Glaze is baked onto terracotta tiles at extremely high temperatures, until it is bonded to the top layer of tile. The protective glaze coating makes terracotta tiles resistant to water damage and staining. However, if you are planning to repaint your terracotta tiles, it is a good idea to remove the glaze first, as paint will adhere better to a porous surface than it will to an impermeable glazed surface. You may also prefer the more rustic look of unglazed tile for a garden or other area where water-resistance is not important.
Sweep the tile clean of debris, and fit a palm-sander with 150-grit sandpaper. Put on a dust mask and protective work gloves.
Sand the tile with the palm sander, holding the sander flat against the surface of the tile. Begin sanding in one corner of each tile, and work the sander in a circular motion around the tile. To avoid burn marks, do not press hard on the sander; allow the weight of the sander to do the work.
Vacuum the tile clean with a shop-vac or handheld vacuum after the initial sanding. Check the sandpaper, and replace if it worn.
Repeat the sanding and vacuuming process until the glaze is removed. Thoroughly sweep the work area, and carefully vacuum any remaining debris before continuing.
Put on a pair of rubber gloves, and wash the deglazed tile with a trisodium phosphate cleaning solution. Dip the sponge into the solution and wipe the tile until it is free of dirt, oil and sandpaper residue.
Prime the prepared tile with bonding primer before repainting. If you prefer the look of raw terracotta, install the deglazed tile in a rustic setting without painting.
De-glazed tile is not water resistant, so mould and rot may develop on the tile. De-glazed tile is not appropriate for installation in a kitchen or bathroom.