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there is a close similarity between the behaviour of many clays and 

 that of a fine concrete composed of Portland cement and fine sand, 

 the freshly- made concrete possessing a considerable amount of 

 plasticity even when the total proportion of colloidal matter present 

 is extremely small. A careful comparison of the structure and 

 properties of such a concrete with those of a plastic clay gives a very 

 clear idea of the possible nature of clay and especially of that of its 

 most characteristic property — plasticity. 



The properties of clays which are most closely allied to those of 

 colloids or mixtures of colloids and inert materials differ according 

 as the clays are respectively in the dry, pasty, or " slip " state. The 

 jjasty condition is produced by reducing the clay to a powder by 

 grinding, and then mixing it mechanically with a suitable proportion 

 of water. Some highly plastic clays' occur naturally in the form of 

 a stiff paste which may be softened by crushing between rolls so as 

 to reduce to thin sheets and mixing this mechanically with water. 

 Clay is converted into a slip or slurry by grinding or crushing it and 

 then mixing with a sufficient quantity of water to keep the clay in 

 suspension. The amount of clay which can be suspended in a given 

 volume of water depends on the physical condition of the clay and 

 the presence or absence of very smaU amounts of alkali, acids, or 

 salts in the water. 



The following properties of clay can be most satisfactorily explained 

 by assuming the jsresence of colloidal matter : — 



Water is absorbed by any clay in fairly definite proportions which 

 appear to have some relation to its plasticity, the lean clays absorbing 

 much less water than the more plastic ones. 



When clay is completely dried without being excessively heated, 

 it is highly hygroscopic and absorbs water readily — sometimes up to 

 15 per cent, of its weight — without becoming appreciably moist. It is, 

 therefore, difficult to keep clay perfectly dry, and most specimens 

 contain a considerable proportion of water which may, in some cases, 

 cause the clay to be tough and plastic. 



The hygroscopic nature of clay distinguishes it from silt and sand. 

 When a piece of air-dried clay is placed in water, the latter enters 

 into the pores, drives out the air, and lifts up the smallest particles 

 of clay, disturbing the structure of the material so that a partial or 

 complete breakdown or slaking occurs. The disruptive action of the 

 water on the solid particles forming the clay mass may be attributed 

 to a molecular attraction between the water and the clay whereby 

 the water wets the surface of the latter and the resulting interposed 

 film of water reduces the cohesion of the clay grains so that they 

 separate easily. The absorption of water is accompanied by a sUght 

 rise in temperature, which though scarcely noticeable is characteristic. 

 The amount of water absorbed varies greatly with different clays; 

 in some cases, it is equal to 80 per cent, of the weight of the clay. 



Rohland^ suggests that this power of imbibing a definite amount 

 of water is due to the colloids in the clay, and that as soon as the 

 clay has absorbed a sufficient amount of water to convert its colloids 

 into the form of a colloidal sol its abihty to absorb water reaches a 

 saturation point and ceases ; this is proportional toJ[the colloids 



