86 FLORIDA GEOLOGICAL SURVEY—I5TH ANNUAL REPORT 
except those cases where the ware is dried by the waste heat from burn¬ 
ing 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-smoking period). He also suggests that 
the term scum be limited to designate all those surface coatings of soluble 
salts which have originated through deposits upon the surface of vola¬ 
tile substances. This distinction is adhered to in the following discus¬ 
sion 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 causes. 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 common 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 be 
brought into the clay by the water used in tempering. In any case 
where soluble salts occur the water used should be considered. 
Parmelee 1 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 de¬ 
termination of the amount of the soluble salts in clays is seriously com- 
J Loc. cit. 
