What is the effect of using different crushed rock when making clay?

by Flip
(Chewelah WA)

Question: I was making clay that I dug up on my uncles farm but I mixed it too wet. The problem is that it is winter here and I did not have any easy access to dry sand. I had some granite with hornblende in it and I crushed it up and screened it and added it to my clay mix. It worked like sand and the clay came out okay.

I was wondering if I crushed up other kinds of rock like green serpentine or black marble or quartz and used that as sand or just an additive to clay, just what would be the result?

Answer: I haven't used any of those additives, so I can't say for sure. I've heard that quartz expands a lot in the firing and therefore wouldn't be the best choice.

The best way to find out which one would work the best is to do small samples and test fire them. Then you can compare the shrinkage and firing results and use the one that works the best.

Those are the joys and frustrations of working with homemade clay!

Hope that helps,
Steve

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