42nd Annual Meeting – Photographic Materials Group, May 31, “Characterization of a Surface Tarnish Found on Daguerreotypes under Shortwave Ultraviolet Radiation” by Krista Lough

Krista Lough, graduate intern in photograph conservation at the Metropolitan Museum of Art and third-year student in the Buffalo State College (BSC) program in art conservation, presented an interesting talk on the presence and potential sources a particular fluorescent tarnish found on many daguerreotypes when viewed under shortwave ultraviolet radiation. In addition to examination and photodocumentation of a set of daguerreotypes that exhibit this type of fluorescence, Lough also used Raman spectroscopy, scanning electron microscopy (SEM), and x-ray diffraction (XRD) to determine that the fluorescent tarnish is copper- and cyanide-based.
The presentation began with a summary of prior research on this subject by Lee Ann Daffner, Dan Kushel, John Messinger, and Claire Buzit Tagni. These studies corroborated Lough’s findings in characterizing the fluorescent tarnish as copper- and cyanide-based. These studies also showed that the tarnish was either removed or its fluorescence quenched when the daguerreotypes were treated with ammonium hydroxide.

Daguerreotype and brass mat from study observed normal illumination (top) and UVC (bottom) to reveal fluorescent tarnish. Image courtesy of Krista Lough

Following a brief review of the phenomenon of fluorescence and its causes, Lough presented the photodocumentation of nine daguerreotypes that were examined during this study. The plates came from two sources—a private collection and a study collection at Buffalo State College—and only those from previously opened packages were examined. Lough’s research focused on determining the source of the fluorescent tarnish and its long-term effects. While the plates varied widely in condition, three primary types of fluorescent tarnish were identified: edge tarnish; rings and circles; and continuous film. The characteristic fluorescence was only observed when the plates were viewed under shortwave UV-C and not under longer wavelengths of ultraviolet radiation. Lough also noted that it was not always possible to associate fluorescent areas with tarnish perceived under visible light. Further, the greenish fluorescence was observed on the verso of some of the plates and along the verso and beveled edges and brass mats that accompanied some of the daguerreotypes. No strong connections could be made, however, between the fluorescence observed on the plates and the corresponding components of their once-sealed packages.
As part of her research methodology, Lough created a number of pure copper and silver-coated copper mock-ups. The mock-ups were treated with both potassium cyanide and sodium cyanide in an attempt to produce the same fluorescent tarnish observed in the 19th-century daguerreotypes. Ultimately, the tarnish only formed in the mock-ups treated with sodium cyanide in areas of exposed, pure copper. The fluorescent tarnish did not form on the plates treated with potassium cyanide or where the copper mock-ups were protected by a coating of silver.
Mock-ups observed under normal illumination (left) and UVC (right). Image courtesy of Krista Lough

To characterize the composition of tarnish, the mock-ups and select 19th-century daguerreotypes were analyzed using Raman spectroscopy, SEM, and XRD. The Raman spectra obtained indicate that the composition of the tarnish was identical in all spots analyzed. SEM was used to create elemental maps of some of the tarnished areas on one of the 19th-century daguerreotypes. A higher concentration of copper, carbon, and nitrogen and a lower concentration of silver were revealed in the areas of tarnish analyzed. Further, a higher concentration of sodium was observed in the areas surrounding the tarnish spots, perhaps an indication of previous treatment with sodium cyanide. Finally, XRD analysis of the fluorescent tarnish on the historic plate produced peaks for silver sulfide and pure silver. Unfortunately, while cyanide was identified on one of the mock-up plates, it was not found on the historic daguerreotype examined and it is thought that the quantities present may be below the detection limits of the XRD instrument.
Lough concluded the presentation with a list of daguerreotype procedures documented in historic literature that could account for the presence of cyanide: electroplating, cleaning, brightening, fixing, gilding, and engraving by galvanism. She also identified avenues for future research including investigation into whether or not the tarnish should be removed, the presence of copper cyanide on brass mats, and potential problems or affects to the daguerreotype that may arise if the tarnish remains untreated. Lough suggested documentation of the fluorescent tarnish could be used to develop a monitoring program for daguerreotype collections and potentially map trends during the examination of larger collections to determine, for instance, if a specific cyanide procedure is common to daguerreotypes from a particular period or location. In closing, Lough summarized the findings of her study in three main points: UV-C examination is a useful tool for understanding the condition of daguerreotypes; the fluorescent tarnish was positively identified as copper cyanide; and the objects exhibiting this characteristic fluorescent tarnish should be handled with caution as the tarnish is toxic.
The copper cyanide tarnish is toxic! Image courtesy of Krista Lough