Black oil is the classic drying oil made with lead. It improves 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.
Since the earliest times, litharge (lead monoxide) was cooked with vegetable oil, such as linseed or walnut oil, to clarify it while removing impurities and imparting faster drying characteristics. Cooking the oil for long periods and at high temperatures allows more significant amounts of litharge to be dissolved in the oil while at the same time darkening the oil. Rublev Colours Dark Drying (black oil) oil is cooked at the lowest temperature possible to affect the solution of the lead while making the palest drying oil possible.
Rublev Colours Dark Drying Oil is prepared by heating akali-refined linseed oil (derived from the dried ripe seeds of the flax plant [Linum usitatissimum, Linaceae]) at the lowest possible temperature (less than 180 °C [356 °F]) for several hours to dissolve litharge.
It is advisable to avoid oil with excessive lead content as it will continue to deposit lead, which may also give rise to problems in the paint film. The lead should not exceed about 3% metal content by weight of the oil. Dissolving lead in oil has always been problematic because it requires heating the oil at certain temperatures and for specific lengths of time. In addition, most companies that prepare this oil for artists do not realize the optimum type of linseed oil they should use to form the lead linoleates readily soluble in drying oils.
You may have wondered why most paint manufacturers today had stopped using black oil as a source of dryers for paint long before lead was regulated out of paint in the 1970s. The reason is simple: lead dissolved in drying oils (lead linoleates) tends to come out of the solution in time. This is why you observe the sediment forming in the oil. It is for this reason and also because lead is no longer used in industrial and decorative paints that lead linoleates, and lead resinates have been abandoned.
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