Originally Posted by The Forge
Do you use a gas kiln to fire the pieces? After you do the initial firing, refire the piece using a 'high temperature reduction'. That usually reduces the piece by 40% and gives a dark color to the clay.
[Have you ever actually done this, or is it something you just heard about? I've done a lot of ceramics and I don't know of any clay that shrinks 40%, no matter what the atmosphere in the kiln is like. And while refiring can cause a clay sculpture to crack, it doesn't normally make it shrink further.
"High temperature reduction" refers to an atmosphere that has a lot of carbon in it, due to the presence of unburned fuel gas. This will darken (unglazed) clay bodies and cause metallic oxides in the glazes to revert to a metallic (de-oxidized) state, but it's not associated (as far as I know) with any further reduction in size, more than the normal 10-15% shrinkage that is normal between the raw and high-fired stages of ceramics.
But I'm always willing to learn new things - do you have any sources for this I can look up, or maybe some before-and-after pictures with yardsticks to show the rate of shrinkage?]