John Hancock needed a check-up. New-York Historical’s portrait of the famed signer of the Declaration of Independence and the first governor of Massachusetts is going to be on view as part of the upcoming exhibition, Beyond Midnight: Paul Revere (opening Sept. 6). However, before it gets installed, the Museum’s Paintings Conservation Lab wanted to learn more about it. New-York Historical actually knows very little about the oil-on-canvas work, which was acquired as a gift in 1945 from Mrs. Edward Van Zandt Lane. It was executed by an unknown artist and is a copy of John Singleton Copley’s 1765 portrait, currently at the Museum of Fine Arts, Boston. But we do have a new tool to aid our detective work: New-York Historical’s first portable X-ray fluorescent (pXRF) spectrometer.
XRF analysis has been available since the 1960s and in use in the museum world since the late 1980s. It works by using X-ray beams to excite the electrons of atoms in a sample of material—in our case, the paint layers on the portrait. When the electrons in the atoms are excited, energy is released. Since the amount of energy released is unique to each element, this technique can identify which elements are present in the paint. Also key: XRF is a non-destructive technique. In the past, our only options for analyzing samples were looking at them under a high-powered microscope (a very slow process) or mechanically removing a tiny sample of paint with a scalpel.
The more we learn about the elements in the paint used, the closer we can come to possibly identifying when the painting was made. Some pigments were only used in paintings during specific time periods. Copley, for instance, used an arsenic-containing pigment called orpiment to paint some of the yellow details in the original. Due to its toxicity, orpiment went out of use in the 19th century. If we can find arsenic in our painting, it would indicate that it was likely painted in the 18th century, not long after the original was completed.
We selected several spots on the painting to analyze, focusing on areas with yellow paint since that is where any orpiment is most likely to be found. After gathering the measurements, I analyzed the data in a computer program that translates the results from the pXRF spectrometer into a graph, with peaks corresponding to different elements found in the paint sample.
The graph above shows the results of one of the samples we analyzed. We learned that this sample contains lead (Pb) and iron (Fe), as well as trace amounts of other elements. The large lead peak probably indicates that the artist was mixing other pigments with lead white. The iron peak indicates the presence of natural pigments such as raw sienna, which contains iron oxide. Although we found several different elements in the paint layers, we did not find any arsenic, which means the artist did not use orpiment to paint our copy. Therefore, it is probable that the copy was painted in the 19th century, after orpiment went out of use.
While the artist’s identity will continue to remain a mystery for now, we are hopeful that new technologies in the future might help us learn more. Until then, the pXRF spectrometer will be an exciting new tool to uncover the enduring mysteries in the New-York Historical collection.
Written by Lauren Conway, Museum Division, Conservation Intern
Generous support for conservation initiatives at New-York Historical is provided by the Sherman Fairchild Foundation