542 NEW TOEK STATE MUSEUM 



that it is the rule, and that high tensile strength always denotes 

 high plasticity. There are clays running low in their tensile 

 strength that exhibit marked plasticity, as for example some of the 

 'New Jersey clays, or some English ball-clays, but on the other 

 hand it can be said that probably no clay of low plasticity has 

 high tensile strength. 



A means of measuring the plasticity of a clay and expressing it 

 numerically for purposes of comparison has been the one aim of 

 clay technologists. Several methods have been suggested, none 

 meeting with universal, and few with even partial adoption. One 

 of the more important is testing with Vicat's needle. 



The clay is reduced with water to a mass of the proper con- 

 sistency for ordinary working. It is then forced into a metal 

 ring, and the resistance which it offers to a steel wire of given 

 size under known pressure is noted. This method is followed in ' 

 cement testing, and the apparatus is known as Vicat's needle. It 

 gives the most satisfaction for comparative testing, that is for de- 

 termining the relative plasticities of several clays examined at 1:he 

 same time. ^ 



A second method is to mold the plastic clay in a briquet 

 mold similar to that used in testing cement, care being taken that 

 the clay briquet is homogeneous throughout and contains no 

 flaws. It is then allowed tO' dry thoroughly, and subsequently 

 pulled apart in a cement testing machine, the tensile strength being 

 expressed in pounds per square inch. As the clay shrinks in dry- 

 ing, it is necessary to measure the cross-section of the briquet be- 

 fore breaking it, and tO' calculate from this the strength of a 

 Lriquet whose cross-section is one square inch. 



Another method is to form the plastic clay into a cylinder, 

 which is placed horizontally in a semicircular channel of the same 

 diameter, and so arranged that a wire can be laid across it at right 

 angles. A weight is attached tO' the wire, and the time which ia 

 required for the wire to cut through the clay observed.-^ 



^ C. F. Binns. Ceramic technology, p. 35. 



