To begin making flat tiles you need to use a heavily grogged clay formulated for sculpture or tile – not a plastic throwing clay. I like my clay on the dry, stiff side as too much water makes it dry slowly and promotes warping.
Most of my tiles are press molded in plaster molds, but if you don’t use molds for your tiles, just roll out clay slabs directly onto a piece of drywall (drywall makes a great work surface – just make sure to seal all of the drywall edges with duct tape to contain that nasty drywall dust) using wooden spacers or dowels beneath the rolling pin for the desired thickness. I prefer half-inch-thick tiles.
Once you have rolled out the clay slabs, don’t move, lift or turn them. If you do move the clay, its “plastic memory” will kick in and it may warp, bend, or curl during drying and firing. Just trim the slabs in place, cutting them to the desired dimensions using a trimming knife and your pattern. After trimming, it is very important to allow the wet tiles to sit on the drywall for 8 to 12 hours (overnight is usually good). Drywall sucks a lot of water out of the clay and the tiles will really stiffen up.
By the next day the tiles should be pretty close to leather hard and stiff enough to handle without flexing. Test a tile to see if it can be picked up safely. At this point, trim and smooth the edges. If you wish to incise or decorate the green tile in any way, now is the time to do it. There is no need to score the backs of tiles unless you want to. Scoring has nothing to do with the warping or drying process, but it helps the tile adhesive cling to the tile and hold it to the wall or floor during installation. I only score my tiles if I know the customer wants them for an installation.
Once the tile is trimmed, place it directly onto a rigid metal storage rack. Because air circulates on all sides of the tile, it dries very evenly and no warping occurs. While your tiles dry, avoid direct sources of warm air like a register vent or portable heater that might dry one area faster than another. You want even drying from top and bottom. I keep tiles on the rack until they are completely dry and ready to bisque.
You should only handle your green tiles about three times: once to roll out and cut the clay; once to smooth the edges and place on a drying rack; and once to put it in a kiln for your bisque firing.