A PRELIMINARY report on clays OE elorida 89 
CHAPTER IV 
PHYSICAL PROPERTIES OF CLAYS 
Clays possess certain characteristics which are of a physical nature 
and which are variable in different clays. The value of a clay for the 
manufacture of clay products depends upon these physical properties. 
Herein lies the necessity for determining the possibilities and limitations 
of a clay by a series of physical tests. These also determine the kind 
of product for which the clay is best suited. The more important physical 
properties will be briefly treated. 
plasticity 
Plasticity is one of the most important properties of a clay, for 
without it the manufacture of ordinary clay products would be greatly 
limited. Plasticity has been defined as the property possessed by a clay 
of forming a plastic mass when mixed with water ; this definition is satis¬ 
factory only when applied to clay because plasticity is not a property 
of clay alone. Ries 1 defines plasticity as “the property which many bodies 
possess of changing form under pressure, without rupturing, which 
form they retain when the pressure ceases, it being understood the 
amount of pressure required, and the degree of deformation possible, wifi 
vary with the material.” 
The degree of plasticity varies greatly in different clays. Clays 
showing a high degree of plasticity are said to be “fat,” while those which 
are only slightly plastic are “lean” or “short.” No satisfactory method 
for measuring plasticity has as yet been devised. The description of 
the plasticity of-a clay is largely a matter of individual judgment and 
varies with the personal equation. 
The amount of water required to develop the maximum plasticity 
in any clay varies with the material. It ranges from eight or ten per 
cent in some to over forty per cent in others. 
The cause of plasticity is not thoroughly understood, and while 
many theories have been advanced to explain it no one of them seem to fit 
every case. 
The structure of the clay particles has been used in several attempts 
to explain plasticity. The fineness of grain theory is that plasticity is due 
1 Ries, H., Clays, Their Occurrence, Properties and Uses, p. 119, 1908. 
