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Plaster/Silica Mixtures for Mold Making

This 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 the series.

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 dry ingredients.)

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.

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Copyright 2005 Brad Walker.  All rights reserved.

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Copyright 2005-2006 by M. Bradley Walker.  All rights reserved.

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