120 



sensitive to the action of electrolytes, a very small quantity of a 

 solution of soda being capable of converting a clay-jiaste into a viscous 

 fluid which, on the addition of just sufficient acid to neutralise the 

 alkali, will again become solid. This behaviour bears a remarkably 

 close resemblance to the action of electroljrtes on the coagulation and 

 deflocculation of colloids. Roliland^ has suggested that the formation 

 of a clay slip (sol) may be explained as due to the action of electrolytes 

 on the colloids present in the " clay." With a negative sol in colloidal 

 suspension, the most powerful factor in coagulation is the positive 

 ion of the electrolyte added, the negative ion having but little iiiiluence. 

 The power of different positive ions appears to be the same for those 

 of the same valency, but divalent and trivalent ions are more powerful 

 than monovalent ones. Thus, Foerster has shown that if a clay 

 contains just enough calcium ions to keep the colloidal matter in the 

 gel state, and sodium carbonate is added, the sodium will combine 

 with the colloid clay so as to form the sol, the plasticity being reduced 

 according to the completeness of the reaction, but if an excess of sodium 

 ions is added they will recoagulate the coUoid. This has been confirmed 

 by experiments on the viscosity of clay slips by Mellor and others. 



The addition of electrolytes to a clay body also affects some of the 

 materials present. Thus, Schurecht* has found that the working 

 properties of mixtures of graphite with sufficient plastic clay to act 

 as a binder, such as are used in the manufacture of plumbago crucibles, 

 are considerably improved by the addition of 0-3-0 -4 per cent, of 

 sodium hydroxide or, in some cases, of hydrochloric acid, according 

 to the nature of the colloidal matter present. 



Kosmann^ attributes the disintegration action of alkaline solutions 

 on clays to the solution of a siliceous film on the particles which acts 

 as a binder. This explanation scarcely seems to account for the great 

 effect produced by so small a proportion of soda. 



When clay is saturated with water and an electrolyte is then added, 

 the adhesion of the particles is reduced, partly as a result of the 

 osmotic pressure of the solution on the porous liarticles^" which then 

 act as a permeable diaphragm and force the water more strongly into 

 the interior of the particles than would be the case if plain water 

 were used. If the basicity or alkalinity of the solution is altered 

 by the addition of an acid, the particles tend to coagulate and adhere 

 to each other with the result that the mass becomes semi-solid. 



When clay is suspended in a liquid having a liigher coefficient of 

 capillarity than water {e.g., acids) the particles tend to precipitate, 

 but in a liquid with a lower coefficient than water {e.g., bases and 

 alkalies), they tend to remain in suspension. This behaviour is 

 attributed to the difference in the adhesion of the fluid particles of 

 the liquid to the particles, the surface of the smallest particles being 

 much greater in proportion to their weight than that of the larger 

 ones. 



Adolph Mayer has determined the limiting power of electrolytes 

 which permit a fine clay (freed from carbonates and soluble salts by 

 treatment with hydrochloric acid) stUl to be kept in suspension in 

 water (100 grammes clay, 500 grammes water). The limits are : — 

 ammonia, 2-5 per cent.; sulphuric, hydrochloric, and nitric acids 



