Photomicrographs Nos. 109, 110, 113 and 35 which show the samples at 75 diameters magnification show the course of the annealing process. No. 109 was taken from an unannealed casting. No. 110 was of the same iron after approximately thirty hours in the furnace. No. 113 shows that after about forty-five hours nearly all of the iron-carbon chemical compound has been broken down into black patches of free carbon, surrounded by the white areas of pure iron. After about sixty hours all of the iron and carbon have been divorced and the annealing operation is complete, as is shown by photomicrograph No. 35. [Illustration: No. 113. PHOTOMICROGRAPH OF NO. 109 WHEN NEARLY ANNEALED ] [Illustration: No. 35. COMPLETELY ANNEALED MALLEABLE CAST IRON ] While heat alone effects the divorcing of the carbon and iron, which is the essential part of the annealing process, in the greater number of cases aid is given by what may be termed chemical means. Reaumur, who about 1722 discovered the annealing process, used iron oxide for the purpose. The white iron was packed in iron ore or mill scale. At the high temperatures employed the oxygen of the ore in some way not yet definitely known, gradually removed the greater amount of the carbon from the casting. It has always been a scientific conundrum how a solid, iron oxide, surrounding another solid, a piece of white iron, could remove from the latter its carbon when neither of them melts nor mingles with the other. Whether some of the oxygen from the ore penetrates the iron and burns out its carbon or whether the carbon of the casting itself migrates is not yet definitely settled. Certain it is that the carbon is gradually removed from the casting, from the surface first and with increasing length of time from greater depths. [Illustration: No. 390b. PHOTOMICROGRAPH SHOWING DECARBONIZED OUTER LAYER The photomicrograph also shows that this casting was not fully annealed. ] In European practice malleable iron castings are still malleableized in this way, i. e., by burning out the carbon. The castings are made as thin as possible and the annealing in “packing” (iron ore or mill scale) is continued for from one to two weeks. At the expiration of this time the castings have a white, steely fracture which is entirely unlike the fractures of malleable iron castings which are made in this country. Photomicrographs of such malleable iron show few or none of the black spots which No. 35 exhibits, and analyses of castings annealed in this way give very low results for carbon. [Illustration: MALLEABLE CAST IRON IN WHICH PRACTICALLY ALL OF THE CARBON HAS BEEN REMOVED BY REAUMUR PROCESS ANNEALING ]