Of these carbon is the most important. It is sometimes free, being visible as black specks, sometimes chemically combined when it is not visible.
The combined carbon does not show in the fractured surface, which is white and bright, the iron being very hard, brittle, and forms when fused a pasty mass, which will not freely fill a mould.
These materials are : -
Cast iron, containing from 2.0 to 6.0 per cent of carbon - a comparatively large percentage.
Steel, containing about .15 to 1.8 percent - a small percentage. Wrought iron, containing, if perfectly pure, no carbon, but practically containing a trace.
Bessemer Pig, a distinct variety, free from impurities, but containing a little manganese and silicon ; made for the Bessemer process (see p. 251).
Foundry Pig, having a fracture of a gray colour, and useful to the iron founder.
Forge Pig, being almost devoid of free carbon, not fit for superior castings, but only for conversion into wrought iron.
Besides the above varieties, the pig iron of commerce is divided into six or eight classes.