A PRELIMINARY REPORT ON CLAYS OE ELORIDA 
87 
plicated by the progressive solubility of clay in contact with water, so 
that the duration of the contact and the temperature are extremely im¬ 
portant factors. 
If pyrite is present in a clay it oxidizes when exposed to air and 
moisture to form the soluble iron sulphate. Its presence may often be 
detected by a yellow or brownish-yellow effloresence on the raw clay. It 
is oxidized during burning to red or brown. The iron sulphate decom¬ 
poses with the formation of sulphuric acid which then attacks the calci¬ 
um, magnesium, or iron compounds to form the sulphates of these ele¬ 
ments. In some cases organic acids present in many clays may attack 
calcium carbonate (often in the form of limestone) to form some of the 
soluble salts. 
The maximum amount of soluble salts which may be present in a 
clay without detrimental results is not known. It is placed by some in¬ 
vestigators at as low as one-tenth of one per cent. Dr. Mackler 1 says: 
“The amount of scum (efflorescence) formed on finished goods bears no 
definite relation to the proportion of soluble salts contained, but there 
is quite a definite relation between the amount of the magnesium and 
sodium sulphates and that of scum (effloresence).” On this point 
Staley 2 writes “that he added calcium sulphate to a fine-grained, red- 
burning shale in amounts increasing to 3 per cent of the dry clay and 
was unable to produce effloresence even with slow drying. When he 
added 1 per cent of magnesium sulphate it appeared in large quantities 
and a 1 per cent mixture of equal parts of the sulphates of calcium 
and magnesium give an even greater amount of effloresence.” He ex¬ 
plains this by pointing out that magnesium sulphate is very soluble in 
water while calcium sulphate has only a slight solubility. He also states 
that calcium sulphate hastened the rate of drying which would act 
toward a decrease in effloresence. 
There are several methods of preventing the troublesome effect 
of soluble salts. The numerous remedies which have been suggested 
and used depend upon the character, origin, and occurrence of the salts 
in the clay in question. In some cases use of the clay in the unweathered 
condition is desirable. In other cases prolonged and thorough weather¬ 
ing will permit the objectionable compounds to be leached out. Rapid 
1 Quoted in Clayworkers’ Handbook, p. 213, 1906. 
2 Staley, Homer F., Use of Barium Fluoride for the Prevention of Dryer Scum 
on Bricks. Trans. Am. Ceramic Soc., Vol. XVII, p. 200, 1915. 
