I discovered asphaltum when I was conserving/restoring two dozen pieces of Mason Monterey furniture for the National Park Service. Being a painter who up until that time primarily used acrylics, I had never heard of it before.
Asphaltum (also called bitumen) was once made of a broad range of naturally occurring substances, namely pitch or petroleum or bitumen of Jerusalem, and occasionally referred to a dark resin from the sap of trees, called burgundy pitch.
Artists were trying to find a good source of transparent black or deep brown oil paint, and generally used a mineral substance based on petroleum for the pigment. The color produced was unreliable because the sources were so varied in mineral content. Two common methods for creating the oil paint were to melt the asphaltum in spirits (turpentine), then add it to beeswax and boiled linseed oil. This produced a paint that lacked body, and so many old recipes added more resins or mastic. A second method was to burn the asphaltum until the highly volatile material was reduced to ash, further crush it into a fine powder, and then add it to the linseed oil. This resulted in more durable and reliable paint.
These paints were typically used sparingly, for subtle shadow especially in nudes.
Mason used asphaltum as part of his glaze in the Monterey line. Did I mention that asphaltum was and is also highly toxic, with lead and other poisonous minerals? Even if I could find it today I was not able to use it in the historic hotel furniture!
I was very happy to find that Gamblin produced a wonderful non-toxic “Asphaltum” of Transparent Mars Red and Bone Black to use in the restoration of the hotel furniture. For more information old masters and their colors, I recommend Robert Gamblin’s article on Earth Pigments, “Evolving Earth.”
As I move into painting with oils, I find asphaltum a wonderful pigment to use as a glaze, in place of the hematite or quinacridones I used when painting with acrylics.
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Images courtesy of MPF Conservation (Mitchell and Kate Powell, Partners!)