78 GEOLOGY OF OHIO. 



pears, the clay turns black in color and porous and vesicular like lava 

 and worthless for any purpose. 



In the light colored clay the iron was present as grains of sesquiox- 

 ide; heat and the influence of the gases of the fire reduced this finally to 

 ferrous oxide; in this condition the fluxing action began to assert it elf 

 and combination between any accessible free silica and the iron ensued, 

 forming black ferrous silicate; this would rapidly continue to absorb such 

 additional quantities of silica from the clay as would satiate its affinities 

 at that temperature; and the iron has now done its worst in this clay. 



In the red clay the iron is assumed to be present as an iron alumina 

 silicate. All parts of the clay alike are equally permeated by it and colored 

 by it. As the heat is increased the iron, affecting every part of the clay 

 causes a fritting or incipient fusion of the clay with its other constituents. 

 When this process is complete the clay presents a glassy fracture, a very 

 hard structure, non-porous and having no affinity for water, and generally 

 a brown red color. If now the heat be still increased the iron begins to 

 be affected by the reducing gases of the fires, and to be reduced to a lower 

 state of oxidation in 'which combination with the alumina of the clay is im- 

 possible. This reaction then continued, results in the complete destruc- 

 tion of the valuable qualities of 'the clay; it becomes porous, light in 

 weight, brittle and rotten in texture, unable to stand either blows or fric- 

 tion. Also the phenomena presented in the changes of color of clays 

 under the action of reducing and oxydizing atmospheres lend additional 

 strength to this theory. 



Take several samples of fire clay, such as is used for pipe or stone- 

 ware, and subject them to a strong reducing or carbonaceous atmosphere 

 for a time; if one sample be quickly withdrawn and cooled it will show 

 itself probably of a gray or bluish color in its whole structure, indicating 

 the presence of a ferrous oxide'in very small quantities. If the balance 

 of the white hot samples now be allowed to slowly cool in a light draft of 

 air they will become yellow, red or brown according to the quantity of 

 this iron, aside from the free iron which the clay contains, which will 

 manifest itself by small blotches of black cinder for every grain which lay 

 exposed to the surface. This change in color is due to a re-oxidation of 

 the iron in its long exposure to heated air during the cooling process. 

 Also it very seldom happens that a clay is found which will yield up its 

 iron contents by solution in boiling hydrochloric acid. Yet any free iron 

 oxides, or ferrous silicates or other iron compounds are readily decom- 

 posed by this means. Hence the inference that part of the iron is present 

 as a double silicate of iron and alumina which like the simple silicate of 

 alumina is not attacked by hydrochloric acid. 



These and other reasons make the assumption a probable one that 

 iron plays its most important part in the chemistry of clays as a chemi- 

 cally combined constituent of the clay base. 



The action of lime and magnesia in promoting the vitrification and 

 fusion of clays is a power ,'ul one if the bases are present in large quantity. 



