86 15L0RTDA GE OLOGICAL SURVI4Y-I5T11 ANNUAL REPORT

except those cases where the ware is dried by the waste heat from burning kilns; all coatings of soluble salts in masonry work in walks, piers, etc., which have originated through the solvent action of water upon materials in the clay wares or has been contributed by, the mortar used; and those coatings of soluble salts brought to the surface during the early stage of burning (water-smnoking period). He also suggests that tile term scum be limited to designate all those surface coatings of soluble salts which have originated through deposits upon the surface of volatile substances. This distinction is adhered to in the following discussion of soluble salts:
 Soluble salts brought out in the drying of the ware are termed "dryer white," those appearing in the process of burning are "kiln white," and those which come to the surface of the finished product after being exposed to moisture are "wall white".
 While soluble salts are present in all clays they may occur in various forms and may result from several cau-ses. Not all of these, however, are harmful. It also sometimes happens that other conditions intervene in behalf of the manufacturer to reduce the harmful effect of the more objectionable ones.
 These soluble salts may occur as the hydrated silicic acid, as aluminum salts, as the sulphates of iron, calcium, magnesium, sodium, potassium, etc., also as sodium chloride or commu-on salt. Some of the compounds of vanadium at times occur as soluble salts. The calcium and magnesium sulphates are the most troublesome and incidentally are the ones most frequently found.
 The soluble salts may be present in the clay in its raw state; they may result from the decomposition of certain mineral impurities in the clay as pyrite, upon exposure to moisture and air; they may be introduced by gases coming from fuel used in drying or burning; or they may he brought into the clay by the water used in tempering. In any case where soluble salts occur the water used should be considered. Parmelee' has pointed out that clay particles are of very minute size and when these are subjected to prolonged contact with water they are more or less taken into solution. Thus, the same writer adds, the determination of the amount of the soluble salts in clays is seriously comlLoc. cit.