80 
FLORIDA GEOLOGICAL SURVEY—15TH ANNUAL REPORT 
8. The presence of several per cent of lime and carbon dioxide 
shows the clay to he of calcareous character, and not only often of buff- 
burning character but with a narrow margin between vitrification and 
viscosity. 
9. Titanium oxide should be determined, especially in fire clays, 
as a small quantity may reduce the fusion point of fire clay several cones. 
It might be seen from the above that the ultimate analysis yields us 
much, and yet it leaves us practically in the dark as to the plasticity, air 
and fire shrinkage, density and hardness of burning, tensile strength, etc.” 
In modern clay investigation emphasis is placed on the physical 
tests and the ultimate analysis is rarely made because it is of little prac¬ 
tical value except in rare cases. 
The rational analysis attempts to resolve the clay into its consti¬ 
tuent minerals, and in one sense of the word gives a more accurate con¬ 
ception of the true character of the material. The ordinary analysis 
divides the clay into: clay substance, quartz, and feldspar. The method 
as at present used is not by any means satisfactory, nor is it altogether 
safe to figure the mineral composition from the ultimate analysis. 1 
Methods of making both the ultimate and rational analysis are con¬ 
veniently outlined in a number of publications dealing with clays and 
clay technology. 2 
CHEMICAL EFFECT OF VARIOUS CONSTITUENTS IN CLAYS 
Silica —Silica may occur in a clay in the free state as quartz or it 
may occur in a combined state in the form of the silicate minerals. The 
common silicate minerals found in clays are kaolinite, hydromicas, feld¬ 
spar, mica, hornblende, etc. The total silica is usually given in the ulti¬ 
mate analysis and includes both the free and the combined forms. Except 
in the case of kaolinite the silicate minerals occur in a more or less sandy 
condition and exert an influence upon the plasticity and shrinkage as 
does quartz. All of these affect the fire shrinkage and fusibility of the 
^Washington, H. S., The Calculation of the “Rational Analysis” of Clays, 
Jour. Am. Ceramic Society. Vol. I, p. 405, 1918. 
2 Washington, H. S., Manual of the Chemical Analysis of Rocks, 1919. 
Hillebrand, W. F., The Analysis of Silicate and Carbonate Rocks, U. S. Geol. 
Survey Bull. 700, 1919. 
Hillebrand, W. F., Some Principles and Methods of Rock Analysis, U. S. Geol. 
Survey Bull. 176, 1900. 
