Josef:
Maroger medium is something originated by a guy named Jaques Maroger, a painter and technical director of the Louvre laboratory from 1930 to 1939. He developed a number of theories and painting mediums based on his research. The most famous of these mediums involved boiling lead litharge in linseed oil to create an enhanced drying oil. It has become known over the years as black oil because the litharge and cooking turns the oil quite dark. His theory was that many of the master painters of old used these mediums and there is some evidence to support his claims.
Most of the medium recipies he had mixed the black oil with a varnish and possibly a resin. The one I use is a professional developed formula that mixes the oil with double mastic varnish. It is available at Studio Products.
This is one of those "don't try this at home kiddies" things however, because if the medium is not formulated correctly, you can end up with bad results - like overdarkening of your painting over time. Some artists know how to make it and do it correctly - Bill Whitaker is one. However, there are several commercial brands out there that are very stable and you don't have to worry about catching your hair or your house on fire, so we buy those instead.
