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-26                       METALLURGY   OF   CAST   IRON.
In pouring test bars, use only "clean iron." Never take iron having slag or dross floating on top of it. Not only should the iron be clean, but a '' clean ladle '' should be used and skimmed off before pouring. While being poured it should be skimmed so as to prevent the oxide, which often rapidly forms on the surface, from passing into the mould.
With the use of round test bars cast on end, an intelligent comparison of one class of metal with another will demonstrate that there is a dividing line between soft and hard grades as to which would be the strongest with il hot " or "dull" poured metal. At present, that chiefly concerning us here is, at what temperature are bars best to be poured. As the founder chiefly makes tests for comparison, either to test his own mixtures or to furnish tests to compare with those of competitors, at the request of a middle party, it seems but reasonable and best that a temperature be maintained that would best conform with that generally used. I would not advise a metal being too " hot " or too " dull," but something that would average about four and one-half inches up in the fluidity testing tips S and S, Figs. 121 and 122, pages 509 and 514.
Some founders might say their iron was hotter and would run up higher to a fine edge than that. I am not disputing these, but I do question whether they will always obtain the same. high fluidity; and then again the iron may come out of the cupola all right, but owing to some "hitch" in the moulder getting to his " floor " ready to pour at some one time, could throw them off in their calculations. All elements and conditions considered, it is decidedly best to pour at a temperature while sure to run and make solid tests is often done by the kind of swabs sometimes used.''