From a test portion all ethanol-soluble matter is removed by extraction with ethanol. In the presence of silicates the solution of the residue is treated with hydrochloric acid and evaporated to dryness. The residue is dissolved in water and the operations are repeated twice. The last solution is filtered and in the filtrate or in the silicatefree solution of the first step the sulfates are precipitated with barium chloride. The precipitate is filtered, washed, heated at 900 degree Celsius and weighed.