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What is "chemical water"?

I've learned something that I want to share! For years, I have heard people talk about the importance of venting chemical water out of a kiln while heating a plaster/silica mold during kiln forming processes. But no one has explained it clearly enough until I came across this youtube video about clay. Shout out to Phil Berneburg at Washington Street Studios in West Virginia for actually explaining what the heck chemical water is! I would recommend watching the entirety of the video as it's interesting, but it's thorough. I really like the series. Phil Berneburg has a wealth of knowledge!

In short, chemical water refers to H2O that is present in a chemical compound. In kiln casting/ kiln forming terms, among other compounds you may have present in the investment mold, you're dealing with plaster, or calcium sulfate hemihydrate (CaSO4⋅ 1/2H2O) and silica, or silicon dioxide (SiO2).

When mixed with water, plaster becomes CaSO4⋅ 2H2O, and will release steam, or "chemical water" when reaching temperatures above 325 degrees Fahrenheit. At this temperature, the plaster becomes a anhydrite or anhydrous calcium sulfate. However, the exact temperature that this transition takes place differs based on its conditions. When kiln casting, I tend to always err on the side of being conservative.

Thus, chemical water is different from the moisture imparted into the mold upon mixing it with water. The wetness that you feel in your investment will burn out as steam above 212 degrees Fahrenheit.. but we all knew that!

I hope this helps!


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