100 FLORIDA GEOLOGICAL SURVEY-I5TH ANNUAL REPORT 
CHAPTER V 
TESTS MADE UPON CLAYS 
The methods of testing clays are now fairly well standardized and 
are somewhat uniformly followed with the result that clay technologists 
and investigators have a reliable basis upon which to compare the prop¬ 
erties of various clays. 
- The measurement of the plasticity of a clay still remains a matter of 
personal opinion, for no satisfactory means of measuring it has as yet 
been devised. The terms employed to describe the plasticity of a clay 
are only relative and will of course vary to some extent with the indi¬ 
vidual. Plasticity is usually described as “good”, “excellent”, “poor”, 
“low”, etc. A clay having good or excellent plasticity is said to be “fat”, 
and one having low or poor plasticity is described as being “lean.” 
The water of plasticity is a more definite factor and can thus be 
accurately measured. The water of plasticity is the amount of water 
necessary to give a clay its maximum plasticity. It is expressed in terms 
of percentage based upon the dry weight of the clay and is determined 
by weighing a test piece at maximum plasticity and again weighing it 
after all the moisture has been driven out by first drying at room temper¬ 
ature and finally at 110° C. until it has a constant weight. The water of 
plasticity test indicates the amount of water necessary to bring a clay to 
its point of maximum plasticity in which state it can be best worked. 
The shrinkage water is that portion of the water of plasticity which 
escapes up to the point where air-shrinkage ceases, or in other words, 
it is the water lost in air-drying. The clay without the shrinkage water is 
at the point of maximum air-shrinkage in which condition the clay 
particles are in contact. 
The pore water is that portion of the water of plasticity which 
remains in the inter-particle spaces after the point of maximum air- 
shrinkage has been reached. It can be driven off only by heating the 
test pieces to 110° C. The pore water plus the shrinkage water is the 
water of plasticity. 
The slaking of clays is the property possessed by them when dry, 
of crumbling into a flaky or pulverulent mass when immersed in water. 
In order to determine this property it is necessary to first dry the test 
pieces at room temperature. Then all the remaining moisture is driven 
off by heating them for two hours or longer at 110° C. They are then 
