OBSERVATIONS ON SPALLING*

Abstract

If a sphere of clay at room temperature be immersed suddenly in a red hot furnace, spalls fly off its surface. These spalls are more or less circular disks, of somewhat definite thickness. They do not form instantly, but a finite time elapses between immersion and spalling. This time interval also is consistent and definite. The hotter the furnace, the shorter the time interval and the thinner the spall. According to the experiments, the thickness of the spall is proportional to the square root of the time interval; it is also inversely proportional to the fourth power of the absolute temperature. The first relation implies, apparently, that the controlling factor in the clay is the thermal diffusivity; the second relation implies that the connection between the clay and the furnace is radiant heat. Both implications seem entirely sensible. The addition of an inert material of relatively high diffusivity accentuates the rôle that this factor plays in the resistance to spalling of clayware. These experiments were carried out by adding Sic flour in increasing amounts to the clay batch. The views of the Refractories Division are sought before extending the work to geological problems.