Actually, of course, mass is not strictly conserved in combustion, since part of the chemical binding energy of the materials is released as light and heat that radiate away from the fire. According to special relativity, it is mass-energy which is conserved as a sum, with their equivalence given by the well-known E=mc². But, as I calculated in “Relativistic Nutrition”, the mass equivalent of energy released in chemical reactions is so small it escapes all but the most sensitive of measurements.