Ralph Mayer speculates that, because sun thickened oil is partly oxidised, it’s adhesive proberties may be compromised to some extent.
Do you know if this has been tested at all?
Thanks Brian, that’s pretty much what I expected.
So as I see it, the take away summary would be, sun thickened oil should have lost some of it’s adhesive properties in theory, but this would likely be insignificant in practice.
Especially considering that the oil, as a medium, should be only a small percentage of the paint film.
Ron Francis
PS. I believe sun thickened oil was widely used in the Renaissance, so if there was a problem with it, I would expect it may be showing up in the conservation area.
Then again, I;m not sure how anyone could tell sun thickened oil was used unless it was documented by the artist.
Thanks again, much appreciated.
My guess is that if this has been studied, it was by the industrial/household paint manufacturers in the early 20th century when these materials were still common in industry. I will take a look at a few old manuals to see if I can find anything. Perhaps sone of our moderators working in the art materials industry know of such a study. As to the basic concept, it is in line with what we believe about drying and adhesion. The greatest adhesion would likely take place when a drying oil goes through its whole drying curve without being disturbed. This is the technical answer. Sun thickened oil behaves differently in actual practice. In most circumstances, it would be used in excess of the minimum for adhesion: painting and glazing mediums, etc. It should retain all the adhesiveness necessary for most uses. Whether it is the best choice for a given duty is another matter. It is sufficiently different in handling from stand oil and other thin and bodied oils making it another tool in the kit.
One of the problems here is that sun thickened linseed oil is such a laborious process and so archaic that it was probably never used in industry (the people that are most likely to have performed serious, repeatable, scientific testing).
The references that I have consulted speak about the film strength of blown and oxidized oils (the closest analog that I can find), stating that those that are short, more oxidized, and perform poorer in the long run than those that are longer, and less oxidized. This comes from Chemistry and Technology of Paints by Toch. He also references ASTM standards but again, these are unlikely to have been performed on such a labor intensive, wasteful, expensive, and unwieldy process as sun-thickening.
There are likely less rigorous tests performed by artists and or perhaps inhouse tests by some art material manufacturers but I do not have access to these if they exist. Maybe others on this forum do.
To make a rigorous and valid test I believe that one would need to start with linseed oil of a known composition, acid number, etc. and then subject samples of it to the various refining, bleaching, and thickening processes. Thicker samples would need to be thinned to create a uniform film thickness (complicated in itself). They would then need to be applied as a clear coating and pigmented with a standardized mixture of pigments. Drying times would need to be calculated. Finally the resultant films would need to be subjected to the various tests used to determine “film strength” (mechanical, chemical, etc) You can see how difficult this would be, especially as the materials have little industrial application today.
Some of the complications regarding film strength are mentioned in this thread:
https://www.artcons.udel.edu/mitra/forums/question?QID=355
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