If you need to make only 6-12 tiles for a custom backsplash, you might opt to just sculpt them directly as a tessellated bas-relief or separate unique tiles, but what if you need 60 -- or 500? Or a variety of different designs within field tile? Enter the press mold.
Usually made of plaster, (although you could make it from clay and bisque-fire it if you figured out the shrinkage rates for both bisques), the press mould/ mold makes it easy for you to make replicas of your original tile design in no time flat.
I made a short video in which you can see the process, step-by-step. It's really quite easy. The hardest part might be figuring out how much plaster to add to the water. There are tables you can find online where the water is measures in quarts or liters and the plaster is measured by weight, but the simplest method is the "island method," which is just basically adding plaster to the water until the islands of plaster sprinkled on the surface take more than 10 seconds to slake. When this happens, the water has become saturated with enough plaster to do its job.
The wooden cottles (the sides which make the "well") should be wide enough to allow 1-1.5" of poured plaster over the highest part of your tile or work. This gives the mold strength so that when you put pressure on it later it doesn't just crack in half. The cottles are clamped together and can be adjusted to any size tile or mold needed.
Have fun making your own molds for tiles, sprigs, applique work, or anything you can think of!