Wood types such as oak contain tannic acid. Lye reacts with this tannic acid. This reaction makes the color of some wood types darker. Natural soap, made of lye and vegetable oil, is a way of finishing light woods preserving their natural color. Soap is a common wood finishing in Denmark and other Scandinavian countries.
In woodworking, base or lye (sodium hydroxide) is often used to remove paint, varnish and wax from wooden furniture. By dipping the wooden furniture in a bath filled with lye, the paint, lacquer and wax become soft and release. This results in clean and natural wood.