View Larger Image In today's video, Holly Goring demonstrates how to make a one-piece plaster mold.

September 26, 2008

Video Tip of the Week: Making a One-Piece Plaster Mold

by Holly Goring | Read Comments (14)

1. Make a form with no undercuts. Make a simple form out of clay with thicker walls than a finished piece would. The extra mass will help the form stay put when plaster is poured in and  will allow it so soak up some of the water in the plaster without collapsing. Your form must have no undercuts. An easy way to check for undercuts is to place the form upside down on a flat surface, look at it from above and run a finger up the side. If at any point the tip of your finger goes behind part of the form, it is undercut and will not release from a mold. Let the form get leather hard so it will be sturdy enough to have a mold made from it.

2. Prepare the mold on a smooth surface. A laminate (Formica) counter top provides a nice smooth surface for pouring plaster molds. It makes the top of the mold nice and even, and it releases from the plaster without the need for mold soap.

3. Prepare the form and cottle boards. Wet the lip of your piece and adhere it upside down to the laminate work surface, then clamp the cottle boards together around the form, leaving about 1 inch on all sides between the form and the boards. Seal all joints with clay coils and press firmly to ensure that no plaster will leak out of the form. Seal all porous surfaces with mold soap (you do not have to soap the clay positive.
 
4. Mix the plaster. See detailed instructions below, or go to the video archives to check out  Holly's "Plaster Mixing 101" video.

5. Pour the plaster. After mixing, tap the bucket on a hard surface to release trapped air. Pour the plaster carefully. Wherever possible, pour plaster carefuly into the deepest area so the slurry flows evenly across the surface of the mold.

6. Let the mold cure and dry. When plaster sets, it heats up because of a chemical reaction. When it has cooled, it is safe to remove the cottles or forms—about 45 minutes to an hour after pouring. Molds must be dry before use. Drying molds properly promotes good strength development, uniform absorption and reduced efflorescence. Dry molds evenly. Don’t set them near a kiln where one side is exposed to excessive heat or the relative humidity is near zero. Place them on racks in a relatively dry location away from drafts.

Use fresh water. The mixing water you use should be at room temperature or 70°F (21°C). If the water is too warm, the plaster will set too fast and vice versa. Use only clean, drinkable tap water or distilled water. Metallic salts, such as aluminum sulfate, can accelerate the setting time, and soluble salts can cause efflorescence on the mold surface.

Use fresh plaster. Plaster is calcined, meaning chemically bound water has been driven off through heating. If the plaster has been sitting around in a damp environment, it will have lumps in it, in which case it is no longer usable. Pitch it. Use plaster that has been stored dry and is lump free.

Weigh out materials. Do not guess about the amounts of plaster and water you’ll need. Once you start the mixing process, you do not want to go back and adjust quantities. To determine the amount you need, estimate the volume in cubic inches then divide by 231 to give gallons or by 58 to give quarts. Deduct 20% to allow for the volume of plaster, then refer to the table.

Add plaster to water. Slowly sift the plaster onto the surface of the water. Do not dump the plaster or toss it in by handfuls. Adding the plaster shouldn’t take more than 3 minutes.

You can purchase a drill  attachment for mixing plaster in the paint section of any hardware store.

Soak the plaster. Allow the plaster to soak for 1–2 minutes maximum. The soaking allows each plaster crystal to be completely surrounded by water and it removes air from the mix. Small batches require less soaking than large batches. If the soaking time is too short, it may contribute to pinholes; and if it is too long, it will contribute to fast set times, early stiffening and gritty mold surfaces.

Mix the plaster. Small batches of plaster can be mixed by hand. Use a constant motion with your hand and you will notice a change in consistency from watery to a thick cream. Breakdown lumps with your fingers as you mix. Mix only for a minute or two being very careful not to agitate the mixture so much that air bubbles are incorporated into the mix. Mixing time affects absorption rates—longer mixing times produce tighter and less-absorptive molds.