CLAYS or NEW YOKE 517 



matter, and this material, if present in a dense wet clay, tO' whicli 

 the air can not gain access, may keep the iron in a ferrous condition. 



Whenever the iron exists in the clay in combination with silica, 

 it is present prohably as a complex silicate, for pure ferric silicate is 

 very rare in nature. 



The presence of ferric hydrate in clay increases its absorptive 

 power for both gases and water, but both it and the carbonate are 

 converted in burning to the oxid. 



While it may be said that the burning of clay in an oxidizing 

 fire converts the iron to the condition of ferric oxid, still this state- 

 ment only holds true up to a certain temperature, depending on the 

 fusibility of the clay, for in every clay the iron see-ms to return to 

 the ferrous condition as the point of vitrification is approached. 

 The change would of course be accompanied by a liberation of 

 oxygen, which would increase with the amount of iron in the clay, 

 and may account for the greater blistering of ferruginous clays as 

 the point of vitrification is passed, and that of viscosity approached. 



While this fact is not unknown, very little attention seems to 

 have been paid to it. 



Remole ^ considers that the greenish color of hard burned clays 

 is due to this cause. Seger^ also notes the ferrous condition of 

 iron at high temperatures, and states that in this form it is a 

 powerful flux. 



The tendency of iron oxid is to unite with the silica and alumina 

 and also with the lime of the clay the moment that fusion begins, 

 thereby forming a complex silicate, whose fusibility is lower than 

 the simpler ones from whose union it was formed. 



The experiments of Berthier (Percy's Metallurgy, refractory 

 materials and fuel, p. 60-75) on mixtures of iron, alumina and 

 silica point out these facts very clearly. These consisted in making 

 up the mixtures given below and subjecting them to a high tem- 

 perature, that of molten steel, with the results also stated below. 



' Wagner. Manual of chemical technology. 1897. p. 634. 

 'Seger. Gts. Schrift. p. 391. 



