Ferrous oxide is not stable, why it can be stable in the green brick

Ferrous oxide is not stable, can be stable in the brick because of the firing process. According to the query related public information shows: brick production technology is a traditional handicraft. The process of making bricks all manual operation, choose high quality clay, cool sun, stirring, to storm the quality, increase flexibility, so three volts two summers, and then screened, with neutral water and mud, mud, batter, made of billet and placed in the shade in the cool place to dry, and finally in the cellar at high temperatures within a month of firing, so ferrous iron oxide is unstable, can be stabilized in bricks is due to the firing process.