114 



There are numerous definitions of the term day, but most of them 

 are incomplete. Some are so inclusive as to be applicable to any 

 plastic material, others involve an erroneous assumption as to the 

 manner in which the " alumina " and " silica " are combined, and 

 no definition has yet been published which is entirely satisfactory. 

 Until a better one is forthcoming the following is convenient, though 

 by no means free from objection : — 



A clay is a naturally occurring earthy material, whose chief physica 

 characteristic is its plasticity, and whose essential constituents are reported 

 in an analysis of the substance to be " alumina," " silica " and " water." 



This definition does not exclude those highly siliceous and plastic 

 materials commonly known as " brick clays " though some of these 

 are known to contam as much as 60 per cent, of materials of a non- 

 plastic, sandy nature, which is certainly not of the nature of clay. 



When a commercial sample of clay is mixed with an equal weight 

 of water and allowed to stand for a few moments, and the liquid 

 decanted through a sieve having 200 holes per linear inch, this treat- 

 ment bemg repeated with fresh water until all the small particles 

 have been removed, the residue will usually be devoid of plasticity 

 and win not possess the properties of clay. Usually, it will resemble 

 sand or a mixture of gravel, sand, and rock flour. When some clayey 

 materials, such as some indurated clays, are subjected to this treat- 

 ment, the whole of the plastic material is not removed, but on 

 prolonged exposure to water, or better still, if the water is made 

 slightly alkaline and boUed in contact with the clay for several hours, 

 the material will be effectively separated into a coarser, sandy, 

 non-plastic matter, along with the smallest non-plastic particles. 

 By a suitable modification of the treatment just mentioned, a " clay " 

 may be divided into a number of fractions, of which all those 

 consisting of particles which will not pass through a No. 200 sieve 

 are obviously not clay. The finer particles are sometimes designated 

 " clay substance," but, though they contain the whole of the plastic 

 material, they are not wholly " clay," as by careful elutriation or 

 repeated sedimentation a further series of non-plastic and siliceous 

 materials may be separated. Seger'^ suggested that the particles 

 which were carried away by a stream of water flowing at the rate 

 of 0-18 mm. per second should be regarded as "clay substance," 

 but this fraction contains a considerable proportion of non-clayey 

 material unless it is derived from a particularly pure clay, so that 

 this use of the term " clay substance " should be abandoned. 



The smallest particles which are obtained by elutriating the 

 materials commonly known as clays are found to correspond more or 

 less closely, on analysis, to a composition which may be represented 

 by the formula H4Al2Si209. In some samples of Cornish clay and 

 some kaolins, the composition is remarkably constant, but many 

 highly plastic clays and most flreclays yield a product richer in sUica 

 and deficient m the elements of water. The constancy of the 

 composition of the better qualities of white-burning clays has led 

 to the supposition that there is in all clays an essential substance — 

 true clay, clayite, or pelinite — on which aU clayey mixtures depend 

 for their chief properties. The existence of this '" true clay " has 



