An observation was made on two types of finished oil paintings: stretched canvas and canvas panel on paper hardboard, that reveal some type of impact heat may have on the thin layer of walnut oil rubbed over the final painting before varnishing.
After the first week of drying time, a nice oily shine appeared on the canvas. At the end of the third week under much higher temperatures (85 to 100 degrees F ), the surface now showed a visible dry and flat appearance as if no walnut oil was applied.
Each painting used a quality canvas, received four (4) additional coats of gesso and quality oil paints were used.
Did heat change the appearance of the walnut oil and it remains on the surface, but not in a shinny wet appearance?
Did the heat evaporate the walnut oil product?
Did the higher temperatures allow for the oil to be absorbed into the painting, requiring additional coats of walnut oil prior to the application of varnish?
Your thought would be most appreciated!
Patrick McGuire
Tucson, AZ
First, I do not recommend the application of unpigmented drying oils over painting unless these sections are to be covered with additional pigmented oil paint. Look through our resources and do a search on oiling out to read about our reasoning regarding this practice.
As to your questions regarding heat. It is true that chemists understand that each rise in 10 degrees Fahrenheit, you get double the reaction rate. In the short term, the temperatures you are describing should have little effect other than to speed up the process that would occur naturally over a longer period of time. In the long term the elevated temperatures may contribute to premature aging, embrittlement, and desiccation of an oil layer, oil paint film.
As far as evaporation, technically, oil films initially gain molecular weight as they absorb oxygen. As they age, they do lose some weight and components overtime. I doubt that this is what you are observing. My guess is that the oil is simply drying more thoroughly and quicker than if the works were stored at lower temperatures.
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