Making a Plaster Mold
This section demonstrates a few techniques on working with clay to construct plaster molds. If you have a one-of-a-kind piece you would like to make, this is a simple, convenient approach. Its only for one-of-a-kind pieces since the mold is destroyed when its removed. The outline here notes the steps taken to create the mold for a vertical, west-facing sundial.
TOPSince the dial needs to be flat I use a piece of dowell (a rolling pin works fine too) and some 1-inch spacers to give a smooth surface and uniform thickness. Just keep adding clay until you have the piece the size you need it to be. The operation is performed on a flat surface covered with a piece of fabric...the cloth makes it easy to lift the piece up and turn it over when the time comes to remove the clay from the plaster mold.
NOTE: The clay you use doesn't have to be anything special. From your local ceramic supplier you can pick up 50 pounds of moist earthenware fairly inexpensively (they might even have some "mixed cone" stuff, which should be even cheaper because it doesn't have a well-defined firing range). I used some 50 mesh fireclay simply because I had some sitting around.
TOPTake your pattern (here, the sundial hour and calendar lines were drawn on a sheet of paper beforehand) and lay it over the clay. Using a knife you can trim the excess clay from the perimeter and using a pencil one can trace the pattern, applying just enough pressure to leave a slight impression on the underlying clay.
TOPUsing a stylis then just go over the pattern and increase the depth of the design pattern to whatever depth you choose. A knife with a blunt rounded edge works well for long, relatively straight lines. For smaller details I just use a piece of tie-wire (i.e. "baling wire") bent into a small loop. Use whatever is handy and experiment with what you've got. Around the perimeter, just use your fingers to smooth the edges.
The image here is of the finished model. I free-handed some zodiac symbols to indicate the time of year and some roman numerals to indicate the time of day, and a simple sunface just because I thought it needed something "extra".
