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 —CaC03. 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 1 
states that this effect is most marked when the percentage of lime is 
three times that of iron. 
Gypsum —CaSCU, 2 H 2 O. 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. Ries 2 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. Ries 3 states that he has 
1 Ries, H., Clays, Their Occurrence, Properties and Uses, p. 87, 1908. 
2 Ries, H., Clays of Maryland, Md. Geol. Survey, Special Publication, Vol. IV, 
Part III, p. 225, 1902. 
3 Ries, H., Private Communication. 
