Monday, May 13, 2013

May 12th Hint of the Week

Making Your Own Press Molds

from the Clay Times Pottery Forming Techniques

·       Handbuilding of similar items is easily accomplished with the use of press molds of various shapes and sizes. But you needn't purchase them ­ it's relatively easy and much more affordable to learn a few simple steps toward making your own mold.

·       By far the easiest method is to make your large, rounded molds by throwing them on the potter's wheel, then firing to bisque at cone 010 for molds that are sturdy, yet porous and absorbent enough to release your hand-built forms as they dry. The molds should be at least 1/2 in. thick, however, to absorb moisture sufficiently. This technique may be used to make as many molds as you wish, in any size you're physically able to throw or build and fit into your kiln. In case of eventual breakage, it's a good idea to make at least two molds of each shape/size so that you have a back-up handy for last-minute projects, in case of breakage.
·       To make a plaster mold, you'll need pottery plaster from your local supplier or a bag of plaster of Paris (from the hardware store), water, a large bucket for mixing, a jar of Vaseline, a flexible rib, and the form to mold, or "model."
·        Start by placing the model on a flat, clean surface, and setting a clay coil around it, roughly 1/2 in. from the form. Next, apply a heavy coat of Vaseline to the mold, the table surface, and the coil. Mix the powdered plaster with water as instructed on the plaster bag and stir thoroughly, then apply to the model and coil when creamy, at a thickness of at least 1/2 inch. Do not allow the plaster to extend beyond the clay coil. As the plaster sets, scrape it smooth with a flexible rib. Once it has hardened and dried (usually about 30 minutes or so), it can be removed from the model and coil, and sanded on both sides. Let set for about 24 to 48 hours to fully dry. The interior may now be used as a "slump" mold; the exterior, as a "hump" mold. The ridge left by the clay coil offers a useful ledge for lifting the mold.
a.      To use the mold, drape with a soft clay slab, then remove the slab when leather hard; trim, decorate, and fire as usual. Be careful, however, not to let any plaster chips remain in the clay, as they will cause large pop-outs during firing. When cleaning up, let plaster dry and put in the garbage ­ don't rinse down the drain, or it will clog the pipes.

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