A PRELIMINARY REPORT oN CLAYS OF FLORIDA 87

plicated by the progressive solubility of clay in contact with water, so that the duration of the contact and the temperature are extremely important 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 decomposes with the formation of sulphuric acid which then attacks the calcium, magnesium, or iron compounds to form the sulphates of these elements. 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 investigators at as low as one-tenth of one per cent. Dr. Mdckler' 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 Staley2 writes "that he added calcium sulphate to a fine-grained, redburning 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 effioresence." He explains 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 weathering will permit the objectionable compounds to be leached out. Rapid

 'Quoted in Clayworkers' Handbook, p. 213, 1906.
 2Staley, Homer F., Use of Barium Fluoride for the Prevention of Dryer Scum on Bricks. Trans. Am. Ceramic Soc., Vol. XVII, p. 200, 1915.