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MIXTURES FOR HEAVY AND MEDIUM GRAY IRON CASTINGS.
Mixtures for heavy gray iron castings may consist of all charcoal pig iron or all coke iron; again, these pig irons may be mixed in almost any proportion, or with scrap. In cases where heavy castings require the best possible strength cold or hot blast charcoal irons are the best, and one may often have old rails, car wheels, steel or wrought scrap mixed with them to advantage. In the case of massive castings and utilizing large, heavy s^rap with pig iron, the mixtures are generally melted in air furnaces. Cupolas are also often used where the scrap is not too large, and some obtain excellent strength in iron by their use; nevertheless, as a rule air furnaces should give the best results.
flixtures for sand rolls are generally made of iron that is of a hard nature, and in some cases the same approximate analysis given for chilled rolls seen in Table 43 may be used. Then again, softer mixtures may be required than those shown in Table 43, and which can be obtained by raising the silicon or lowering the sulphur and manganese as shown. Sand rolls are often cast with cupola iron, and such can be made to give good service in many cases. over % of an inch variation in the depth of chill in like sections of the rim. In making the mixtures, it must be remembered that Tables 45 to 50 show analyses of the iron after it is remelted or in the castings, so that the iron before being charged' must be higher in silicon and manganese and lower in sulphur, after the principle described in Chapter 45.