Plaster/Silica Mixtures for Mold Making
is part three of a multi-part series on using mixing plaster and
silica for mold making. Click here to go to the beginning of
Start the actual mixing process by adding the plaster/silica
mixture it to the water. (Always add the dry ingredients to the
water, not the water to the dry plaster and silica.) Add the dry
mixture a little at a time. It's not necessary to stir (some
advise that you don't), but it's usually ok to occasionally gently
stir the mixture by hand. Some people use a sifter to evenly spread the dry mixture on
the surface of the water, but simply sprinkling the particles
across the surface will work also.
Continue adding the investment until it "peaks." Peaking
occurs when the mixture sinks slowly and dry investment islands
appear in the container. (If you measured out your ingredients
properly, the mixture should "peak" just as you use the last of the
Once the investment peaks, give the mixture one final stir to
mix in all the dry ingredients. Then stop mixing and allow the
mixture to sit undisturbed for about five minutes. This process,
called "slaking," helps ensure that the investment particles become
saturated with water.
As the mixture slakes, it should become thicker. Watch
carefully until it begins to thicken, then slowly pour it into the
mold box that surrounds your model. To minimize bubbles and
distortion of the model, pour the investment along the edges of the
box, rather than directly on the model. Do not pour leftover mixture
down the sink, as it will harden and ruin your plumbing.
After the mold has air-dried (this may take several days), you
should be able to remove the model and begin the casting process.
Click here for information on speeding up the
mold drying process by kiln (rather than air) drying.
Copyright 2005 Brad Walker.
All rights reserved.