Kathyv
I have an old wooden shortbread mould that I use. I estimate it has been in use since at least 1900 by various women in my family! It is a round of wood with a recessed thistle in the middle and a kind of 'v' shaped circle around the outside. The wood has darkened over the years, but I think it started life as Scots pine! I recall seeing similar shortbread moulds in the Scotch House in Kensington, many years ago.
I find that the secret is to lightly dust the mould with a small amount of sifted plain flour (or cornflour) then a sharp tap to remove almost ALL the flour. This seems to help the mould 'release' the very buttery dough.. I never make less than 2 or 3 'rounds' of shortbread at a time, and I have a pefectly 'clean' mould for all three.
I would think it MIGHT work with stone or pottery moulds, too. Let me know if it works!