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296                       METALLURGY   OF   CAST   IRON.
ing" up '' the general proportion and shape of the whole casting, judge more readily of the " grade " in the massive parts than if it came to his yard in a haphazard form.
We are compelled to analyze pig metal (as shown on page 178) simply because it is deceptive in showing its true t i grade '' to the certainty that scrap iron will permit, on account of its being a re-melted product. If one wishes to grade scrap by the plan suggested on " pages 292 to 294, in this chapter, it. is best to follow a silicon formula for a base, owing to the fact that silicon is the element generally largest in gray castings excepting carbon and affords a larger range or margin in guessing percentages, which if not close to the actual silicon contents cannot so greatly result in injury as it could if one used a guess of the sulphur for a base, and should err much. As scrap with many founders constitutes a third and often two-thirds of their total mixture, this chapter cannot but be of benefit to any who may be desirous of conducting their mixtures of scrap iron with the best assurance of obtaining desired results without resorting to analysis.
Much oxide of iron, or rust on scrap iron, is very injurious in lowering the silicon of a mixture and thus cause a hard iron where a soft one was expected. Burnt annealing boxes, old grate bars, etc., give off a great deal of oxide of iron. The good iron melts more readily than the oxide of iron. If any of the latter is not reduced to iron and is carried with the molten metal into castings, as it may be, blow holes may be formed which are generally to be found in the top surface of castings as they are poured. Where there is any apprehension of such difficulty, it is often well to addo break, he can, by " siz-