127 



added to induce the cohesion to a point where it can readily be 

 ovci'corae by the pressure of the worker's hands, i.e., to 1-3 lb. per 

 square inch, so that it is a balancing of forces producing a peculiar 

 combination of tluidity and rigidity in the mass of wet clay; under 

 a light pressure it acts as a rigid body, under a heavier pressure, it 

 acts as an imperfect fluid. The rigidity is attributed to friction 

 between the clay grains, so that a mass of clay retains its form until 

 acted on by a force sufficient to overcome this friction and produce 

 distortion. The fliiidity of the wet clay is due to the freedom of the 

 individual particles to move over each other, after cohesion has been 

 partially ncutrahsed by the addition of water. 



The theory that plasticity, instead of being a special jjroperty, 

 issimply the result of molecular attraction, and that all bodies which 

 are made up of laminated particles must become plastic when they 

 are reduced to sufficiently impalpable powder has been confirmed by 

 \'^ogt as regards mica, which is highly laminated, being made up of 

 thin layers, and when reduced to an impalpable powder becomes 

 distinctly plastic if water is added. The insistence laid by Bourry^* 

 on the laminated structure of the particles has been frequently over- 

 looked, and the suggestion that, because burned clay may be ground 

 equally fine and yet never become plastic his experiments are not 

 conclusive, is irrcievent. 



Segeri, and independently Schumacher, consider plasticity to be 

 due to molecular differences in the clay particles, and Bischof agrees 

 with the latter in considering that clay has undergone great changes 

 in density during deposition, and a kind of " felting " of the particles 

 has resulted so that they adhere much more closely to each other than 

 the quartz and other particles in which this felting process has not 

 taken place. 



Wolff has calculated the attraction of the particles of various 

 substances to each other on the assumption that they are spherical. 

 He finds that the mutual attraction of the clay particles is vedy high 

 and that the ratio between their mutual attraction for each other 

 and for water is much higher than for any other substances examined. 

 He stated in confirmation of this theory that other substances can 

 be made plastic, if they can be made sufficiently small, as by 

 precipitation.* He also pointed out that the combined water in a 

 clay particle increases the ratio considerably and is accompanied by an 

 increase in plasticity not only in the clay, but in alumina and iron 

 oxides. Zschokke confirmed this theory, and has show^i that clay 

 l^articles have a thicker film of water around them than particles of 

 non-plastic materials such as sand. 



It is extremely difficult to find satisfactory reasons for attributing 

 the plasticity solely to the plate -like or lamellar structure of the 

 particles or to purely mechanical or chemical characteristics in the 

 atoms and molecules of the clay and water, though these are 

 undoubtedly important. Nor has the effort of Le Chateher to fuid 

 the source of plasticity in the presence of small amounts of impurities 

 proved really helpful. The smallness and shape of the particles 

 appear to be important, as clay ground in a pan-mill is more plastic 

 than wlien a ball-mill is used, as the former flattens out the material, 



