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are sometimes used instead of plain water in pugging the clay. 

 Such additions do not usually increase the plasticity of the clay 

 to more than a small extent and they greatly increase the difficulty 

 of burning the ware satisfactorily. 



(ii) The addition of weak acid such as acetic acid, oxalic 

 acid, or tannin (gallotannic acid) to neutralise the acid present 

 or to discharge any hydroxyl-ions formed by the dissociation 

 of salts present as impurities in the paste. One of the most 

 interesting proposals in this connection is that of Acheson and 

 Ries who have found that on the addition of a 2 per cent, 

 solution of tannin to certain clays the plasticity may be greatly 

 increased and the clay was apparently deflocculated and broken 

 up into smaller particles, whilst the tensile strength was greatly 

 increased. The use of a substance definitely known as an acid 

 is not necessary ; any substance which acts as a corresponding 

 electrolyte of either organic or inorganic nature is advan- 

 tageous because it neutralises the charge on some other electro- 

 lyte present in the paste and so effects a definite increase in the 

 plasticity of the clay. 



The great importance of souring is not appreciated as it should 

 be, because the phenomena which occur are largely misunderstood 

 and consequently, it is often omitted where it would be most useful. 

 In the manufacture of bricks, and coarse goods, omission may not 

 be serious, but it should never be omitted in the preparation of a 

 paste for the manufacture of tiles, glazed ware, earthenware, fine 

 pottery, and porcelain. 



Ware made from a properly soured paste is less sensitive to sudden 

 changes of temperature, can have thinner walls, does not break so 

 easily, and is easier to produce as the souring increases the plasticity of 

 the paste. 



It is usually necessary to pug the paste after it has been soured 

 so as to form it into a compact and homogeneous paste. 



Certain clays have long been used as absorbents for grease and 

 similar materials especially in fulling cloth and for medicinal purposes. 

 Curiously enough, these clays are among the least plastic. Their 

 usefulness depends on their absorptive and adsorbent properties. 



Conversely, the use of clay in the preparation of ultramarine 

 depends partly on its chemical composition, but chiefly on the colloidal 

 nature of the product. 



It should be remembered that clays can only be used for removing 

 basic dyes and colours such as malachite green, as their removal from 

 solution depends on the negative electric charge carried by the 

 colloidal particles in the clays. Incidentally it may be noted that 

 clays containing much colloidal silica or colloidal alumina give 

 uncertain results when treated with malachite green. 



Clays are also used in the preparation of several other colloidal 

 substances including Portland cement. 



The clarifying power of clays, when mixed with turbid fluids, also 

 depends on the colloidal nature of the finely suspended particles 



