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Plaster can be very useful for making molds and stamps. And it is actually quite easy. The beauty of plaster is that it is porous, so clay doesn't stick to it. Here are some tips for mixing and using plaster.
There are almost as many methods for mixing plaster as there are potters who use them! I will tell you one method I recently learned which works quite well.
Buy plaster. Pottery Plaster #1 is best because the particle sizes are small and will capture detail the best. But if you can't get this, normal Plaster of Paris will work.
Put on a dust mask.
Add water to a bucket. It should be between cool and room temperature. You will learn through trial and error how much water to use, but most people tend to underestimate the amount of water needed. If you want to avoid cleanup, line your bucket with a plastic bag, then discard the whole thing at the end.
Using a cup or scoop, start adding plaster to the water. Sprinkle the plaster out evenly over the whole surface of the water. You don't have to be in a big hurry, but don't work too slowly or the plaster will start to set. Some people prefer to sift the plaster into the water. At first the plaster will sit on the surface of the water very briefly before it disappears beneath the surface of the water. (This is called "slaking".)
Keep adding plaster, sprinkling it in the same manner. After a while the plaster will be visible for a few seconds before it disappears into the water. You are getting close... Keep adding plaster. When you add plaster and can count to 5 before it disappears, you have enough. Let it sit for 1-2 minutes to wet the plaster particles (this helps reduce air bubbles).
Mix. You want to make sure that you aren't adding air to your plaster, or you will get bubbles which will cause problems later. So if you use an electric mixer make sure to keep the blade deep in the plaster. Or, just take your hand (a glove is useful) and place it at the bottom of the bucket, and slowly move your hand back and forth across the bottom of the bucket. This slowly wets all the particles.
When you can draw a line in the plaster and it doesn't immediately flatten back out, the plaster is ready to pour.
The goal in pouring the plaster is to avoid introducing air pockets, particularly at the surface that you are going to be using.
Some people allow the stream of plaster to run off the palm of their hand, thus slowing it down.
It is helpful to apply a thin coating all over the surface and allow it to set a little before pouring the final amount necessary for the mold. If the air bubbles come to the top of the thin surface, they will be away from the actual surface you will be using. (This is especially useful if you are pouring upside down, i.e. will use the bottom surface of the mold you are pouring.)
After pouring, pick up your container and softly tap it on the table or the floor. Or if it is too big, tap the edges and shake from side to side. You are trying to get air bubbles to the surface.
And perhaps the ultimate trick. Use rubbing or denatured alcohol in a spray bottle (some people dilute, some don't). When the plaster mixing is almost complete, spray a couple bursts into the plaster. This breaks down the surface tension of bubbles on top and they disappear. After pouring you can repeat this when there are bubbles that have risen to the top of the mold. (This is especially useful if you will be using the top surface of the mold you are pouring.)
Smooth the top of your plaster item with a rib or something like that, and let it set. Clean your bucket and tools at this time.
If you are pouring a very large mold, or for some reason didn't mix enough plaster in the first batch, immediately start mixing the second batch. When it is ready you can pour it over the existing plaster. Scratch crisscross lines into the top of the first layer before pouring the second layer to help them adhere together.
There will be a point where the plaster is hard, but still wet enough to easily carve. This is a good time to put the item on your wheel and trim it, or trim off the rough edges by hand, or start carving your designs.
It can take many months for a large plaster mold to completely dry out, so it will continue to get lighter as that happens. But it is usable immediately. If using the same piece of plaster over and over, it will eventually become too wet to release the clay. Let it dry and it will again work fine. If you can't wait for the plaster to dry, you can sprinkle with talc.
Here is a beautiful thing about plaster. If you ever want to carve into it later, you can soak it in water, re-wet it, and it will become soft enough to carve into. You can do this at any time.
Bags of plaster should be kept in a dry place. If they absorb a lot of water in the bag, they set up much faster and are more difficult to work with. If your plaster has absorbed water, you can refresh it by heating it to around 700 deg. F. to remove the moisture.
There are actual suggested ratios of plaster to water for various plaster mixes, and some people use those. But most people I know use some rule of thumb similar to the one here to achieve the correct consistency.
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