134 



(6) Noting the length of the threads, produced by expressing 

 the clay from a vertical pug mill, before they break off by their 

 own weight {tensile strength and extensibility). 



(7) Forming balls of clay and pressing them untU the edges 

 crack {crushing strength). 



(8) Bending cylinders of clay into a ring {bending moment). 



None of these characteristics taken alone can give a measure of 

 the plasticity of a clay, though several of them are closely related 

 to each other. The most reliable measure of plasticity appears to 

 be that devised by Zschokke (p. 133) or by Rosenow, who multiplies 

 Zschokke's figure by the percentage of water added to the dry clay 

 to make it into a workable paste, i.e., by Rohlancl's figure (p. 133). 



The binding power of a clay is the property it possesses of uniting 

 with non-plastic material and water to form a luiiform plastic paste, 

 and is consequently closely related to the plasticity. This absorption 

 of non-plastic material with the spread of plasticity throughout the 

 whole mass has been attributed to the power of the saturated colloids 

 (gels) to retain the non-colloidal particles in a state of pseudo-solution. 

 Other coUoids are known to possess the property of preventing insoluble 

 matter from settling, and this is, in some senses, a parallel case. The 

 binding power of a clay may be determined by measuring the tensUe 

 strength of mixtures of clay with varying amounts of sand, but a 

 skilled clayworker can teU by the " feel " whether such mixtures are 

 strong enough to be useful. In order to determine how much lean 

 clay or non-plastic material can be added to a clay without unduly 

 destroying its value for moulding into shajDe, Bischof's test may be 

 used. In this, the two materials are mixed in various proportions 

 and the same measured quantity of water is added to each. The 

 pastes are then rolled into small balls as equal in size as possible, and 

 allowed to dry. They are then rubbed gently between the finger and 

 thumb, or with a small " camel hair " brush. The mixture which 

 just resists the action of rubbing may be taken as the standard. 

 Some authorities make up balls of mixture in this way and then 

 notice to what length a cylinder can be rolled from each without 

 cracking. 



Clays with a high binding power are known technically as " fat " 

 clays ; " lean " clays are deficient in binding power. 



Some writers appear to consider that binding power and jalasticity 

 are sjmonymous ; this is by no means the case, as a clay may be very 

 plastic and yet not be able to bind much non-plastic material into 

 a uniform jilastic paste. At the same time, there is clearly some 

 relationship between these two properties of clays. 



The dehydration of clays is accompanied by changes which are 

 remarkably similar to those which occur in the dehydration of colloidal 

 gels. The most important of these changes is the shrinkage or 

 contraction of the mass, the production of a hard material which — 

 if the dehydration is accomplished by heat — may result in^the 

 production of a material comparable to an irreversible gel. • 



Plastic clays, like coUoidal gels, shrink greatly when dehydrated 

 and possess both a drjdng-shrtnkage and a kiln-shrinkage. By mixing 



