What causes char on laser-cut wood, and how do I reduce it?

Char is carbonized wood left where the beam lingered — slow passes, weak air assist, and smoke re-depositing along the kerf all feed it. Some char is the medium; a black greasy edge is a settings problem.

The reduction stack: mask the surface with transfer tape, run air assist hard on cuts, prefer faster multi-pass strategies over one slow crawl, lift the work on a honeycomb bed, and clean your lens. Each step is small; together they change the product.