After playing around with all the variables trying to get level tiles, I found what turned out to be a pretty simple solution for this particular piece. The slipcast tiles weren’t really warping too badly, so all they needed was a bit of extra weigh to hold the thinner corners down.
It turned out the these small magnets were perfect for holding down the corners. They weren’t heavy enough to damage the leatherhard clay, but they were enough to keep the tile from warping.
I moved the tiles to my workbench to make them more visible. Normally they dry on a sheet of drywall, but the gray color made them hard to see in photos. I was tempted to use a cookie sheet so the magnets really held, but it turned out that the weight alone was enough.
I knew when I had to buy a package of 50 magnets just to get the two my son needed for a project that I’d find some use for the rest. I just never imagined they might be useful in the studio!