The procedure is based on evaporation to dryness of a test portion previously acidified with hydrochloric acid. Dissolution of the soluble salts, filtration and washing of the insoluble matter. Second evaporation of the filtrate and washings under the same conditions, further dissolution of the soluble salts, filtration and washing. Calcination and weighing of the two lots of insoluble matter simultaneously. Volatilization of silica by heating with hydrofluoric and sulphuric acids and weighing of the residue after calcination. The*difference in mass represents the silica present in the test portion.