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.     MIXTURES   FOR   CHILL   ROLLS,    CAR   WHEELS,    ETC.     265
a solid, hard surface free of all defects, that will not spawl or shell off by usage, and a depth of chill which will permit the face being trued up occasionally until the chill is nearly worn off.
The character of iron used for chilled rolls consists largely of cold and hot blast charcoal iron, often mixed with broken rolls, car wheels, and sometimes steel scrap. Cold blast charcoal combines strength with ductility more than any other iron and excels all other brands for the manufacture of chilled rolls. Charcoal iron of. Salisbury and Muirkirk brands are generally considered as . excellent irons for chilled rolls, car wheels, etc. Many in making rolls will use a good deal of old car wheels and steel scrap in their mixtures. For an example, the author has used a mixture of 1,300 pounds of old car wheels and 300 pounds of steel rail butts for making rolls about 14 inches in diameter that required i%-inch thickness of chill. Wherever scrap is used in mixture with pig, care must be taken to have it of as uniform a grade as practical. Another mixture consisted of 1,000 pounds of car wheel scrap, 500 pounds of No. 4, and 500 pounds of No. 5 charcoal iron. It is to be remembered, wherever we refer to grade numbers, that they are supposed to contain silicon and sulphur agreeing with table 22, page 152; and by referring to the analysis of the car wheel seen on page 268 one can perceive about what constituents the above scrap should contain. For further information on adding steel scrap to iron mixtures and melting it, see " Moulder's Text-Book. " Some select car or other chilled scrap by the thickness of the chill, but since it has become known that the pouring temperature can 234 in "Moulder's Text-Book."from the white to the grey in chilled castingsly the same depth.... Shop scrap     ......	15            x ro           x	1.50 i. So	22.50 iS.oo