145 



colloidal gel surrounding particles which are of a non-plastic or 

 colloidally inert nature ; in some cases, they may be rich both in 

 alumina and silica — as in china clays and ball clays — whilst in others 

 they may be almost wholly silicious, as in fireclays and many brick 

 earths. 



The particular kind and amount of gelatinous matter present, 

 the size and shape of the grains of non-coUoidal material, and the 

 relative proportions of large and small grains are important factors 

 in determining the various physical properties of clays, particularly 

 their binding power, compressive strength, tensile strength, and 

 air shrinkage. 



Some Technical Uses of the Colloidal Properties of Clays. 



Although the nature of the materials to which clays apparently 

 owe their colloidal properties is unknown, great use is made of the 

 properties in various industries, as will be seen from the following 

 notes : — 



In purifying clays and similar substances, the suspension of the 

 material in water, followed by a process of elutriation or sedimentation, 

 whereby the coarser impurities are removed whilst the partially 

 purified clay is carried off in suspension, has been in use since ancient 

 times. More recently — especially in America — electrolytes, such as 

 sodium carbonate, caustic soda, &c., have been added to the water 

 employed, so as to ensure a maximum amount of clay being held in 

 suspension in a minimum quantity of water. The use of such 

 electrolytes also ensures a sharper separation of the inert sandy 

 material present. The suspension is run off into suitable vessels, 

 the added alkali neutralised by the addition of sulphuric acid ; the 

 clay is allowed to deposit and afterwards removed and dried. The 

 separation of the impurities is due to the fact that whenever a charged 

 colloid particle in suspension meets another similarly charged particle 

 they mutually repel each other and so remam in suspension. When 

 tiro particles of opposite charge come into contact, the charge is 

 neutralised, and the two particles unite and are precipitated. The 

 electrolj^e added must, therefore, be one which ^\'ill increase the 

 negative charge of the material as a whole, so as to effect the precipita- 

 tion of the impurities, (which are chiefly electro-positive) and retain 

 in suspension the electro-negative particles of clay and, along with 

 them, some silica which is also electro-negative. The coarser particles 

 of silica, pyrite, felspar, mica, &c., do not become charged, but settle 

 on account of their size and weight. If the liquid is too viscous, 

 the impurities will not settle properly, and it must then be diluted 

 until the density and viscosity are such that sufficient separation 

 is effected Avithout the loss of clay by sedimentation. 



A similar process of purification is used as a preUmtnary stage 

 of the Schwerin electro-osmosis process-^, but instead of the clay 

 being allowed to settle it is caused to migrate to a rotating electrode 

 immersed in the liquid and is scraped off in the form of a stiff paste. 

 Schwerin found that all clays did not behave thus, and only migrated 

 so when they carried an electric charge. This charge is supplied, 



X 11454 K 



