A PRELIMINARY REPORT ON CLAYS OF FLORIDA 75

and when such coal is used as fuel in firing clay products it may cause the same injurious scumming effects as when it is an ingredient of the clay.
 Calcite-CaCO3. The presence of this mineral in clays can be easily detected by the application of a few drops of hydrochloric acid which causes the calcite to effervesce briskly. Calcite is quite abundant in some clays and may occur as disseminations, as concretions, or as fragments of limestone. These larger pieces can be separated by hand in mining. Calcite acts as a flux in burning and if present in fine lumps is converted into lime (CaO) which slakes upon being exposed to air and causes the ware to crumble unless hard-fired. If the calcite is present in a finely divided state and evenly distributed through the clay it has no especial injurious effect. Some of the Florida clays are quite calcareous. At higher temperatures the lime resulting from the burning of calcite will unite with the other elements of the clay, especially the alumina and silica, giving a reaction which has a marked effect on the color as well as the fusibility of the clay. If the lime be in excess of the iron, when iron is present, it has a tendency to give the ware a buff color. Ries' states that this effect is most marked when the percentage of lime is three times that of iron.
 Gypsum-CaSO4, 2H20. This mineral, the calcium sulphate, is not widely distributed in clays, but in some deposits it occurs abundantly. It is regarded as an injurious ingredient when present. Ries2 says: "The effect of gypsum on clay is that of a flux, especially if the brick is burned to vitrification, but if the clay is not burned sufficiently hard to drive off the sulphuric acid which the gypsum contains combined with the lime, then soluble sulphates may be left in the clay, which will be brought to the surface of the brick where they cause an unsightly white coating upon evaporation of the moisture." Gypsum has a pearly lustre and occurs in plate-like crystals or in fibrous form and exhibits no effervescence when acid is applied. It may be secondary in origin in clays and is formed by the action of sulphuric acid upon calcium carbonate. Gypsum specks in burned clay do not cause the ware to slake as is the case when calcite is present. Ries3 states that he has

 'Ries, II., Clays, Their Occurrence, Properties and Uses, p. 87, 1908.
 2Ries, H., Clays of Maryland, Md. Geol. Survey, Special Publication, Vol. IV, Part III, p. 225, 1902.
 3Ries, H., Private Communication.