The free “C” content was calculated as mole fractions of SiO2, SiC, Si3N4 and “free” C in SixCyNzOwHv ceramics using their chemical composition. It was assumed that N and O bond to Si to form the equivalents of Si3N4 and SiO2 respectively. Si-H bond formation was assumed to be negligible. The rest of Si bonds to C to form SiC, thus the amount of “free” C was estimated as the remaining C which did not interact with the Si matrix according to table 2.