This International Standard specifies a method for dissolving sodium or potassium silicates that are not easily soluble in boiling water at atmospheric pressure, and also a method for the determination of matter insoluble in water. NOTE — When the method described in this International Standard is used, the masses of sample laid down in ISO/R 1690 and ISO/R 1691 must be replaced by taking corresponding quantities of the solution, and the expression of the results must be modified accordingly.