64 METALLURGY OF CAST IRON.
composition, it is generally an indication of a production of iron which will be well up in silicon and low in sulphur, with low iron in the slag. Degrees in color and solidity of the slag between the two extremes may vary according to the difference found in the grade of the iron. Foundry irons generally produce a slag more silicious or '' stony '' than Bessemer irons. The use of high manganese or manganiferous ores generally produces either a green or brown slag. A green, glassy slag, from such ores, indicates that the furnace is working well, but a brown slag denotes the reverse. These grades of slag are generally produced in the making of spiegeleisen and high manganese iron.
The slag called "scouring cinder" is generally the worst slag which comes from a furnace. It is of a reddish brown color and is chiefly caused by a slip or some bad working of a furnace, causing ore to pass down to the fusion zone in an unreduced state. This class of slag is very cutting to the lower lining of a furnace, owing to its containing so much oxide of iron and being very basic, a combination most effective in dissolving the silica in the bricks forming the lining. Some furnacemen are having their slags analyzed at every cast, as a guide in regulating their furnace. This proves very satisfactory in assuring a furnaceman as to the character of the iron he may expect, or whether any changes are taking place which might call for prompt attention in making alterations in the manner of charging or working of his furnace. Some expert furnacemen can greatly vary the grain of an iron by methods of fluxing or, in other words, cause like percentages of silicon, sulphur, and carbon to make some casts open-grained and others close-grained iron. This shows still further why the appearance of fractures in pig iron is so often deceptive.mina in proportion to the amount of heat or friction they are if j