Hindsight is 20-20, as they say.
This week Art Daily* reported that a widespread preservation treatment, developed to help canvases survive humid environments, actually makes paintings more vulnerable when humidity levels soar.**
“The wax-resin treatment was enormously popular in Europe and the U.S. during the 1950s and 1960s,” says Cecil Krarup Andersen at the Royal Danish Academy of Fine Arts, who made the discovery. “Many masterpieces, such as Rembrandts and Van Goghs were preventatively treated with wax-resin linings to help protect the artwork from humidity degradation. The treatment does exactly the opposite.”
Anderson has just wrapped up her PhD work on the topic, a research project that began because museum staff at Statens Museum for Kunst were trying to figure out why Danish Golden Age paintings treated with wax-resin were not resisting the insults of time as well as they should.
I needed a little background on wax-resin treatment which Andersen kindly provided: It was popularized in the 1800s by a Dutch restorer named Nicolaas Hopman. One of the first masterpieces to be treated was Rembrandt’s Night Watch in 1851.
The overall motivation was logical: Hopman thought that coating the back of a canvas with beeswax and an extra layer of canvas would act as a protective support for the painting. Later on, he and others began mixing tree resin in with the wax because it added stiffness. Throughout the 20th century, the treatment gained popularity. Until the 1970s.
That’s when conservators started talking about the importance of reversibility, the idea that any conservation treatment on artwork should ideally have an undo button, just in case a treatment turned out to have unforeseen, negative, long-term impacts or in case a better treatment came along sometime in the future.
At a conference in Greenwich, England, in 1974, a group of high profile conservators decided that wax-resin treatments were not reversible and should be discontinued, Andersen says. Wax-resin treatments were gradually phased out, but it was too late for thousands of masterpieces that had already faced the hot iron.
Initially conservators used irons to melt the wax-resin on to the back of paintings, upon which they adhered the extra canvas layer. Then in the 1950s, specialized heating tables were invented. These tables could uniformly heat the wax-resin and seal the back lining to the painting “in no time,” Andersen says. They made it easy for conservators to overdo it, she says. (As an aside, Andersen says the treatment additionally flattened out the texture in some paintings.)
Another reason 1970s conservators became nonplussed with wax-resin was that the treatment actually changed the color of paintings. Sometimes the hot wax and resin would melt right through the canvas, passing though spaces in the paint where it would leave a dark oily stain.
Despite all its downsides, conservators still thought that—at least—the wax-resin treatment protected artwork from humidity. Until now.
Here’s why: Humidity hurts canvases because the textile fibers—often a kind of flax linen—absorb water, albeit not evenly. The humidity makes the diameter of canvas fibers expand by as much as 50%, she says. Meanwhile the fiber length barely changes—at most by 0.1%.
The increase in diameter means each thread in the weave has a longer path around neighboring threads; the canvas textile shrinks or warps to accommodate the now thicker threads. Warping of the canvas can lead to cracking paint. It can also loosen the paint’s adherence to the canvas, so that little, or big, chunks of paint fall off. Not good.
However if the canvas weave is not too tight, it can accommodate the swelling of fibers that occurs in humid environments. The problem with wax-resin is that it fills the gaps between fibers, leaving no wiggle room for swelling.
Andersen said that conservators had always expected that the wax would prevent humid air from swelling the fibers but sadly, over time, the humid air still gets in. And it causes more damage because the wax is there, Andersen says.
So what to do?
Conservators are loathe to remove the wax-resin because it doesn’t ever entirely come out. They are also hesitant to bring a 65° C (150 °F) degree iron in close proximity to million-dollar masterpieces in order to melt the wax.
Andersen is still working to better understand the shrinking that takes place in these wax-resin treated paintings. Perhaps she’ll find a remedy for the harmful interventions of past conservators.
*I was somewhat amused that the image used to illustrate the Art Daily article on wax-resin treatments is an oil painting made on oak wood, not canvas. Thus the image is entirely incongruous with the story. Ah well.