The presenter, Linda Stiber Morenus, began her discussion of these complex prints with a description of the printing process. Chiaroscuro woodcuts were intended to emulate chiaroscuro drawings, which were comprised of black chalk shadows and white chalk highlights on colored paper. Color oil-based printing inks were first used to print 14th-century textiles, being used on paper by the mid 15th-century. The chiaroscuro woodblock prints required two to five separate woodblocks, inked with different shades lighter and darker than the midtone colored paper.
In order to better characterize the media, Morenus collaborated with art historian Takahata, and conservation scientists Eng and Rimbaldi from the Los Angeles County Museum of Art (LACMA). In addition to prints at LACMA, the team studied prints from the British Museum and Library of Congress. Out of over 2000 surveyed woodcuts, 72 were studied in depth, with X-ray Fluorescence (XRF), Fiber Optic Reflectance Spectroscopy (FORS), and Raman spectroscopy. Inorganic compounds were indicated by XRF analysis. FORS was especially helpful for detection of indigo. Raman spectroscopy provided additional information about organic colorants.
Renaissance artists’ manuals, such as Cennino Cennini’s Libro dell’Arte guided the research by providing information on the most likely colorants for printing inks. Inorganic pigments included lamp black, lead white, ochres, vermillion, verdigris, and orpiment. Organic pigments included indigo and a variety of lake pigments.
After providing background information, the presenter began to focus on deterioration and conservation of the chiaroscuro prints. The prints from the Niccolo Vicentino workshop had a high lead content. The inks typically had a low vehicle-to-pigment ratio, tending to turn gray around the edges, due to the presence of lead sulphide. Verdigris corrosion was also a common problem, as found on “Christ Healing the Paralytic Man” by Giuseppe Niccolo Vicentino, as well as 13 other prints from the same workshop. Typical copper-induced paper degradation included yellow-brown halos around inked areas and cracks in the paper.
Fading and discoloration were major problems for the organic colorants, such as indigo and the yellow lakes. Morenus compared copies of Ugo da Carpi’s “Sybil Reading a Book” in the British Museum and the Library of Congress, finding clear evidence that the indigo in the British copy had faded. The British Museum had confirmed the presence of indigo through Raman spectroscopy. At least 8 of the prints were found through XRF to have high levels of calcium in the same areas where indigo had been identified, suggesting the presence of chalk-based lakes. Organic greens had shifted to blue or brown where organic yellows had faded or become discolored.
The presenter concluded with suggestions and caveats for conservation treatment. First, she advised conservators to exercise caution in aqueous treatment, in order the preserve the topography of the prints. The woodblock creates a relief impression in the paper, and the layering of the inks adds another level of texture that might be altered by humidification, flattening, washing, or lining treatments. The low binder content also makes the inks more vulnerable to saponification and loss during alkaline water washing. Morenus warned that the hydrogen peroxide color reversion treatment for darkened lead white would be particularly risky, because the white lead sulphate end product has a lower refractive index than basic lead carbonate original pigment. This means that treated lead white becomes more translucent, and the lower “hiding power” shifts the tonal balance of the print to appear darker overall.
For exhibit recommendations, Morenus suggested that we should always expect to find fugitive organic colorants in chiaroscuro prints, so exhibit rotations should be planned accordingly. Maximum exhibit conditions should be 5 foot-candles (50 lux) of visible light for 12 weeks of exposure, no more often than every three years. She also indicated that overmatting should be avoided to reduce the risk of differential discoloration.
During the Question and Answer period, Morenus clarified the color order used in printing. Some prints were inked from dark to light, but most were printed with the lightest color first.
I thoroughly enjoyed learning about these beautiful prints, but I think that the discussion of the lead white conversion treatment-induced refractive index shift was the most important “take-away” from the presentation.