Inoculation is the addition of a small quantity of inoculant — typically a ferro silicon-based alloy containing trace amounts of calcium, barium, aluminium, or strontium — to molten iron shortly before pouring. These trace elements provide nucleation sites that promote the formation of fine, evenly distributed graphite during solidification. In grey iron, effective inoculation produces uniform flake graphite that improves tensile strength and machinability, and prevents the formation of hard white iron (chill) at thin sections. In ductile iron, inoculation — used after the magnesium treatment that converts graphite from flake to spheroid form — prevents undercooled graphite structures and carbide formation. Inoculation fades rapidly in the ladle (typically within 10–15 minutes) so consistent timing and dosing are critical process controls. In-stream inoculation — adding inoculant to the metal stream as it fills the mould — is preferred for critical castings because it minimises fade.