John Delaney discussed the “yellow filter” model for virtual cleaning and compared it to experimental results from measurements of cleaned paintings before and after treatment. To estimate the result of removing a discolored varnish, conservators sometimes correct the color using imaging software, “subtracting” out the varnish as if it were a yellow filter. The experimental values were collected using FORS (fiber optic reflectance spectrometry) of specific points and hyperspectral imaging of the entire surface of the paintings. The specific values for the “yellow filter” for a given painting is taken from the absorbance spectrum of the removed varnish and then applied to the rest of the painting. After examining the spectra before cleaning, after cleaning, and the estimated virtual cleaning, it was clear that the yellow filter model for virtual cleaning was not enough to explain the spectral changes seen after cleaning. After varnishing the cleaned area, there was no change in color, only an increased brightness indicated by a rise in the near-infrared signal, so that alone did not explain the additional component missing from the yellow filter model.
I was thinking that even if it wasn’t spectrally perfect, could the yellow filter model at least be a good tool for an estimation of results? The example of the 15th century Tuscan panel convinced me otherwise—while the yellow filter worked well for the blue areas, the model was a poor estimate for the red and brown areas. One component that is not taken into account with the yellow filter model is the effect of scattering by the varnish. Aged varnishes don’t just yellow, they also increase the scattering of light. Even with a scattering model included, the yellow filter model still did not accurately reflect the results of removing a yellowed varnish.
While virtual cleaning with simple “yellow filter” color correction in image processing software such as Photoshop is cheap and can be somewhat informative, it certainly is not a perfect model for varnish removal, even when the yellow filter’s spectrum is taken from the actual discolored varnish. The study has not considered selective cleaning, grime removal, or other nuances of cleaning paintings, which would only further complicate the spectral correction.
In his talk, Lance Mayer introduced “Gallery of the Louvre” by Samuel F.B. Morse and detailed the treatment and history of the painting. The main condition problem with “Gallery of the Louvre” was the overall yellowed appearance caused by a discolored varnish and the extensive yellowing and darkening of the artist’s original glazes. Almost preemptively, Lance discussed (and dismissed) the possibility of simply “thinning” the varnish, the choice of many paintings conservators in this situation—if the varnish cannot be safely removed in total, partial reduction of the coating may be possible without significantly effecting the layers below. Lance showed examples of several treatments where he had been able to reduce or remove varnish coatings above sensitive paint layers that included wax and bitumen, but this approach was not feasible in this case. Analysis of “Gallery of the Louvre” revealed mastic and some oil in the glaze layers, but it is not clear if this was due to the use of mastic varnish in the paint or the use of a megilp medium. There was no cleaning system that could distinguish between the overall varnish and the extensive glazing, and there were plenty of examples of previous cleaning tests which either removed glazes entirely or solubilized both the varnish and glaze layers and mixed the two together. Several of the paintings depicted in the composition had been selectively cleaned previously, leaving the overall tonality of Morse’s painting out of balance with starkly overcleaned pictures hanging in the yellowed room. In addition, there were areas of discolored retouching and several areas of reworked paint over large, flake losses. Morse rolled the painting for transport from France and perhaps the painting stuck to itself—the retouching in these areas is thought to be by Morse himself because they are not well-defined and have a distinctly different appearance when compared to other restorations present.
This treatment and the problems presented fall into a grey area for conservators, and the goals for treatment were realigned accordingly—the goal after the thorough examination was not to clean the painting and remove materials, but rather to “undo” previous cleanings and restorations by adding materials to allow it to be read as a harmonious and coherent whole.
An overall grime layer was removed, and the painting was varnished overall with MS2A. “Bright,” overcleaned areas were toned back, as were abrasions and selected craquelure, particularly in dark areas. The flake losses thought to be repainted by Morse were not filled, and previous discolored retouchings were toned to blend with the surrounding areas. The painting, while still yellow overall, is more harmonious after treatment and the compositional space is much easier to read, particularly the hallway that recedes into the background.
Lance concluded with some discussion of Morse’s techniques, citing his contemporaries who said Morse was too fond of process and often glazed paintings until they looked soiled. Thomas Cole criticized Washingtion Allston, Morse’s teacher, saying “Those pictures which anticipate the beauties of time are pregnant with the seeds of decay,” which was true for Morse’s “Gallery of the Louvre” as well.
Judith Bannerman and a team of researchers at the Tate, London have been developing microclimate frames to reduce fading and preserve sensitive works of art, particularly photographs and works on paper. While we know that light fades materials and low-oxygen environments can reduce this phenomenon, the research team set out to design a functional microclimate frame and then measure the impact of various environments on sensitive works of art.
First, the microclimate frames were introduced; the frames are designed to be versatile, compact, reusable, and fit into existing installations and frames. Following a conference at the Tate in September, 2011, the frames should be commercially available through KeepSafe Microclimate Systems in 3 sizes: A1 (841 x 594 mm / 33.11″ x 23.39″), A2 (594 x 420 mm / 23.39″ x 16.54″), and A3 (420 x 297 mm / 16.54″ x 11.69″). The frames allow for easy access to the gas valves and artwork for unframing while maintaining a good seal through a sandwich design with the use of elastomer “o” rings. Questions were raised about the composition of the elastomer and adhesives used in the frames, and while they could not be disclosed at this point, Judith assured that more technical information would be available after the patents are finalized.
With their prototype frames, the Tate began to study the impact of various oxygen levels and relative humidities on the fading of various art materials. Three oxygen levels were chosen: air at 21% oxygen, hypoxia at 4-6% oxygen, and anoxia at 0-1% oxygen. Temperature and humidity (at 40% and 50% RH) were monitored inside the frames during testing, and a microfadeometer with a 0.4mm spot size was used to study the accelerated fading of various materials. Generally, when the oxygen level was brought to 5%, the fading was halved for most materials at both 40% and 50% RH, but at 0% oxygen some materials improved slightly while others faded more. The study determined that 5% oxygen at 40%RH was best for a work on paper with iron gall ink and a dyed basket, and 0% oxygen at 50% RH was best for the digital photographs tested. Some composite objects, like the iron gall ink on paper, required a compromise since one set of conditions might reduce the fading of one component but increase the fading of another. During the question period, someone mentioned that some pigments fade more rapidly in anoxic conditions, so it is important to remember that conditions should be carefully considered for each object.
Future topics of research include studying long-term display or storage in the microclimates and pressure-testing the frames. They may also develop larger cases for objects, but at this time the three frame sizes are slated to be available in September.