A student, Martha Mixon, recently brought in some slab plates she had been working on using these square cardboard plates that you can buy at the grocery store. My version is in B-mix 10, and carries the impression of a lace doily on half of the plate.
This is not a new idea, of course, and I have used it often with kids' classes, but I had forgotten about it, and the shape is new. I like square plates better than round, anyway, and if I am going to make round plates, the wheel is a better choice. The key to slab work, IMHO, is to keep the slabs nice and thick -- thicker than your first inclination, by a lot. So many slab-built pots -- plates and platter especially - end up feeling flimsy, fragile, and cheap, just because they are too thin.
I press the slab into the cardboard mold using a sock filled with sand, and wrapped in an old bit of nylon hose.
This avoids (mostly) causing lumpy fingerprints that have to be tediously ribbed out afterwards. It's also important, once the plates are leatherhard, to remove them from the molds and lightly paddle the edges, to help prevent cracking.
Thanks Martha! I am thinking of celadon for these; or else copper red in the texture only, and otherwise, just soda: a riskier choice, as my reds have not developed well in the soda kiln. But if it turned out...