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A simple test to see shrinkage rate may be to make a quick clay ruler. Roll
out a simple slab of each type of clay and then make inch measurement marks
on the two different clays. You will be able to tell how much they shrink
when they get to the bone dry stage very quickly. This is a good indicator
of their compatibility.
In the past, I have gotten away with mixing different clays that would
normally not work together at the low fire temperatures. This is often the
case with application of slips too.
If you are not firing the clay up to its vitrification point you often have
more leeway with the piece.