Ch. 25] AREAL EVALUATION OF CLAYS 471 



alkalies and alkaline earths in feldspar, etc., will reduce the refractori- 

 ness. 



Clays composed of a single clay mineral usually have a shorter 

 temperature interval between the beginning of vitrification and com- 

 plete fusion than clays composed of a mixture of minerals. This seems 

 to be more applicable to illite and montmorillonite clays than to 

 kaolinites. Factors other than mineral composition influence the 

 vitrification range. Thus the vitrification range tends to increase as 

 the range of particle size increases. The presence of alkali-earths fre- 

 quently tends to shorten the vitrification range, and this is one reason 

 why many calcareous clays are difficult to burn. 



Similarly, clays composed of a mixture of clay minerals are apt to 

 have lower shrinkage during burning than substantially monomineral 

 clays. This again is more applicable to illite and montmorillonite 

 clays than to kaolinite clays. 



Certain clays tend to bloat or swell on firing when they are heated 

 to a temperature at which considerable vitrification takes place. Il- 

 lite and montmorillonite clays, particularly if they are relatively pure, 

 have a tendency to bloat. Calcareous illite and montmorillonite clays 

 seem to bloat very easily. A low-fusion temperature and short vitri- 

 fication range are among the factors that favor bloating. 



INDUSTRIAL APPLICATION OF INVESTIGATIONS OF THE 

 COMPOSITIONS OF CLAYS 



Studies of the composition of clays are of practical importance in 

 several ways in the solution of problems involving the use of clays for 

 ceramic and other purposes. 



Evaluation of Clays for Ceramic Use in an Areal Study 



Frequently there is the problem of determining if the clays in a given 

 area are particularly suitable for any ceramic use. Obviously such 

 a problem can be solved by making complete ceramic tests of samples 

 of all varieties of clays. This procedure is expensive and time-con- 

 suming if there are many varieties of clays to be tested, and if the 

 area is large. A simpler and more rapid procedure is to determine 

 the mineralogical composition of the clays and then select (on the 

 basis of the analytical data and the general relation between com- 

 position and properties) those clays that appear to be particularly 

 suited for certain ceramic uses. The ceramic properties of those clays 

 can then be determined in detail. The determination of the composi- 

 tion of the clays does not take the place of determinations of ceramic 



