II!) 



process of casting. In ihc former case, the articles to be covered 

 are immersed in the sUp, and in the latter, the slip is poured into plaster 

 moulds and allowed to remain for a short time, after which any 

 superfluous slip is jjoured aAvay. On allowing the mould to dry, the 

 w ater is absorbed by the plaster, and the clay article may be removed 

 in due course. 



In both cases, it is necessary that the proportions of clay and 

 water should be carefully adjusted, in order to obtam the best results. 

 When a suitable mixture has been obtained, it will usually be suflicient 

 to weigh exactly one pint of it accurately, and to dikite other mixings 

 with a stronger slip or with water, vintil they reach the same weight 

 per pint. The specific gravity of the slip may be determined with 

 great exactness in a pycnometer, if desired, but this involves unneces- 

 sary trouble for most purposes. 



Schwerin has found that water and alkalies in the clay slip may 

 be removed by electro-osmosis by connecting the bottom of the tank 

 containing the slip with the negative pole and the cover with the 

 positive pole of a battery when, on passing a suitable electric current, 

 the water and alkali will collect at the bottom, and the slip will become 

 very stiff and apparently — though not actually — dry. 



The hygroscopicity of dried clay is very marked, up to 20 per cent, 

 of water being absorbed from a damp atmosphere by some claj^s. It 

 does not necessarily prove the presence of colloidal gels, but if the}' 

 were present such hygroscopicity would be anticipated. 



Miscibility. — It is a remarkable fact that highly plastic clays, in 

 addition to ha\'ing a limited power of absorbing water, are incapable 

 of forming a uniform mixture with less plastic clays. According to 

 Rohland^, this is due to the fact that when colloids in clay are 

 coagulated they form gels which cannot be brought into solution by 

 the addition of more water, and resist the absorption of water. They 

 are also incapable of taking up anj^thing from a second colloid. Hence, 

 if the colloids are coagulated, as in very plastic clays, they will not 

 absorb more than a certain amount of water, will not take up other 

 plastic clays, and will not mix homogeneously with them. Many 

 objectionable qualities of a higlily plastic clay may be obviated by 

 saturating it with water and then adding a suitable amount of non- 

 plastic material. In this way, also, highly plastic clays gain the 

 power to be mixed thoroughly with other plastic clays and with 

 felspar, which forms coagulable colloid solutions. 



Rohland has found that plastic clays in which there is only a small 

 proportion of colloids, and these not coagulated, may be uniformly 

 mixed with other similar clays. 



Deflocculation . — Clays usually exist in large masses which are not 

 readily affected by water, but smaller pieces may be broken down 

 or " slaked," as just described, in a manner which is very similar to 

 the deflocculation of colloidal gels. If a suitable electrolyte such 

 as sodium hydroxide, carbonate or silicate, or baryta is added, the 

 amount of suspended matter is increased, as with \\'ell-known colloids, 

 and, if an acid is added to the suspension, the clay particles are 

 rapidly precipitated like a coagulable gel. Clays are remarkably 



