A question has been asked about using tiles in addition to standard kiln shelves to fire glass upon. Yes, you can use the unglazed backs to fire on, assuming they are not ridged or in other ways not a regular surface.
It is important to have flat shelves, as ones with even small shallow depressions can promote bubbles at higher temperatures. Tiles for walls and floors do not need to be flat to do their intended job and so are not checked for be flatness.
|A magnified view of a shelf surface that is not perfectly even|
You can do a quick check for flatness, by placing a ruler on edge across the tile or shelf to see if any light comes through underneath the ruler. The light areas are the places where the surface is lower than the rest. If these are few and small you can make corrections in the surface of the tile by grinding.
You can make sure they are flat by putting two tiles back to back and grinding them together. The initial grind will show you the high spots as they will have the grinding marks there.
You can eliminate these higher areas by rubbing the tiles together with a coarse grit (ca. 80) between the tiles to speed the grinding. If you are concerned about the dust or don’t have good ventilation, you can make a slurry of the grit by adding water. When the whole surface has the same marks, both will be flat. To double check, scribble with a paint marker over one and let it dry. Then add grit between to grind again. When all the paint marks have come off they are both flat on the back.
This sounds time consuming and lots of effort, but you will be surprised at how quickly you can achieve flat smooth surfaces even on larger tiles. This also works for larger kiln shelves.