Dry distillation is the heating of solid materials to produce gaseous products (which may condense into liquids or solids). The method may or may not involve pyrolysis/thermolysis. The products are condensed and collected. This method usually requires higher temperatures than classical distillation. The method has been used to obtain liquid fuels from coal and wood. It can also be used to break down mineral salts such as sulfates through thermolysis, in this case producing sulfur dioxide/sulfur trioxide gas which can be dissolved in water to obtain sulfuric acid. By this method sulfuric acid was first identified and artificially produced.