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138                        METALLURGY   OF   CAST   IRON.
the iron at that spot. And the same is to be said where the swab or ill '' tempered '' sand causes one spot or portion of the mould to be different from another, or the venting is inadequate for the free escape of gas or steam. Hard grades of iron are more liable to an ill /diffusion of the carbons than soft grades, especially so where the former is melted or poured dull. Light castings are also much more liable to an ill diffusion of the state of the carbon than heavy castings. The above statements also give additional reasons why test bars as small as one-half inch square, or any having square corners, are not the best standards for making comparison of mixtures, etc.
By re-melting pig iron we effect a mixing process in which the chemical constituents of the castings will be uniform unless they are distorted by means of dull iron, hard ramming, wet sands, ill venting, ori i chills, as above stated. The metalloids most liable to segregate are the carbons and silicon. Chiefly with the first named lie most of the phenomena which effect segregation in castings, and which are defined simply by one part being higher in graphitic or combined carbon than another. Some have claimed the existence of '' sulphur spots '' in castings. With iron melted or poured dull these may exist, but with the reverse conditions the writer has reason to believe, from analyses which he has conducted, that sulphur will generally be found uniformly distributed throughout a casting that has not blown or from any cause been chilled.using different proportions of combined or graphitic carbon A rammer should never be allowed to hit a pattern, as this causes a hard spot on the mould which, in light castings, can change the character of the carbons orhe one analysis which may be given is simply an average of the whole, generally taken from the two ends andith the uncertainty of furnace workings when in urgent need of ten hundred             jon of iron; and Sir........................    2,720     "