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 makes the carbon (as graphite) form small nodules (dark spots). They are surrounded by smooth envelopes of ferrite (body- centred cubic crystalline iron). These envelopes are held in a matrix of grainy pearlite spheres (iron carbide). Interference contrast illumination. Magnification x30 at 35mm size.