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causes of plasticity which meets all the needs of the case. Plasticity 

 varies with different samples and on different occasions, though no 

 raw moist clays are entirely devoid of plasticity. Clays which are 

 quite dry are not plastic, but become so when mixed with a suitable 

 proportion of water so as to form a paste. Hence, the amount of 

 plasticity developed is dependent on the proportion of water present. 



Liquids other than water may be added to the clay to produce 

 plasticity, but they must usually contain water, and even then, 

 sometimes produce quite different characteristics. Thus, glycerine 

 may be used, but it prevents the clay from drying, and Krupsay has 

 pointed out that if plastic masses made from clay and glycerine and 

 clay and water respectively be kneaded together the resulting mixture 

 is non-plastic. Fatty Uquids, such as oils, seem to make a more 

 plastic body than with water, especially if the clay has been dried 

 so as to take away from it the hygroscopic water, but alcohol, ethej, 

 and turpentine produce bodies with little or no plasticity. 



The nature of the plastic product formed when liquids other than 

 water are used is worth further study. In the case of an oil, the 

 plastic mass is quite different from that produced with a liquid such 

 as anhydrous nitric acid, anhydrous sulphuric acid, absolute alcohol 

 and glycerine. Each of these fluids is soluble in water, and is, therefore, 

 able to wet the hydrated clay grain with its attached water molecule 

 and to separate the grains sufficiently to produce a plastic mass. 

 In each case, the clay may be " dried " again and made plastic with 

 any of the other fluids. According to R. F. MacMichael^* only those 

 liquids which " wet " the clay particles can produce plasticity. 

 Water and fatty oils do this, but ether, gasoline, kerosene, engine oil, 

 and similar fluids which do not "wet" the clay grains are either 

 unable to penetrate between them and so do not develop plasticity 

 in the clay or they form a film of such a nature around the clay grain 

 as to prevent cohesion, so that the mass acts like sand and water, 

 but there is no gradation or balancing of the forces, as is necessary 

 in order to obtain true plasticity. 



Plasticity also depends both on the nature of the fluid and that 

 of the solid. Thus, while both water and oil wet quartz sand, water 

 under suitable conditions will easily displace oil films from a mixture 

 of sand and oil. On the other hand, both oil and water wet zinc oxide, 

 but in this case the oil will readily displace the water films, forming 

 paint or putty. The resulting mass, in this case, may be said to be 

 oleated, in very much the same manner as clay is said to be hydrated. 

 The same principle is employed commercially on a very large scale 

 in the flotation of metal- bearing ores. 



The possible plasticity^ of clay or other substance cannot be 

 developed by commercial methods of grmding unless the material 

 is in a state which may be regarded as dormant plasticity. This has 

 been regarded as an objection to the view that plasticity is due to 

 the colloidal properties of clay, but the objection may be met by 

 the difficulty of reducing some clays to so fine a state as is required 

 to produce the requisite amount of colloidal matter. 



Plasticity also varies with the presence of certain other substances ; 

 thus, the following soluble substances reduce the plasticity of clay : 



