be applied very carefully, the remaining coils must be applied neatly, but with less precision. Continue adding coils until a rectangular wall blocks off the lower half of the object (Figure 2). This wall must be at least 1" from the farthest projection of the piece on all sides. Smooth each coil into the preceding one on the side on which the halfway line is visible. The under side does not need to be smoothed. When and if the walls become unstable, brace under­neath with coils ½" to 1" in diameter. Do not work into clay figure.

Figure 2-Add coils until the lower half of the object is blocked off.

Figure 3—Build a cardboard enclosure around the object.

3. Now one side is ready for casting. First, how­ ever, an enclosure which fits snugly about the clay rectangle must be made, and the enclosure must rise 2" above the highest projection of the object (Figure 3). A corrugated cardboard box is usable if the area to be cast is no larger than 10" x 5" x 5". If it is larger, a wood frame should
   be constructed. In casting the rabbit, a card­ board box is amply strong. One corner is care­ fully sealed with gummed paper, the box pro­vides two sides, the other two are specially made. All corners, inside and out, and bottom edges, are carefully sealed with clay. For additional
   support, brace the exterior with bricks.
4. Mix plaster (see Project XVIII). A volume of water equal to or slightly more than the volume to be filled is adequate. The cardboard enclosure should be filled to at least 1" above the highest point of the object. If any plaster is left after pouring, additional bats may be made.
5. When the plaster is set—15-30 minutes—re­move the cardboard enclosure, then the clay wall from the plaster and object. For the sake of good craftsmanship, you can trim and smooth the outside of the mold with scraper and knife.
6. On the surface of the plaster from which the wall was removed, two or three "keys" must be made (Figure 4A). These are hollow, half-round holes which serve to hold the two pieces of the mold in place when casting with clay slip. A melon scoop is ideal. Hollows no deeper than a 1/2" hemisphere, and at least ½" from the ob­ject, should be scooped out. Beware of under­ cuts when scooping.
7. This same surface must now be soaped or greased to prevent the next side of the mold to be cast from sticking to the first. Apply carefully, thoroughly, and evenly, vaseline or a  heavy, dark, liquid soap (English Crown) used especially by mold makers (Figure 4B). Remove any excess vaseline or soap bubbles.
8. Place a whole coil, ½" to 1" in diameter, in the center bottom of animal (Figure 4C). This coil later  provides  a  hole into which  the  slip  is poured.
9. If possible, use the first cardboard enclosure, or make a new one. Be sure to have enclosure 2" higher than highest projection of bottom of ani­mal.

Figure 4—      A. Make "keys" with melon scoop.      B. Apply vaseline or soap to inside surface. C. Place a coil in center bottom of object.

10. Repeat Step 4.
11. Allow plaster to set for one hour. Enclosure may be removed sooner. Tap the side that was cast first lightly with a mallet until the two pieces of the mold separate. When the original model has been removed, the inside of the mold should be clean and free of clay (Figure 5). If bits of clay adhere to the mold, it may reveal the pres­ence of undercuts. These should be discovered and corrected by carving them with a knife. Do not touch the surfaces where the two pieces of mold come together. Leave the mold open on a radiator, or in some other warm place, over­ night.

You are now ready to make a slip casting. Fit the mold together by matching the "keys." Cut notches on top and bottom edges to hold string in place (Figure 6A). Do not allow string to cross over hole. Tie the cast tightly together and place the mold on the table, hole side up. Mix a quan­tity of slip, from dry clay, to the consistency of molasses (Figure 6B), (see Introduction, page 14). Strain it through a screen, 36 holes to the inch. Place funnel in hole and pour slip until slip is even with the top of the hole. Remove funnel im­mediately. The slip will sink in a few minutes as the water is absorbed by the plaster. Continue adding more slip until a wall of about 1/8" forms at the top edge of the hole. Pour out the excess slip. Remove clay wall from hole with a knife. Wait for twenty minutes to an hour, or until the clay starts to shrink away from the mold, then separate the pieces of the mold carefully. You should have a perfect reproduction of your or­iginal sculpture. The slight ridges where the two pieces of the mold meet can be scraped and sponged when the piece is firm. Repeat the proc­ess until you have as many casts as you wish.

The cast forms are ready for firing, and after the first firing, for glazing. You can decorate them with slip if you wish before the first firing (see Introduction, page 14). Cast pieces can be fired in a test kiln (see Introduction, page 17).

Figure 5-Separate meld and clean in­side.

Figure 6— A. Cut notches on edges and tie with string. B. Mix slip and pour into hole of cast through a funnel.