Dark Drying Oil or black oil is a fast drying oil made by heating linseed oil with lead oxide (litharge) and used in historical oil painting. The lead content of the dark drying oil is about 3% by weight.
Black oil can improve the handling and drying of oils and can be used in recipes to make megilp, Maroger and Roberson's mediums and traditional oil varnishes, such as copal. This is the classic drying oil made with litharge (lead oxide). Since earliest times, litharge was cooked with vegetable oil, such as linseed or walnut oil, to clarify it while removing impurities and imparting faster drying characteristics. This dark drying oil is cooked at the lowest temperature possible to dissolve the lead while making the palest drying oil possible.