19. PRESS MOLDS-Jewelry, Tiles

Materials — plaster of Paris; jewelry clay, or pottery clay

Tools         — cardboard box, or cardboard; gummed paper tape 1" x 2" wide; enamel pan; spoon, metal pallet

It may be desirable to make several copies of ceramic jewelry or tiles for gifts or sale, or for the purpose of achieving identical copies, as in the case of earrings, buttons, or tiles for a table. These are best made from a press mold. The first consideration is to make the model simple and well planned in the interest of good design and construction. Simple geometric forms (Figure 4), animal or floral motifs, present good possibilities. The decoration may be incised lines or flat areas (i.e. a line or shape cut into the surface about 1/16 or more), or modeled in relief (i.e. raised in various places 1/16" or more above the sur­face). The major concern is that the modeling have no undercuts—hollows or projections that catch the mold and break off bits of the model­ing (see drawings right). To insure against un­dercuts, make all incised and raised parts at right angles to the piece, or you can round or slant hollows or projecting parts, if you check them to see that they will allow the mold to slip off without catching any of the modeled parts.

When the piece, jewelry or tile, is modeled, lay it in a cardboard box about 2" high and allowing at least 1" on all sides (Figure 1). Mix plaster and pour to 1" from the top. When it is set (hard) break away the box and tap the mold face down on the table. If the modeled piece does not drop out of the mold, press a piece of fresh clay against it and pull it away. The mold will be the reverse of the original piece (Figure 2).

Figure 1—Lay piece in cardboard box.

Figure 2—The mold is the reverse of the original pic

Figure 3A—Make impression of clip, pin, or ear screw on back of piece before re­moving it from mold.

Figure 3B—Remove the pin with a fresh piece of clay.

Wedge fresh clay, jewelry clay for jewelry, gray or terra cotta clay for tiles (see Introduction, page 12). Press the clay into the mold using slightly more clay than is needed so that the clay stands about ⅛" to ¼" above the mold. Scrape the surface even with a metal pallet. Make an impression of clip, pin, or ear screw on back before removing the piece from the mold (Figure 3A). Tap the mold on the table, face down. If the piece does not drop out, draw it out by forcing a piece of fresh clay against it and exerting a pulling motion at several places (Figure 3B).

The pieces should be left to dry and be fired in a test kiln, decorated with glaze and refired (see Introduction, pages 14-18). The greenware (un-fired pieces), especially the tiles, may be deco­rated with slip, fired once and refired with a transparent glaze. (See Project XV for decora­tion, and Project XVI for finishing backs and applying findings.)

Figure 4—Pottery stamps, Mexico. Photo from American Museum of Natural History, New York.

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