Posted 25 January 2013 - 11:27 AM
Ball clay would add plasticity, but here's the problem: By adding raw clay- fire clay, ball clay, whatever- you're changing the formula of the clay so that the proportion of clay to feldspar to silica gets out of whack and the clay may not fire as tight as it should. This is a bit less of an issue with cone 10 clays, but can be a big problem with cone 6 and low fire clays. When I worked as a tech for a clay supplier, I regularly received calls from teachers who suddenly had major problems with their glazes. They had been adding ball clay every time they recycled their low fire white clay, and after a few rounds the clay formula was all messed up and their glazes started shivering. No good way to fix that, either. The clay had to be replaced.
SO, if you're going to add raw clay to your mix, if it's a cone 10 stoneware clay I would also add about 5% feldspar and 5% flint. For cone 6 clay go with 10%. When working with one clay body, the best thing to do is either buy some dry mix of that same body, or even easier just keep a bucket of dry trimmings to mix in with the slop.
Kiln Repair Tech
L&L Kilns Distributor
Owner, Neil Estrick Gallery, LLC