Is it True you can not pour porcelain slip in a mold that has used ceramic slip ? Why is this?

Answer 1

If a mold is clean and dry, you could probably use it for either, but there would be two issues I would worry about. The first is that any soluble fluxes like soda have not soaked into the plaster.

The second problem is much greater and that is that the low temp talc ceramic is the same color texture etc as the porcelain but do very different things at normal porcelain firing temperatures.

I have seen a piece of talc that was mistakenly thought to be porcelain that was fired to that temp. It melted and ran (fortunately) into a cup below looking like the cup was full of creamy coffee! If the smallest bit of talc got into the porcelain it would ruin not only the piece but possibly the kiln or kiln furniture as well.

Basically I would never allow both to be in the same business, or building because of the problems of telling them apart.

In general in any Clay studio all the clays used should be the same firing range in any case just on general principals.