13S 



the angle through which a bar of claj'^ can be bent without rupture ; 

 (4) elasticity, or the extent to which a jDiece of clay can be stretched 

 and yet return to its original length when the tension is removed. 

 Many plastic clays show slight elasticity, though it is usually too 

 small to be measurable. 



The tensile strength of a clay is its resistance to torsion or to being 

 pulled apart. The non-plastic materials influence its strength 

 inversely as the diameter of their grains, so that fine-grained clays 

 ■wall usually be the strongest, though an excess of very fine or very 

 coarse grains will cause the clay to break prematurely. In support 

 of the theory that the grains of clay interlock to some extent, Ries 

 found that mixtures of two clays can be made which have a higher 

 tensile strength than either clay taken separately. This fact has 

 long been kno^^Ti by the makers of crucibles for steel-melting in this 

 country, as many as four different clays being sometimes used to 

 produce a sufficiently strong crucible. The tensile strength of the 

 clay has, in fact, an important bearing on its resistance to accidents 

 in the process of manufacture, particularly from the commencement 

 of drying to that of firing. It has sometimes been stated that the 

 tensile strength of a clay enables it to carry a large quantity of 

 non-plastic material, but this is rather confusing the effect with the 

 cause. It is the binding power of the clay which enables it to carry 

 such a large quantity of added material and still retain a sufficiently 

 high tensile strength. Olschewsky has proved that there is no direct 

 relationship between the binding power of a clay and its tensile 

 strength \A^hen dry. It was at one time thought that the tensile 

 strength of clays is proportional to the jjlasticity, but this is only 

 true, if at all, when the pieces are tested in the moist (plastic) state. 

 If air-dried, the definite relationship ceases. 



The tensile strength of dried raw clays dej)ends on the proportions 

 of the grains of different sizes. Equal-sized grains caimot be packed 

 into a dense mass. An excessive proportion of the finest clay particles 

 or a large percentage of sand grains (0-5-1 • 0mm.) weakens the 

 strength of an air-dried clay. 



Fissility — or capability of being split up into thin slabs or flat 

 pieces, or even into flakes or foliations — is characteristic of many 

 indurated clays, especially of shales. If the splitting can be effected 

 so as to form plates 6f extreme thinness, the material is said to be 

 laminated ; if the tendency to spht is strongest in the direction of 

 bedding, the material is termed sJialey; if this tendency is strongly 

 marked in any other direction it is said to be fissile, as are slates and 

 certain hmestones and sandstones. 



Sectility, or capability of being easil}^ cut, is a characteristic of 

 clays which occur in a plastic condition, such as baU clays and many 

 surface clays. This property often serves as a means of distinguishing 

 " clays " from other minerals, though the " clays " so found may be 

 too impure to be of any commercial value. Anyone constantly 

 engaged in examining clays soon learns to recognise some varieties 

 by their sectility and by the sUghtly glossj'' appearance of the freshly- 

 cut sui'faces, though these cannot be clearly described. 



