This phenomenon called “plaquing” occurs due to non-uniform reactions of gas release during the firing of the clay, which is related to sudden changes in temperature. These vapors or gases form a flat bubble on the face of the pieces in contact with the tile.
To avoid its formation you can take the following precautions:
1) Condition the clay slab well, spread it on the tile avoiding encapsulating the air.
2) Slide a rolling pin over the dough before cutting.
3) Preheat the oven
4) When placing the pieces in the oven, protect them with an aluminum tray to distribute the heat indirectly.
5) Bake twice: once for the time recommended by the manufacturer, cool and return to the oven for an additional 10 minutes.
While these recommendations will help reduce air plates, they will not completely eliminate them. To remove them from your pieces, use different grades of sandpaper and acetone soaked in a microfiber cloth.
You can also work the back of your pieces with different textures that will help to obtain a more professional finish.