144 



silica which does not possess the properties of the original clay. 

 Various other attempts to synthezise " true clay " have been equally 

 unsuccessful. 



It is generally considered that one of the most important sources 

 of clays is the mineral felspar which can be obtained in the form of 

 pure crystals, should form a good starting point. The felspar to 

 which certain clays are commonly attributed is orthoclase, wliich 

 has the formula KjALSieOie. This is a minimum formula and may 

 be more correctly represented by K . Al .; Si ;< sOg < • Keeping the simple 

 form, the decomposition may be represented by — 



K2Al2Si60i6+ 3H2O = 2K2O + H.ALSi^Os + 4Si02, 

 which assumes that the felspar is a potassium alumino-sihcate with 

 china clay as the correspondmg acid, but containing much less silica. 

 Fireclays, on the contrary, even after such limited purification as 

 can be effected, often contain more silica, and indicate a slightly 

 different equation and they appear to have an entirely different origin 

 both chemically as well as geologically. Unfortunately, this decom- 

 position of felspar has never produced clay when carried out in the 

 laboratory. Water produces under great pressure a white body 

 which has little or no plasticity, which may be clay, but cannot be 

 identified as such. 



Another attempted synthesis was that of Pukall,-" who decomposed 

 a pure china clay with sodium cliloride and treated the product with 

 carbonic acid. It then jHielded a salt deficient in silica and soda 

 and corresponding to 2Na,0 4H2O 6AI2O3 lOSiO,. The same salt 

 when treated with a strong acid (HCl) dissolves and from the solution 

 ammonia precipitates a material wliich PukaU mistook for synthesized 

 clay, but which contains rather more hj'drogen and oxygen and 

 dehydrates readily at 350° C. instead of 500° C. Apparently, an 

 isomer of clay had been formed. 



At present, it seems quite impossible to be certain of the composition 

 of the substance or substances to which clays owe their chief character- 

 istics. The reporter favours the view (p. 125) that age-long grinding 

 has produced a film of coUoidal matter on grains of non-plastic 

 material as probably accounting for most claj^s, but he also believes 

 that no single cause can account for the formation of all kinds of 

 clay, and that other explanations, such as those given on p. 126 seq., 

 may be equally correct in some cases. The fact that the finer 

 particles of all plastic claj'S correspond more or less closely to the 

 formula HoAl^SiaOg does not necessarily invaUdate the theory that 

 clays are simply a product of mtensely ground rocks, as there are 

 many clavs which do not correspond to the formula just mentioned 

 and some of these which correspond to it more closely are only feebly 

 plastic. The similarity in composition of materials regarded as 

 clays may, possibly, be merely a coincidence due to the predominating 

 proportion of alumino-sUicious rocks in the material of which the 

 earth's crust consists. 



Whatever its nature and origin, it is now fairly well established 

 that many of the properties of clays are closely connected with the 

 coUoidal matter present, such matter being in the form of a film of 



