Insulating Clay is Epic with a total supply of 998
548 left (of 998) - Known to be found in Macabre Crypt, Haunted Catacomb, Shattering Chamber, and Dank Descent
Clay tempering is an art form as well as a science and a good blacksmith will always have some Insulating Clay in his workshop.
The objective behind clay tempering is to achieve both hardness and flexibility by covering the blade with the clay to insulate the parts of the metal that you want softened so that when the weapon is heated and quenched, the insulated part will cool more slowly.
This results in strength in the section where it is needed but flex in the rest of the weapon.