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254                     METALLURGY OF CAST IRON.
best brands of charcoal iron melted in an air furnace, which is superior to a cupola in giving the best grades of iron for such castings. Nos. 39 and 40 are made of what are called chilling irons, and which may be composed of a mixture of charcoal and coke irons, or of all charcoal iron. The rolls are best made of iron melted in an air furnace, although many are cast with iron melted in a cupola. Chilling irons differ most radically from the grades or brands generally used in the specialties Nos. i to 38. For information on making mixtures for specialties herein described, see Chapters XXXVI. to XLIIL pages 255 to 292.d white, or very brittle grade of metal. Nos. 25 to 29 are a class of castings that will generally require a different mixture and a harder iron than those ranging from Nos. 10 1^.22. Nos. 30 to 35 are specialties which generally call for as strong grades of iron as can be finished in lathes, planers, etc. Strong grades of iron can be made so hard as to make it difficult to turn or plane them in finishing such castings. Charcoal iron is often largely used in these latter grades, whereas, in. Nos. i to 29 it is rare that such is used, as coke iron can generally be made to answer all purposes. Nos. 36 and 37 require a grade of iron very distinct from the other specialties shown, owing to such castings having to stand radical, changes of temperatures, which cause an action of alternate expansion and contraction while the castings are in use. Iron of a medium soft character and low in phosphorus, or what is termed regular Bessemer, is found best for such castings. The cannon of No. 38 calls for a grade of iron that should be of fair ductility, but at the same time possess the greatest strength to be obtained. Cannons are generally made from the com-o.n. in ladle	1,772 Ibs.	.100"	-326 "	I.IOO	.242	3