Bitumen/Asphaltum/Gilsonite Nbk6 pigment in oil paint

Asphaltum and bitumen are broad terms for a wide range of substances based on high-molecular hydrocarbons. From the viewpoint of current art historical research, bitumen represents a large group of organic substances, which consist of an indefinable mixture of high-molecular hydrocarbons. Bitumen either occurs naturally or is obtained from the synthetic distillation of petroleum. Depending upon its place of origin or technique of manufacturing, bitumen possesses a composition of different characteristics.
The English term “bitumen” is used to designate a wide variety of hydrocarbon substances, just as pitch is a term used to designate bituminous substances based on petroleum (such as “glance pitch”) and substances from the sap of trees (such as “Burgundy pitch”). However, the term as used in painting usually designated a substance based on hydrocarbons derived from petroleum (at least after the sixteenth century).
Asphaltum typically designates a species of bitumen, including dark colored, comparatively hard and non-volatile solids; composed of hydrocarbons, substantially free from oxygenated bodies and crystallizable paraffin; sometimes associated with mineral matter, the non-mineral constituents being difficultly fusible and largely soluble in carbon disulfide; the distillation residue yields considerable sulfonation residue. This definition includes Gilsonite and glance pitch.
Historically, there were two basic methods of preparing asphaltum in oil. One method was to melt the asphaltum in turpentine and then mix this liquid with oil, beeswax and Venice turpentine. Another method was to burn the asphaltum to a cinder (thereby removing most of the easily volatile matter), crush it to a fine powder and then grind it with a drying oil, typically boiled linseed oil.
Although many aspersions have been cast upon the use of any asphaltum in oil painting, it is interesting to note these comments by Church:
‘The disadvantages attending to the use of these coal-tar browns and of ordinary asphalt are two-fold. Not only are they treacherous on account of their easy fusibility, but they are liable to stain contiguous pigments by reason of their solubility in oil or varnish. When used successfully by the older artists they were always introduced sparingly, or were largely commingled with more solid paints.’ (1901, op. 236)
Church had distinguished between the source of asphaltum earlier in his 1890 edition by endorsing the use of native asphaltum when properly prepared:
‘The operation of roasting native asphalt—keeping it over a slow fire ’till it will boil no more and becomes nearly a cinder’—was recommended by Williams in his “Essay on the Mechanic of Oil-Colours” (1787), and furnishes a perfectly satisfactory and safe product.’ (1890, p. 208)

There is a lot of misinformation about the use of bitumen by the old masters, especially Rembrandt. There is actually, little or no evidence of the use of asphaltum or bitumen in his works. Commentators who mention his use of this material are generally relying on visual evidence (usually based on examining paintings with very old and degraded natural varnishes or extremely outdated “analytical” techniques).
I just returned from a conference on the conservation of Rembrandt paintings and it is amazing how the appreciation of his work was surrounded by an aesthetic preferring drastically darkened varnishes for literally hundreds of years, starting in the 18th century.
Bitumen is not a pigment as it is soluble in the solvents and binders into which it is mixed. As such, it is more like a dye that simultaneously functions as a binder. The major issues with bitumen used in oil paint are multifold. First, it remains forever soluble in organic solvents. It will always be disturbed by future cleanings. Second, it loses mass over time and can fissure and fracture into dark unsightly islands often called alligatoring. One can see historical examples of this fissuring where the aperture between islands can be as much as ½ and inch. This can be even more problematic if bitumen is mixed with pigments that do solidify though oxidation and can cause extensive cracking that extends throughout the paint layer and is not just a surface defect.
The colorant is organic and may fade a bit over time but that is a non-issue compared to the disastrous effects that can happen when using it admixed with oil paint. This can cause global cracking issues.
There is no doubt that thinned glazes of bitumen are quite beautiful given their golden brown glow. Anyone who had worked with etching zinc and copper plates is familiar with that effect. Much of that results from the fact these layers have no pigment/binder complex and are truly transparent and not simply translucent.
I generally do not advise to NEVER use a material in painting, but will do so here. If you need this effect, please use bitumen  as a final glaze layer and not as a component of a mixed oil paint application. However, one can very closely emulate the qualities of bitumen glazes by the use of very transparent, modern organic pigments in oil.