G. D. Hubbard — Colloids in Geologic Problems. 107 



are present, which interfere with, the flotation, by adsorb- 

 ing the oil and preventing its nse by the sulphides. Here 

 too, the trouble increases if the oil is emulsified, for the 

 finer the oil particles are, the more readily do the clay 

 colloids adsorb them. Usually the ore is not ground fine 

 enough so that its slime is at all colloidal. Electrolytes 

 are sometimes used as an aid in flotation, for they help 

 to prevent emulsions, or coagulate them if formed. 



Plasticity in Clays. — This subject has been the theme 

 for many interesting papers, and of some heated discus- 

 sions, but the air seems to be clearing, and Ashley 17 writes 

 that it is pretty generally believed by the students of 

 ceramics that the control of plasticity in clays is a matter 

 of the control of colloids. A number of other writers 

 can also be quoted in support of this position. A French 

 ceramic chemist, T. Schlosing, in 1888, P. Rohland in 

 various papers from 1902-9, A. S. Cushman, quoted above, 

 and F. W. Clarke in the Data of Geochemistry, all agree 

 that the plasticity of clays can be very directly traced to 

 their colloid content. Schlosing shows, according to Ash- 

 ley, that the amount of colloidal material in the best clays 

 is small, and rarely exceeds one and one-half per cent. 

 As the percentage of material in the colloidal state 

 decreases, so does the plasticity, and a clay with one-third 

 of the amount mentioned is a lean clay. Likewise, if the 

 amount is greatly increased above the one and one-half 

 per-cent, the clay becomes less plastic, and more sticky. 



It is well known that the plasticity of clays is destroyed 

 by ignition and that the colloids are also made to change 

 state by the same treatment. On the other hand, many 

 thoroughly ignited clays absorb water quite as well as 

 unignited ones. This makes it clear that the colloidal 

 material is not the cause of the absorption of water, nor 

 is the absorption of water vitally related to plasticity. 

 Series of experiments have been made with lean clays to 

 increase their plasticity, and it has been found that very 

 poor clays can be made plastic enough to be worked suc- 

 cessfully by adding small quantities of certain colloids; 

 agar-agar .08% increases the plasticity of different clays 

 as much as 40 and in some cases 60%, and aluminum 

 cream produced the same effects by adding in much larger 

 quantities, e.g. about 3%. 



17 Ashley, H. E., Bur. Stand., Tech. Paper 23, 1911. 



