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(indirect reduction or dilution). (3) The material may be heated to 

 200° C. or other suitable temperature. 



For the first method, any basic material, either organic or inorganic, 

 may be used though lime water is the cheapest. If lime is too weak 

 in hydroxyl-ions, caustic soda may be used, as may any salt composed 

 of a strong base and a weak acid, such as sodium (or potassium) 

 phosphates or silicates, all of which readUy hydrolyse and yield 

 hydroxyl-ions, though the cation constituent of the salt may exercise 

 a considerable effect. Thus, borax reduces the influence of the 

 hydroxyl-ions and potassium carbonate increases it, yet both are 

 salts composed of a strong base and a weak acid. The concentration 

 of the alkaline or basic material added is also of importance, and it 

 may be necessary to render sulj^hates and other soluble salts insoluble 

 by the addition of baryta, as suggested by Weber. Certain clays, 

 as Weber has shown, act in precisely the reverse manner. These are 

 free from sulphates, and appear to be rich in colloidal matter. 



Certain clays containing organic acids of a fatty nature are saponified 

 on treatment with alkali, and the soap so produced increases, instead 

 of diminishing, the plasticity, owing to the coagulation effected. 



The reduction of plasticity by raising the temperature considerably 

 is described later. A comparatively small rise in temperature produced 

 by the action of mechanical stirrers — will reduce the plasticity of 

 clay if free hydroxyl-ions are present. 



The addition of non-plastic material, such as sand or grog, effects 

 a reduction of the plasticity in an entirely different manner, by 

 separating the clay particles from each other. It thus reduces the 

 strength of the material, but by diminishing the shrinkage, it enables 

 the clay to be used in a manner which would, otherwise, have been 

 impossible, and the strength is seldom reduced sufficiently to make 

 any notable difference to the user of the material. The proportion 

 of non-plastic material to be added depends on the size of its grains 

 and on the binding power of the clay. As the latter is closely connected 

 with its plasticity, it will usually be found that the more plastic the 

 clay, the larger the proportion of non-plastic material which may 

 be used. 



Some sands are quite useless for this purpose, so that great care 

 is needed in their selection. For some clays, chalk, flints, or grog is 

 preferable to sand. 



The measurement of plasticity is a problem which has not yet been 

 satisfactorily solved, probably for the reason that plasticity is the 

 result of the united action of several forces some of which may not, 

 as yet, have been recognised as important. Early attempts to measure 

 plasticity usually resulted in only measuring one or more of these 

 forces. Thus, Bischof added sand until the mixture was so soft that 

 it could be rubbed away between his finger and thumb. Bischof's 

 figures are, however, a measure of the binding power of the clay, but 

 not of its plasticity. Measurements of tensile strength, viscosity, the 

 amount of water required to produce a mass of given consistency, 

 the consistency, or the depth to which a Vicat needle will penetrate, 

 Sokoloff's slaking test and other single characteristics are all useful 

 in their way, but they fail to include all the properties involved in the 



