The addition of small quantities of a solution of certain substances to a stiff clay paste usually reduces its stiffness, and in some cases turns it into a liquid. The alkalies are particularly powerful in this respect and their action may be strikingly illustrated by mixing a few drops of caustic soda with a stiff clay paste. In a few moments the mixture will be sufficiently liquid to pour readily, but it may be rendered quite stiff again by adding sufficient acid to neutralize the alkali previously used. Weber (12) has utilized this characteristic to great advantage in the production of sanitary ware and crucibles for glass-making by a process of casting which he has patented. The effect of adding water to a dry clay is curious. At first the particles in contact with the water become sticky and plastic, and if the proportion of water added is suitable and the mixing is sufficiently thorough a plastic mass will be produced, the characteristics of which will depend on the nature of the clay used. This process of mixing clay with a limited amount of water is known as 'tempering.' The proportion of water required to make a paste of suitable consistency for modelling appears to be constant for each clay. If, however, a larger proportion of water is added the particles of clay will be separated so widely from each other that they lose their cohesion, and instead of a plastic mass, the material will form a liquid of cream-like consistency. If a piece of stiff clay paste is suspended in a large volume of water without stirring, disintegration will still occur (though a much longer time will be required) and the clay will be deposited as a sediment at the bottom of the vessel. The leaner the clay or the larger the proportion of non-plastic material it contains, the more rapidly will this disintegration take place. A highly plastic clay will become almost impervious and will retain its shape indefinitely. If a mixture of clay and water in the form of a cream or slurry be allowed to rest, the larger and less plastic particles will settle, but many of the particles of true clay will remain suspended for several hours and some of them for several days. Some particles of clay are so small that it is doubtful if they would ever settle completely unless some coagulant were added, and as they readily pass through all ordinary filtering media it is extremely difficult to collect them in a pure state. These turbid suspensions of clay may be rapidly cleared by the addition of sodium chloride which increases the surface tension of the solution. The fine particles behave in the same way as colloidal substances, _i.e._ as if they possessed an electrostatic charge. Hence the addition of a salt (electrolyte), whose ions annul the opposite charges of the electric double layer assumed by Helmholtz to be present, enables the particles to coagulate in accordance with the ordinary laws of surface tension (14).