Light micrograph of spheroidal cast iron in thin section. Cast iron (or pig iron) is iron containing 2.5% to 4% carbon. The addition of 0.05% magnesium causes the carbon, as graphite, to form small spheres (dark spots). They are surrounded by smooth envelopes of ferrite (body- centred cubic crystalline iron), seen here as orange. These envelopes are held in a matrix of grainy pearlite (iron carbide spheres), shown here as yellow/brown. Interference contrast illumination. Magnification x30 at 35mm , x60 at 6x7cm size.