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the addition of a highly plastic clay. The addition of electrolytes 

 to such clays is only of value when their low plasticity is due 

 to the clay gel present having become hardened or coagulated, 

 but is still capable of being revived or deflocculated by means of 

 an electrolytes or other simple treatment. 



(2) By keeping the clay in a moist damp cellar. This is 

 termed " ageing " or " souring." 



(3) By the addition of colloids such as colloidal silica, alumina, 

 or iron hydrate, hot starch, dextrin, tannin, rubber, sumach, 

 inulin, caramel, gelatin, gum, glycogen, or various ferments and 

 enzymes, the plasticity of the clay may be increased, but care 

 must be taken to avoid confusion between true polasticity and 

 the pseudo-plasticity caused by the addition of materials of an 

 oily, gelatinous, or gummy nature. 



Some very interesting experiments . by Acheson and Ries 

 on the effect of a 2 per cent, solution of tannin (gallotannic acid) 

 on clay show that the addition of this substance notably increases 

 the plasticity of clay, and at the same time apj^arently 

 defiocculates it and breaks it up into finer particles. The tensile 

 strength of the clay was nearly doubled. 



In a later patent, Acheson first adds tannin and alkalies or 

 ammonia and stirs the clay into a fluid state, and then by the 

 addition of a suitable quantity of acid he coagulates the colloids 

 and forms a stiff paste. 



(4) By reducing a sufficient number of particles to so minute 

 a state that they assume colloidal properties in the presence 

 of .water. Thus, by very prolonged grinding with water many 

 hard clays^ become appreciably more plastic. 



The softer materials become exceedingly smooth and plastic ; 

 the harder ones yield less readUy to the treatment, but stUl 

 develop marked jDasticity, very similar to that of normal clay 

 heavily overloaded with sand or grog. 



By selecting the materials and method of grindmg, many 

 degrees of plasticity may be obtained, from that of a very 

 smooth plastic clay to that of a very short sandy clay, indicating 

 that the difference is one of degree and not of kind, the essential 

 characteristics being that the clay or other materials shall occur 

 in a state of very fine subdivision, and that their surfaces are 

 readily wetted by water. The chief practical difficulty lies 

 in grinding sufficiently fine, as the smallest particle that can 

 be seen under the microscope does not by any means represent 

 the limit towards which the grinding should proceed. The 

 j)lasticity produced by artificial grinding depends on the size 

 and shape of the particles, and only indirectly on the material 

 of which the plastic mass is formed. 



To reduce plasticity. — (1) Hydroxyl-ions may be added and the 

 temperature raised (direct reduction). (2) Non-plastic material may 

 be added so as to spread the plasticity over a larger volume of material 



^ Plastic material has been formed in this way from slate, plaster moulds, 

 iron ore, ashes, lava, limestone, sandstone, bui-ned brick, silica, mica, felspar, 

 and even glass. 



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