To prepare green billiard chalks, mix together 5 parts of powdered magnesite and 1 part of china clay, and add 1 part of mineral green or terra verte; for a blue chalk, substitute 1 part of artificial ultramarine. Make the mixture into a very stiff dough with the least possible quantity of water, allow to stand for several days, roll it out, into a cake of the thickness required, then cut it into squares with a fine wire; impress a hemispherical indentation on each square, then separate them and dry them very slowly fust in the air, then in a warm oven. If the squares are shaped in brass moulds the material should be made very stiff, almost dry in fact; the chalks will then be harder. If the chalks are too soft, add more china clay; the colour can be made to suit by trial.