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294                        METALLURGY   OF   CAST   IRON.
where lie attempts such, results by mixing promiscuous scrap with the pig metal. The loss of a few castings ofttimes more than counterbalances the difference in the price of pig and scrap metal, and in some cases, if the question of gross tons in pig metal is considered, the difference will be found strongly in favor of the straight pig mixture, as against that of a combination of scrap, which is generally sold by net tons.
In grading scrap that shows evidence of having been chilled, such, as that in car wheels, rolls, dies, crushers, plows, etc., it is as essential to consider the texture of the grey body of the casting or scrap as it is that of the depth of the chill, for the reason that the depth of the chill part can be deceptive in denoting the true grade of the iron, from the fact that degrees in the pouring temperature of metal, as well as the thickness cf the chill to the limit used for forming the chill part of the casting, has an effect in forming the depth of the chill, factors more clearly defined in Chapters XLI. and LVL*
About the worst class of scrap to pass judgment upon, in an effort to grade it, is that coming under the head of '' white iron.'' Where bodies of scrap are all white, the silicon contents may, in castings say from *' stove plate " up to two inches thick, contain silicon all the way from .50 up to 1.50, and in more massive castings than three inches thick, it is generally safe to conclude that the silicon can range from .10 up to 0.40, with sulphur in any of these thicknesses ranging all the way from .050 up to .200. As a basis to guide the founder in an effort to grade such irons for mixture with softer metals, it can be taken for granted that the sulphur is generally very high and the silicon low
* For a discovery showing that chilled parts give a softer re-melt than gray parts of the same casting, see pages 338 and 339.