Lustrous Carbon Explained: Why It Matters for Castings

Lustrous carbon is a thin, shiny graphite-like film that forms on the mould surface when hydrocarbons crack at high temperature. It is the key to good surface finish in iron castings.

How it forms

When metal is poured, volatile hydrocarbons from additives like gilsonite move toward the hot metal, crack in the reducing atmosphere and deposit carbon as a lustrous film.

Why it helps

• The film is not wetted by molten iron
• It reduces metal penetration into sand pores
• It helps the casting release cleanly from the mould

Measuring lustrous carbon

Laboratories measure lustrous carbon yield as a percentage of the additive. Gilsonite has one of the highest yields among common additives.

Getting the right level

Too little lustrous carbon causes burn-on and rough surfaces. Too much can cause lustrous carbon defects (folds or wrinkles) on the casting surface.

Factors that control lustrous carbon formation

Lustrous carbon defects

If too much lustrous carbon forms, it can be trapped as wrinkles, folds or laps on the casting surface, especially on thin sections or in ductile iron. Balance additive level against casting geometry.

Measuring in the foundry

Track LOI and volatile content of system sand along with visual inspection of castings.

Use foundry grade gilsonite for high lustrous carbon at low dosage. See gilsonite in foundry sand.