252 FLORIDA GEOLOGICAL SURVEY-I5TH ANNUAL REPORT 
and, therefore, any expenditure of time or money in investigating the 
deposits is useless. These tests are of no value if the clay is being con¬ 
sidered for Portland cement manufacture. 
These tests are simple and easily applied, but the results must be 
interpreted with caution. For example, a relatively non-plastic clay 
may be satisfactorily worked in a dry-press machine, or it may be pos¬ 
sible to add quantities of other plastic clays, thus making a blend which 
has sufficient plasticity to be worked by the stiff-mud process. 
The plasticity of the clay may be judged by taking a small moist 
lump and working it in the hands. If it can be easily molded into 
various shapes which are retained without cracking as the piece is al¬ 
lowed to dry, the plasticity may be considered satisfactory. The pres¬ 
ence of sand, or other gritty matter, may be easily detected by tasting 
a small piece of the clay. A small amount of sand or other non-plastic 
material is not necessarily deterimental for use in structural materials, 
while, in high-grade clays, such impurities as sand, etc., may be profit¬ 
ably removed by washing. Calcium carbonate (lime) may be detected 
by applying a few drops of hydrochloric acid to the clay, which will 
cause it to effervesce if lime is present. Calcium carbonate is detrimen¬ 
tal, causing the product to slake after firing. 
By far the safest and most satisfactory procedure is to send a 
clean sample of the clay, weighing about twenty or twenty-five pounds, 
to some ceramic laboratory making a specialty of testing clays, where 
the physical properties of the clay can be accurately determined and its 
adaptability to various uses and types of machinery can be ascertained. 
Many concerns manufacturing clay-working machinery make a prac¬ 
tice of testing small samples of clay submitted by prospective purchas¬ 
ers of equipment, and render an opinion as to the suitability of the clay 
for use in the types of machinery manufactured by them. Also, ar¬ 
rangements may oftentimes be made with a brick manufacturer at some 
distance, so that he will not fear competition, whereby the clay-owner 
may ship a sample of clay, usually several barrels, to be tested by the 
manufacturer for working and burning in his plant. This method has 
the disadvantage that the clay to be tested may require entirely different 
treatment from that given the clay used at the plant where the test is 
being made, and, in many cases, the conclusions derived would be er¬ 
roneous. High-grade clays can only be recognized by the method first 
listed. 
After the presence of a clay deposit has been ascertained, it is next 
of prime importance to know its thickness and the extent of the area 
