4-6 POTTERY IN ARCHITECTURE. Sometimes an endless band is placed under the mill, as in SEPARATION OF FOREIGN SUBSTANCES. It is preferable, for many reasons, not to use clays which contain too much foreign matter, — stones, sands, pyrites, etc., — but there are cases in which one cannot do… … Fig. 21. — Conical MixingMill (Boulet). the kaolins ; generally washing is used to remove the sand and gravel, and stone-removing machines for the larger stones. Washing- of Clays. — This is an excellent but a costly process for freeing clays from the heavy products contained in them. It is used…