Smooth ceramic tiles are tricky to deal with when it comes to resist thickness since it is hard to anticipate the thickness of the glaze that you have to blast through. My recommendation is always to get a few extra tiles to try blasting them with the standard photoresist thickness. If that fails, you know you will have to switch to the heavier material. Blasting pressure is between 30-40 pounds of pressure, depending on how deep you want to go and how detailed the design is; the more detail, the less pressure you want to select. Ceramic tiles always give you a two-tone effect: the color of the glaze and the color of the ceramic exposed through the blasting. Color-filling is not necessary, but optional.
—Ruth Dobbins, Your Professional Glass Consultant