Posted 16 February 2013 - 09:18 PM
An update on results of several ideas presented here, and others - :
1. Changed to a heavily grogged clay, Std 420 and Std 547 (red). The 547 seems pretty good, but have not found a glaze that looks decent on 420. Amazing shades of ugly so far. Don't like the huge sand particles, tricky to smooth edges and backs or cut hanger-hole at just the right time in the drying process when still damp enough to smooth, but firm enough to move. Really would prefer a less coarse grog.
2. Made sure when rolling out clay to have minimal movement under the fabric I was using (bedsheet cotton). I believe the surface fibers of clay were adhering to the fabric while the deeper layers were getting rolled unevenly. I'm now beginning with a slab not much thicker (1/4") than the desired thickness (3/8"), roll out, pull fabric away and flip slab/roll a couple of times so there is no 'drag' on the surface from the fabric. Using dowels for thickness guides and kitchen rolling pin. Will consider going to a local studio to use the slab roller again if I have a big order.
3. Moving, flipping slab by sliding a piece of luan plywood under the fabric and turning without distorting the slab.
4. Drying on an open wire rack with no plastic or covering. Even leaving wet tiles on the damp sheeting on the wire rack influences drying/warping.
5. The drop method suggested was not useful, at least not with wet clay. The edges slumped making an uneven thickness between center and edges of tile. Did not try after clay had firmed up a little.
6. Changed to a pizza cutter instead of fettling knife. Less distortion and drag on wet clay when cutting out tiles.
7. Tried the 'score and break apart later' method, which worked very nicely, but with the heavy grog in the clay it was too difficult to smooth the edges once dry.
8. Firing on tile racks (large size to accommodate larger thickness), and found a sale on them!
As a result of these changes (and I believe primarily #1, 2 and 4), I have only 10-15% warping, vs >60%. Better, but I still do not like the clay bodies (too gritty), and have not found a good glaze for 420. Hopefully someone can benefit from my results, and suggestions are always welcome! Thanks so much to this most generous community of artists!