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248                       METALLURGY   OF   CAST   IRON.
bincd with the carbon. It is by a proper combination of these four elements that the best fluidity and life in molten metal is obtained. Very high carbon or silicon can cause metal to be sluggish or thick on the surface, at either the furnace "or foundry. Such iron can often be seen evolving a great deal of kish at the furnace, or a scum at the foundry, and makes it very difficult, when in iron, to obtain clean castings.
To obtain a thin or clean iron and one which will run quickly while it is hot, in making gray castings, use a mixture which will give castings having carbon 3.00 t 3-75> phosphorus .80 to i.oo, maganese .40 to .60, silicon 2.50 to 3.00; sulphur to be below .07. Such an iron, while running thin as long as it retains its heat, could be made softer and have longer life by increasing the carbon and silicon above the limits here shown, but by doing this the thinness, or quicksilver action, would be reduced unless phosphorus was increased, which would be liable to make the castings brittle. The higher the total carbon, the less silicon is required to maintain the grade and the higher can the carbon be held in a combined or graphitic state, other conditions being equal. vSee pages 280, 282.em the highest carbon. When the exception to this rule takes place and they get low carbon in low silicon irons, which many prefer, they notice its heat effect in a very pronounced manner. Iron with less than i per cent, silicon may have carbon up to 4.50 per cent, while over 4.00 per cent, silicon iron may often not exceed 2.00 per cent, carbon.