﻿426 cox. 



temperature, so increased temperature causes clay to "run" and hence decreases 

 plasticity. Tt appears likely that the organic constituents of clay cause this 

 liquefaction as soon as a certain limiting temperature has been exceeded. The 

 increase in temperature may be brought about mechanically by stirring. 



There is no uniform way of determining plasticity. Some workers 

 employ Bischof's method of forcing the moist clay through a die and 

 measuring the distance it projects ; others use the Yieat needle, 1C the 

 method of Grout, 17 etc. There is a certain degree of relationship between 

 the plasticity and the amount of water necessary to be added in order to 

 form a workable paste. A fine-grained clay usually requires more water 

 and as a rule is more plastic than one which is coarse. In the experi- 

 ments recorded in Table TI water was added until the Yicat needle 

 showed normal consistency, the •water being afterwards determined by 

 allowing the sample to air-dry. 



Tensile or breaking strength is the resistance to rapture which is 

 offered b} r the air-dried clay. This is an important factor to be deter- 

 mined in order to know the amount of nonplastic material that can 

 safely be added and the sample still resist the shocks and strains in 

 handling to which it is always subjected in the process of manufacture. 

 This may be determined on any standard machine such as is used for 

 the same purpose in cements. In earning out the test the tempered 

 clay is molded into briquettes, just as is the case with cement, the 

 briquettes having a cross section of 6.45 square centimeters (1 square 

 inch) when molded. They are allowed to dry, first in the air, then 

 at 100° ; then placed in the testiug machine and a uniformly increasing 

 load gradually applied so as not to produce a shock, until they break. 

 Care must be exercised in placing the briquettes in the machine to 

 prevent all cross strains from improper centering, and from other causes. 

 Theoretically, a briquette should break across its smallest cross section, 

 but failure to take any of the above precautions may result in anything 

 but this. 



Shrinkage is of two kinds — that which occurs in the air after the 

 clay has been molded in a wet condition and that which takes place 

 during the operation of burning. Both of these quantities vary greatly 

 and are very useful in estimating the value of the raw material. The 

 decrease of volume in the air is due to the drawing together of the 

 particles, when the water added to afford a workable mass evaporates. 

 Even in the same clay, this factor is often somewhat variable in propor- 

 tion to the pressure given the material in molding. The amount of 

 shrinkage of a clay upon burning depends not only upon the quantity 

 of volatile constituents such as organic matter, water of constitution, 

 carbon dioxide, etc., but also upon the texture of the clay and the tem- 

 perature at which it is burned. Some clays containing high percentages 

 of calcium and magnesium carbonates, even expand during certain stages 



" Langenbeck, Karl: Chemistry of Pottery. Easton. Pa. (1895), 19. 

 17 Jour,,. Am. Che,,,. Soc. (1905), 27, 1037. 



