I fix that common problem with cobs using super glue gel mixed with some dust from a sacrificial cob. 20150417_095432.jpg
I also being very well versed in the lore of fixing places of fire use Water Glass (Sodium Silicate) and food grade charcoal to make my coatings and pipe mud. The Germans invented it a century ago to line pipes negating break in and find it takes care of cobs that want to burn out. Dries very hard, odorless and safe. 20150426_125655.jpg