Grind test sample and mix thoroughly. Transfer convenient amount (50-200 g according to consistency of test sample) to 500 ml volumetric flask, add H2O to make ca 400 ml, and shake until mixture becomes uniform. Add 2-5 g CaCl2 and shake until dissolved. Make distinctly alkaline to litmus paper with ca 10% NaOH solution, dilute to volume with H2O, shake thoroughly, let stand≥2 h, shaking frequently, and filter.