90 FLORIDA GEoLoGIcAL SURVE1Y-15T1 ANNUAL REPORT

solely to the fineness of the clay particles. Other substances, however, when ground equally fine do not have the plasticity that clay has. The plate structure theory is that clay is made up of very fine plates which afford plasticity when mixed with water. All clays, however, do not show this platy structure. The interlocking particles theory is that the tiny particles interlock and thus afford plasticity.
 An attempt has also been made to explain by virtue of the presence of hydrous alwimintim. silicates, that is to say the plasticity is due to the hydrous aluminum silicate condition, and that the application of heat drives off the chemically combined water thus destroying plasticity. The degree of plasticity, however, does not stand in any relation to the chemical composition.
 Efforts have likewise been made to explain plasticity by molecular attraction and by the presence of colloidal matter. This supposes that these colloids take up water and thus become jelly-like and plastic.
 It does not seem likely, in the examination of a series of clays, that any of the theories as yet suggested is the sole cause of plasticity. It is more probable that plasticity is due to a combination of them.
 Some clays are too highly plastic to be handled with maximum facility in the common types of machinery. In such cases a non-plastic substance, as sand, is added to the clay to reduce the plasticity.

 COLOR
 The color of a clay is quite variable, ranging from white through gray, yellow, brown, red to black. Vegetable matter and the nature of the iron compounds present usually determines the color of a clay, though manganese is sometimes an important factor.
 The color of the raw clay is not always indicative of the color of the burned product. If the raw clay is red because of the presence of iron compounds, the burned product is likely also to be red. The presence, however, of sufficient calcium carbonate would tend to neutralize the coloring effect of the iron and cause a buff or cream color instead. The presence of carbonaceous matter will have little influence, if any, on the color of the burned product. It moreover masks the true color of the raw clay. The colors in burned clays are not as variable as in the raw ona, White, cream, buff, yellow, and red are the common colors in wrned products,