Preparing for the exhibition A Curious Hand: The Prints of Henri-Charles Guérard that is currently on view on the third floor of the Library's Schwarzman Building, Madeleine Viljoen, Curator of Prints, came across an autograph drawing by Guerard in the Avery collection. She contacted me because she was interested in exhibiting the work and knew that it could not be shown in its present state.

Guerard drew this self portrait in graphite on blue paper with  white gouache highlights. Color change is a common problem with lead white gouache (also known as opaque watercolor). The lead white pigment reacts with hydrogen sulfide gas, which is present in small amounts in the air, and creates a new colorant: lead sulfide, which is brown or gray in color. (When lead white is in oil paint, as opposed to gouache, this problem is much less commom because the binders in the paint and/or varnish protect it from the atmosphere.)

Conservators treat lead white darkening by applying hydrogen peroxide. The hydrogen peroxide converts the lead sulfide to lead sulfate, which is not the same pigment that was originally applied, but is also white, and is more stable¹. There are many ways to apply the hydrogen peroxide, such as in liquid, gel, or vapor form. It is important to find a method that will result in the most complete color reversion with the least disruption in paint or paper surface. 

After testing many methods, the one that I found to be the most successful for this drawing was to add a 7% solution of hydrogen peroxide to a gel made of agarose, a polymer extracted from seaweed. The gel was then placed on the darkened areas with a thin layer of Japanese tissue in between to make it easy to remove and limit residue. After 3 - 5 minutes, the gel was removed. The area was blotted with a second piece of Japanese tissue, and then a small blotter square and weight were placed  on top to ensure that the paper did not wrinkle. If further lightening was needed in spots, 7% hydrogen peroxide was applied with a fine-point brush and allowed to sit on the area until dry. Here are photomicrographs of three areas before and after treatment. 

This treatment was only done in areas that had already turned brown. There may still be some basic lead carbonate present in other places, which means that this drawing may darken in spots in the future. After exhibition, the object will be placed into a sealed package for storage to limit air exposure. 

Athough small amounts of brown can still be seen in the pigment, especially under the microscope, the overall tone is white. I felt that this appearance was acceptable, rather than risking more treatment, which can result in disruption of the surface of the pigment layer. 

Here is the drawing after treatment. It now has the appearance that the artist intended. it can be viewed in this state until the exhibition comes down on February 26, 2017. 

¹ Vincent Daniels and David Thickett, "The Reversion of Blackened Lead White on Paper," in Sheila Fairbrass, editor, the Conference Papers Manchester 1992(London: the Institute of Paper Conservation, 1992), pp. 109-113.