The generally accepted composition of grey cast iron is 95% iron by weight, plus 1% silicone to 3% silicone and 2.1% carbon to 4% carbon.
The high amount of silicone in grey iron produces graphite when heated. Graphite is the element responsible for gray cast iron’s coloring. You can see the gray graphite when gray iron fractures. When it fractures, it produces graphite flake, evidence of its graphitic microstructure. In its natural form, the graphite flake looks like a literal flake. On a polished surface, cast iron graphite flakes look like fine lines. Graphite flakes crack easily, a quality which actually makes gray iron more castable. All in all, gray iron is between 6% to 10% graphite by volume.
The presence of carbon alters tensile strength and hardness. The higher the amount of carbon, the higher the tensile strength and hardness.
The exact appearance of individual grey iron depends on both temperature and timing. Other than appearance, variable properties of castings include: thermal conductivity, tensile strength, hardness, energy dissipation, resistance to wear and deformation, and melting point.
Additional elements may appear in grey iron castings. Some may have been placed there intentionally by manufacturers to alter specific properties. Others, like graphite, may have arrived by happenstance. Prominent among these are manganese, carbon and sulfur. When added to molten grey iron, manganese encourages the increase pearlite formation during the cooling stage, while sulfur increases hardness. Other elements may be present, but to a lesser degree.