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FLORIDA GEOLOGICAL SURVEY— 15TH 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 aluminum, 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 chemi¬ 
cal 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 pf 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 pres¬ 
ence, 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 
ones. White, cream, buff, yellow, and red are the common colors in 
burned products. 
