The microscopic image analysis has been applied for determination of p
orosity of hematite grains reduced to magnetite at 450 and 850 degrees
C, At 450 degrees C, two kinds of voids were analysed: cracks and mic
ropores inside subgrains created by crackling of the initial grain. At
850 degrees C, porosity profiles across the magnetite layers were vis
ible. Also a temporal dependence of the local porosity has been observ
ed. The porosity determined by mercury porosimeter agrees reasonably w
ith microscopic data.