A few days ago, one of my colleagues sent me this image on behalf of an artists struggling to understand what happened to this painting. The artist informed us that he was instructed to “heat the back side of the ground” to make the painting dry faster (Ugh…). With that great advice in hand, he proceeded to heat the back of the canvas with a big light bulb only to find the painting’s appearance change to what can be seen in the image on the right. He wants to know if there is some way to undo what he has done. Having never seen anything like this before* (nor have I ever engaged in such a practice), I turned to Natural Pigments Technical Director, George O’Hanlon for some insight.
(*NOTE: This is NOT to be confused with “sinking” which some also describe as a “haze” forming on parts of an oil painting. To learn more about that phenomenon I recommend these posts):
Mr. O’Hanlon stated, “The appearance is similar to haze seen on modern oil paintings. One theory is that the heat caused stearates or other soaps to migrate to the surface more quickly than it would if it had dried naturally. Haze is a common problem of modern oil paints.” He then went on to share a bit about this phenomenon: https://www.naturalpigments.com/artist-materials/haze-modern-oil-paintings/
A whitish surface appearance on oil paintings is a phenomenon of modern oil paintings. While such hazes have traditionally been described by painters as blooming or blanching, nomenclature has not yet caught up with the different causes. Other terms currently used are: efflorescence, exudation, fatty acid deposit or migration, saponification, crystallization, chalking, mold, and ghost images.
It is believed that several factors cause the appearance of haze, principally the migration of free fatty acids and/or metal soaps to the surface of the paint film. The following have been identified as factors that increase the likelihood of haze on paint surfaces:
Slow-drying paints (carbon black, alizarin crimson, titanium dioxide) (Schilling 1998, 1999)
Walnut or poppy oil medium (Schilling 1998, 1999)
Extender pigments and additives in modern paints, such as aluminum, magnesium and zinc stearates, which themselves contain high percentages of free fatty acids (Schilling 1998, 1999, Ordonez 1998)
Excessive amounts of bodied oil added to paint (Koller 1990)
Zinc white in the ground or paint layer exacerbated by moisture (Koyano 1987)
Addition of wax to paint (Ordonez 1998)
Painting on low- or non-absorbent substrates so that free fatty acids are not absorbed (Singer 1995, Rimer 1999)
Lack of oxygen and air flow, such as paintings stored wrapped in plastic (Chan 2013)
Artists Peter Höhsl also shared a GREAT resource on the chemistry of drying oils and the potential for solvent disruption which he believed may contain two clues as to what went on with the aforementioned painting.
11.Tumosa.SCMC3.Mecklenburg.Web.pdf (645.4 KB)
Peter writes, “The effect in the hydrolysis examples, where the spots are made up of freed fatty acids, looks strikingly similar to the above painting. One could assume that the heat from the lamp combined with possible humidity may have accelerated this process and maybe it did, however I find the section on solvents more significant because it states that apart from solvents, heat can help evaporate saturated fatty acids.”
I hope that this may help anyone here that may be contending with a painting that it presenting such issues. (a big thanks to both George O’Hanlon and Peter Höhsl for putting forward some answers here for us.)