Quote from FE Church
"Kenneth, I believe you're right about it being asphaltum. I've only had asphaltum for a few months, but I've made good use of it. According to a post by George, Asphaltum is pigment melted into turps; Bitumen is pigment ground in oil. Asphaltum in mass is black; yellow brown when very thin; orange brown in middle thickness. Pretty much the color of Pepsi. I have not used Bitumen, but I did mix up a very small batch to study. Like Asphaltum it is black in mass. When thinned it becomes a slightly warm grey, with a barely discernable undertone similar to Asphaltum's thinned tone. I imagine figure painters (Bouguereau) would have had more use for Bitumen than Asphaltum. Asphaltum with no varnish mixed in dries rapidly (like 10-20 minutes thin and forms a skin in that time when thick) and is sticky and goopey. It will not hold impasto as placed and tends to puddle on the palette. I used cheap hardware store turps to make mine, so this might explain why mine is still soluable in turps after drying (it might also have to do with the fact that I let the sun heat it and not properly cook it), or maybe that's just a universal quality. Asphaltum that dried in mass is very brittle and looks like obsidion. The supposed problem is that Asphaltum hardens, but never truly dries. With Copal, it is more resistant to turps, it doesn't puddle as much, but still can't be used for impasto. Also, it takes some effort to mix copal with Asphaltum-it clotts before mixing fully. I recently tried Groves' Fir varnish- it mixed easily into the Asphaltum and gave it a "smooth" feel with mo stickiness or clotts. It did not puddle and it held perfect impasto. Also, when dry, it did not shatter when I pressed a fimgernail into an impasted area. It was still weak to turps scrubbing, like Copal, but didn't re-melt when gone over lightly with turps. I was also able to overpaint with normal oils with no cracking when applied fat or lean (Asphaltum is essentially a spirit varnish).
The fact that Rubens' works are 'alive and well' proves that Asphaltum in the underpainting can be stable. I imagine Rubens added something to make it easier to brush and harden enough so as not to remain soft while overpaint enbrittles over time. I don't think he used the wax and Copal combo, which was primarily an 1800s practice.
As for the color looking too warm, remember that Asphaltum is very transparent. I once underpainted a rock with white, yellow ochre, and Ercolano Red. When I glazed over with Asphaltum, the white area had the orange brown tone; the red had imtensified, almost like I glazed over with Madder; the yellow became an intense orange. If Rubens glazed Asphaltum over an earth color imprimatura, it would have taken on the earth colors' warmth. I, too, have never seen an Umber that looks exactly like that. Also, I have never seen an Umber that is totally black when applied thickly.
And if Rubens found a way to control Asphaltum, who's to say he didn't mix it with a warmer color. Frederic Church's sketches, which appear to have paints doused in Copal, often have a rich brown green color for grass and leaves. I think Church mixed Asphaltum with green. Another sketch has a rich brown grading into a red brown.
I'm not sure if they had it in Rubens' time (I'm pretty sure they did), but they didn't in Church's: the Legendary Syrian Asphaltum. For all we know, it was warmer than Gilsonite and other Asphaltums.
In page 17 of the old Underpainting and Layering Theory and Practice topic in the Oil Forum, there is a sketch(?) by Rubens with a yellow brown line drawing with black impasted areas occasionally. It looks exactly like Asphaltum to me. Furthermore, there's a section of the beast's foot where the fur has the effect of being glazed over the former lines. I imagine he did the entire thing at once, so only a color that sets fast, like Asphaltum, could achieve that effect (unless he used a varnish that caused fast set for all colors).
As for the books, I've always wondered whether these conservators actually used lab tests to determine colors or just looked at the pictures and guessed according to historical knowledge (I've seen them do both- I mean, we don't want to hear 'bright red,' we want them to tell us the artist used 'Vermilion'). If they did use tests, Asphaltum being a spirit varnish may not have been identified among the pigments."
Continuing our discussion of asphaltum, bitumen or whatever. I received my Gilsonite the other day and as predicted, I had it all over everywhere do to my less than careful opening of the bag. Very light weight stuff. I was initially interested in a few different combinations. I made a paint out of the Gilsonite by adding Natural Pigments Venetian Medium which since it contained glass would aid in giving body to the Gilsonite, drying capabilities, and sheen. It made a very rich paint with nice body. I then made a paint utilizing 1/2 Venetian Medium and 1/2 Groves Fir Wax. This made an exceptionally buttery and nice paint from a viscosity point of view. It could be controlled and applied with a hogs bristle brush and many of the translucent and textural qualities that I see in Rubens work up above could be duplicated. However it was impossible to thin out using a solvent. It did not react well to the precise kind of controlled painting that I see in the Rubens asphaltum appearing areas. I also tried Oleogel with the Gilsonite but it was also not controllable to the extent that I wanted it.
I think FE Church may have hit on the solution after experiencing first hand the problem of using Gilsonite and making it a paint. I think that it is likely that Rubens used a combination of pigments for this all pervasive rich brown that appears in his painting as well as in Andrea De Sarto and Vasari's underpaintings which remain unfinished. The working properties of Gilsonite as a single pigment do not appear to me to offer what is seen in so many Rubens underpaintings. However, the saturation, translucency, sheen and clumping of strokes and general visual qualities lead me to believe that some kind of Gilsonite, bitumen or asphaltum is part of the pigment combination. My next investigation is to take other possible suspects in the pigment world and combine them with the Gilsonite to see if I can more exactly replicate the qualities of this Rubens paint.