A Study of Iron in Fired Clay : Mössbauer Effect and Magnetic Measurements

A potters clay from the Gorgan plain, in N. E. Iran, has been fired successively at temperatures up to the melting point 1 200 "C in an atmosphere of air or 5 % CO in moist COz. Mossbauer spectra at 296 K and 4.2 K allow the determination of the relative amounts of iron in well-crystallised haematite or magnetite, of poorly-crystallised and fine-particle ferric oxide or hydroxide, and of iron diluted in the ferrous or ferric form in silicate minerals. In an oxidizing atmosphere there is no ferrous iron in firings above 500 "C and at 1 200 "C, all the ferric iron is diluted in the silicate matrix. A peak in the magnetization which reaches 5 emufg. Fe near 1 100 "C is associated with a decrease in the size of ferric oxide fine particles before their disappearance at 1 200 "C. In a reducing atmosphere, some ferric iron persists up to about 1 200 OC, 12 % of the iron is in the form of magnetite between 600 and 800 "C, and its progressive disappearance from 850 "C accounts for the drop in magnetization from 16 emulg. Fe at 800 OC to zero at 1 100 "C.