An interface layer for separating first and second microelectronic ceramic substrates during firing includes a thermally degradable binder, preferably a polymer, that degrades at temperatures above room temperature and below a firing temperature for the microelectronic ceramic substrates, and a separating material, such as boron nitride or graphite. The method of making the interface layer includes mixing the binder, one or more solvents for the binder, a plasticizer and the separating material for a sufficient period of time to form a homogeneous mass, then casting the material in a thin sheet to form the interface layer.