74 GEOLOGY OF OHIO. 



Chemical analyses of clays generally state the ultimate quantities of 

 the various elements represented, but fail to give any idea of their minera- 

 logical structure. This can be in part remedied by such an arrangement 

 of the elements as was first devised by the late Prof. Cook, of the New- 

 Jersey Survey, and which was followed in Vol. V of the Ohio Survey,, 

 and has been used largely in the present report. This method of report- 

 ing an analysis classifies the clay into kaolinite base, sterile impurities 

 like quartz and titanic acid, and deleterious or fluxing impurities includ. 

 ing the strong bases such as iron, lime, magnesia and the alkalies. This- 

 system requires, however, that an extra determination shall be made for 

 each analysis, separating the silica into the combined and the free. The 

 chemical processes employed in this separation are far from satisfactory*, 

 however, and since the essential facts of the kaolinite base theory have 

 been generally adopted it seem not inadmissible to divide the silica into- 

 free and combined, by calculation of the kaolin ratio from the percentages- 

 of alumina and water present. This has been done in most of the new 

 analyses made by the Survey for the. present report. No table of analysis 

 has been prepared for presentation in this section of the report, but special 

 pains have been taken under each separate industry described, to present 

 such analyses as were available in that particular branch. Attention is 

 therefore invited to the succeeding tables to illustrate the general state- 

 ments relative to composition of clays made here. 



(c) Properties. — The properties of clay which make them indis- 

 pensable to mankind are three fold: 



1st. Plasticity when wet, by which it can be formed into the 

 multitude of shapes that suit our convenience. 



2d. Permanence when burnt, which enables us to fix these useful 

 forms in material at once cheap, ornamental and durable. 



3rd. Refractoriness or the ability to stand high and long-continued 

 heat without fusion or loss of form. 



Plasticity is a quality which is shared by very few other minerals- 

 and very few other artificial compounds capable of standing heat. It has 

 never been satisfactorily explained by any one who has worked upon the 

 subject. The late Prof. Cook, of New Jersey, has done more for the sub- 

 ject than any one else. He found out by microscopical examination of 

 kaolins that the mineral generally presents a distinct tendency to crys- 

 tallization. Some kaolins are composed of masses of hexagonal plates or 

 scales piled up in long bundles or faces and masses of unattached scales. 

 When they present this appearance they are very little plastic. By grind- 

 ing a sample of such clay fine, and working and kneading it with water, it 

 gradually becomes plastic however, and an examination under the gla?s. 

 then shows that the crystalline structure is much broken up, and that the 

 still perfect scales or crystals are embedded in a homogenous matrix com- 

 posed of the waste of the other crystals which have been destroyed by 

 the grinding and kneading. Other clays, already naturally plastic, show 



