A PRELIMINARY REPORT ON CLAYS OV VLORIDA 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 satisfactory only when applied to clay because plasticity is not a property of clay alone. Ries' 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, will 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
 'Ries, H., Clays, Their Occurrence, Properties and Uses, p. 119, 1908.