This article does not cite any sources. (December 2009) (Learn how and when to remove this template message)
A semi-drying oil is an oil which partially hardens when it is exposed to air. This is as opposed to a to drying oil, which hardens completely, or a non-drying oil, which does not harden at all. Oils with an iodine number of 115-130 are considered semi-drying.