For me, this presentation was one of the high points of the conference – a thoughtful re-examination of a typically undertaken protocol.
The Winterthur Museum collection of silver numbers around 11,600 and over 2,000 of those pieces are on display in period rooms and galleries at the museum. Indeed, as characterized by Bruno Poliot, this is a massive task. To reduce wear and handling, silversmith and then conservator Don Heller began a program of cleaning then coating silver in 1974. After a period of experimentation, Don chose to use Agateen Air Dry Lacquer #27, a cellulose nitrate, for coating, suggesting that it should be replaced every 10 years but could last up to 28 years in a museum environment with careful handling. Over 1000 silver objects were thus treated between 1982-1987. While the conservators at Winterthur experimented other coating materials, including Acryloid B-72 and Acryloid B-48N in the decade that followed, research undertaken by conservation scientist Chandra Reedy and others, presented in the OSG Specialty Group at the 1999 AIC annual meeting, indicated that cellulose nitrate prevented tarnish better than acrylic resins, and Agateen #27 became the only lacquer used for silver at Winterthur from 1997 onward.
In 2009, the collection on display was surveyed systematically to assess the condition of the lacquer coatings. They found that:
- the lacquer on 42% of the objects had moderate to major problems and would require cleaning and re-lacquering
- condition of the coating did not directly correlate with age, method of application, or composition of the silver alloy
- condition of the coating did correlation with how well the lacquer was applied and the complexity of the object’s surface
- the degree to which the coating had yellowed was difficult to assess, but it was found along with tarnish
In 2011, the museum received a 2 year grant from IMLS for a re-lacquering project. The objects to be treated were grouped into one of three categories:
- objects treated in 1985 or earlier,
- those with coatings which were defective or otherwise failing,
- objects which had never been lacquered before. These were often complex.
Procedures for re-treatment were standardized in order to reduce the amount of application defects. Two conservation assistants were hired for the project, and they underwent a two month training program, working on simpler objects before moving on to more complex ones. Bruno did go into a fair amount of detail about the treatment procedures, and I’m sure his paper will also explore those in greater depth than I will here. One key feature of their procedure is examination one hour and then again 24 hours after application of the lacquer to look for imperfections and iridescence, which would indicate that the coating is too thin. Then a final examination of groups of objects is undertaken 2-3 weeks after the lacquer has been applied to ensure that all solvent has evaporated out of the film and to ensure that the coating is free of defects. Touch-ups to the coating can then still be done, and this period allows enough time for the solvent to completely evaporate, ensuring that it is safe to return them to display.
The team also investigated how Agateen #27 ages, looking both at objects coated with the lacquer in Winterthur’s collection as well as from the collections of two individuals who kept their pieces in home environments. The coatings on the pieces from the private collections exhibited a particular pinkish-brown discoloration, not found on any of the museum pieces. Though analysis of degraded coatings with FTIR and XRF and SEM-EDS provided information about the breakdown of the nitrocellulose polymer and elements found within the degraded films, it was the results of Time of Flight-Secondary Ion Mass Spectroscopy (TOF-SIMS), used to further characterize corrosion within degraded nitrocellulose films, that provided some unexpected results.
In TOF-SIMS, a high energy primary ion beam bombards a sample to ionize molecules on the sample surface. These ionized molecules are then characterized using mass spectroscopy. Looking at the lacquer films from the silver from the private collections, silver sulfide, silver oxides, silver chloride, silver sulfate, sulfate anion and silver cyanide were identified. Except for silver cyanide, all of these corrosion products were also identified in tarnish and corrosion products on uncoated silver.
Bruno pointed out that silver cyanide ranges in appearance from colorless to grey-white in color but could result in the pink-brown color noted in the lacquered objects if mixed with cuprite, which is another corrosion product found on silver objects due to a low copper content. They will next determine whether silver cyanide remains on the silver once the degraded lacquer has been removed and whether this corrosion product is detrimental to the silver.
This pink-brown discoloration was the topic of discussion following the talk. One conservator noted that she had also encountered this condition on lacquered silver from a private collection, and that the condition re-occurred within a few months following re-treatment. She mentioned that Agate, the company which produces Agateen, suggested that this might be a result of exposure to direct sunlight. Another conservator suggested that one potential source of cyanide might be a dip that was used in the past to plate or clean the silver, and that these dips may form complexes on the silver surface which may affect or react with a lacquer treatment. We need to look at how a past procedure may affect a current treatment.
Bruno concluded his presentations with a list of recommendations based on the 550 silver objects completed, and as the team at Winterthur contemplates re-lacquering 500 more pieces in 2015.
- The condition of nitrocellulose coatings on complex silver objects should be re-assessed every 5-10 years, given their finding that the majority of the coatings on complex silver objects failed within a 10-15 year time frame.
- When considering application of cellulose nitrate lacquers on silver, resources and time to maintain a silver lacquering program need to be considered as the time needed to remove and replace a cellulose nitrate lacquer is significant.
- Research into how degraded nitrocellulose lacquer affects silver needs to continue, as does finding alternative options for protecting silver against tarnish.