Care should be taken to make sure the colors are kept clean. If they are kept wet they may be stored in jars indefinitely for later use. If they dry out, they must be reground. Let the pigment spin overnight, remove the ceramic balls, and let the pigment settle, which may take some days, depending on which pigment being ground, and pour off the extra water. The pigment can be left in the jar, or stored in another container.
Small jars are best.
They should be sterilized in boiling water. Several colors can be ground at once, if the tumbler can accommodate the jars. Dimitroff, Stephen Pope, Apprentice to Diego Rivera in Detroit and fresco workshops manual, Stephen Pope Dimitroff, 1986. Mora, Paolo, et al, Conservation of wall paintings, Glasgow, Butterworths, 1984. Fresco Painting American Arists Group, inc.