118 



present, and probably, roughly to the plasticity of the clay. It may, 

 however, be proportional to the capillary spaces between the clay 

 particles. 



In the manufacture of articles from clay paste, it will be found 

 that each kind of clay requires a definite proportion of water for its 

 efficient manijDulation. If more is added it will become too weak, if 

 less it will become too short. This water is known as " water of 

 formation," and its amount has a theoretical as well as a practical 

 importance, being closely related to plasticity. Unfortunately, there 

 is no certain method of ascertaining the consistency of the clay paste, 

 nor of ascertaining when the correct proportion of water has been 

 added to a clay. The ordinary method consists in adding such a 

 proportion of water that when the mixture is worked up into a paste 

 it readily receives the impression of finger-prints, but does not adhere 

 to the skin, the amount of water required being found by trial. 

 This procedure is too rough for scientific purposes. 



If water is added to a moderately plastic, dry clay in increasing 

 quantities, the clay can at first be moulded with difficulty, then more 

 easily, and later it may be moulded with the greatest facility. If 

 the proportion of Avater is stUl further increased, the clay becomes 

 sticky, then fluid, and it is eventually impossible to form it into any 

 definite.shape. 



If the same experiment is rej)eated with a more plastic clay, using 

 the same proportions of clay and water as before, it will be observed 

 that it will adhere to the fingers and will allow of no further shaping 

 unless its plasticity is diminished bj^ adding non -plastic material or 

 altering the proportions of clay and water. 



An excessively lean clay, on the contrary, only acquires the desired 

 plasticity when it has a very soft consistency, which does not allow 

 it to remain in any given form, and it must, therefore, be rendered more 

 plastic if it is desired that it should be sha] ed by hand. If the 

 formation is done by mechanical means, in which the clay is subjected 

 to much stronger pressure, less water must be added to the body in 

 order to give it the required plasticity, and it A\ill be expedient to 

 make it of a stift'er consistency. Pressure, in this case, plays the 

 same part as water in the plastic qualities of clays ; the one can be 

 partially replaced by the other, so that if the amount of pressure is 

 increased the proportion of water should be diminished and vice 

 versa. 



If a sufficient quantity of water is added to a clay to form a slip 

 or slurry, the latter will have certain characteristics, according to 

 the proportion of water and clay, to the nature of the clay and the 

 purity of the water. If the proportion of water is very large and the 

 particles of clay difficult to separate, they may fall to the bottom 

 very soon after the mixing ceases, or the greater part of them may 

 so faU, leaving only the smallest particles suspended in the water for 

 many hours. With high grade clays, such slips have marked colloidal 

 properties (see Viscosity, Adsorption, t&c). 



Slips containing about an equal weight of water and clay are 

 largely used in various branches of clay-working, for covering other 

 clays of inferior quality when burned, and for making objects by the 



