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286                      METALLURGY   OF   CAST   IRON.
sulphur under .060, then the managanese might be permitted to go higher. Manganese is somewhat deceptive, as it will permit a casting to arrange its crystals in large grains, giving the iron the appearance of being high in graphite when at the same time the metal is much harder than if the large grains were all the result of silicon in giving the iron large grains.
By a -study of this Chapter it will be observed that . the state of the combined carbon is the chief factor in determining the utility of a casting for electrical purposes. We have stated that it is desirable that combined carbon should not exceed .70 in any .casting. It is to be remembered that the thickness of a casting and the time it takes the molten metal to solidify have also a great influence in determining what percentage of combined carbon a casting will contain. The more quickly a casting cools the higher will be its percentage in combined carbon. For this reason it will be evident that thin castings would require higher silicon and lower sulphur, also manganese, than thick castings. With all the above elements to influence the formation of combined carbon, it is evident that it would not be practical to here attempt to prescribe what percentage of sulphur and silicon a mixture should contain. All that can be done is to illustrate the fundamental principles involved, and these, as here stated, taken in connection with the effect re-melting of iron has in increasing or decreasing the chemical properties of a mixture, as outlined- in Chapter XLV., page 302, will permit any founder making a study of this chapter to intelligently formulate a mixture which will work well for any thickness of castings to be used for electrical purposes.castings in the cupola that would stand the tests of the United States Government for gun carriage work rightfully belongs to Messrs. Robert Poole & Son Co. of Baltimore, Md., and Muirkirk pig iron made by me. This was in 1893. The War Department at first refused to accept cupola iron as gun iron, but when it was fully demonstrateduois Iron Co., Illinois Steel Co., Jefferson Iron Co., Kittan-ning Iron & Steel Co., C. A. Kelly Plow Co., Lebanon Furnace, Longdale Iron Co., Lackawanna Iron & Steel Co., Logan Iron........................   16,720     "                                     t-