Overfired glass or glaze can embed into the surface of a bare kiln shelf. If the shelf has been coated with kiln wash, you can usually pop off the contaminant without damaging the shelf.
Place the shelf on a surface that has been cleared of all debris, such as a flat sheet of cardboard on a table. Wearing clear safety glasses, hold a putty knife at a sharp angle against the edge of the glaze or glass. Lightly tap the putty knife handle to dislodge the contaminant. It should come right off along with the kiln wash underneath. Recoat the shelf.
If the contaminant has penetrated into the shelf, removing the contaminant may leave a gouge in the shelf. You can sometimes smoothen a small gouge with a heavy application of kiln wash. After it dries, sand the kiln wash. This is only a temporary fix. If you turn the shelf over to fire on the other side, first remove all the contaminant. Otherwise it could melt during the next firing and drip onto ware below.
For future firings, make sure the kiln wash on the shelf is thick enough to protect the shelves. Glaze and glass can work through a thin coating. And, of course, monitor the kiln to avoid an overfire. The higher the temperature, the deeper the glaze or glass can penetrate a shelf.
