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306                   METALLURGY OF CAST IRON.
observe whether there would be any difference in the strength of iron taken from the beginning, middle, and end of "heats," where a uniform mixture was used throughout a heat. Results received affirm that some would obtain the strongest test at one part, while others would receive them from another part of a heat. In this practice the author cannot conceive of any uniformity being obtained unless the management is such as to insure a like temperature and fluxing at every part of a " heat," and in this quality generally lies the secret of the difference between one founder and another. One may have a cupola giving the hottest iron at the beginning of a heat while another will obtain this at the middle or the end. According to the variation of temperature when re-melting iron, so is the combined carbon affected by changes in the silicon, sulphur and manganese; and taking this view of the subject the author believes that all can understand why we find, founders disagreeing in such tests.
As the humidity of the air can, to some extent, produce changes in the smelting or melting of iron, one heat from another, the author appends the following* excellent article written by Mr. A. Sorge, Jr., M. E., in the Foundry, April, 1896:
That variations in the humidity of the atmosphere and its temperature do affect the operation of melting iron in a cupola, will be conceded by foundrymen who have observed the difference in melted iron on different days. Iron is liable to be cold and sluggish with the same charges of fuel on cold and moist days, while it is hot and fluid on warm and bright days.
It is therefore reasonable to look for one cause of poor melting to the atmospheric conditions. Let us assume that we are melting at a ratio of eight iron to one coke on an ordinary bright day,ose of a cupola, and in both cases it is a subject as necessary t<> he understood, in order to obtain desired ends, as is that of knowing the chemical properties of the iron before it is ehanjvd.