Several ways to do repairs.
Make a paste of kiln wash and push it into the divot. lightly sand to smooth. It will have to be re-done periodically.
Make a paste of cement fondu, smooth it while wet, because it gets hard when cured. Cure by firing in kinl to about 720C
Make a paste of cement fondu and vermiculite. Treat it the same way as cement fondu on its own. This combination holds less heat than cement fondu on its own.
Stephen Richard
blogs at: http://www.verrier-glass.blogspot.com/ and http://www.glasstips.blogspot.com/