This is a black varnish which is a favorite with microscopists and amateurs. Being cheap it is also used to blacken ironwork, grates, etc. The formula for the best article is as follows: -

In an iron pot, over a slow fire, boil 45 lbs. of real asphaltum (not that made from gas-tar) for six hours; and during the same time boil in another iron pot 6 gallons of oil which has been previously boiled. During the boiling of the 6 gallons introduce 6 lbs. of litharge gradually, and boil until it feels stringy between the fingers; then ladle or pour it into the pot containing the boiling asphaltum. Let the mixture boil until upon trial it will roll into hard pills; then let it cool, and mix it with 25 gallons of turpentine, or until it is of a proper consistence.