I recently received a commission to make a fish plate as a leaving present for someone working for the Marine Stewardship Council. The order was for the fish part of their logo to be included, as large as possible, with the rim ends higher than the sides.
I made some test pieces with the fish impressed and in relief, experimenting with several clays and glazes. I emailed photographs of the test pieces, and the impressed design on porcelain was chosen for the plate. I explained that the kiln dimensions limited the length of the plate; the maximum is 16 inches.
I made a hump mould of the oval using a 16-inch long oval plastic bowl from the pound shop. Next, I made a cardboard copy of the fish logo and rolled it into the slab of clay, before laying it onto the mould. Then I allowed it to dry slowly. When it was stiff (leatherhard), I removed it from the mould and worked on the rim with a surform to create the higher ends and lower sides.
I made three test plates in porcelain using this method, one out of white St. Thomas, and a couple of white earthenware as last resort backups. I was nervous as I had never made anything so long and flat, and suspected they could crack easily during the firings. The impressed design meant the base thickness was uneven.
I dried the plates very slowly and transported them with the help of a friend with a car to a large kiln. I used plenty of alumina powder rolled flat on the kiln shelves to allow for the shrinkage that happens when clay is fired. (This is due to the molecules of clay that lie in plates which slide over each other in the raw state, moving so close together that they join up during the firing, a process known as vitrification).
The first firing was 100% successful, but during the glaze firing, one of the porcelain plates cracked. Possibly because of firing too soon after glazing, and not allowing sufficient time for the glaze to dry thoroughly, so then the sides collapsed on one another. Fortunately, one came out successfully!