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Problematic Polishing: An investigation in gilded silver conservation

With the help of the Walters Art Museum conservators and conservation scientist, Our team is studying a conservation problem presented by a nineteenth-century Turkish nargile (commonly referred to as a hookah or waterpipe) in the Walters’ collection. The majority of the piece is made of a gilded silver alloy, decorated with intricate patterning. At the base of the

nargile is a Chinese blue and white porcelain water jar. Decorating the rest of the metalwork are rubies and emeralds as well Persian-style enamel on the upper bowl. Although missing its essential hose, this nargile is a functional object, designed to be used. It was acquired by the Walters Art Museum Founder, Henry Walters on his 1903 trip to Istanbul. Some records from

the dealer who sold it to Walters state that it was an object for royals. Today, thanks to Henry Walter’s gift of his museum to the city of Baltimore, this precious object can be enjoyed by all.

As any silver object does, this beautiful nargile tarnishes over time. Tarnish is a natural product from the reaction of silver with certain gases in air, primarily hydrogen sulfide gas. The tarnish causes the object to change color, darken, and lose its shine. Routine polishing is necessary for it to maintain its intended appearance. There are many different ways to remove

silver tarnish; however, this object presents additional complexity. Even the gentlest cosmetic sponge, while rubbing away tarnish, removes the very thin, original layer of gold (gilding) on the surface of the nargile. Our group within the Baltimore SCIART program is investigating which conservation methods are safe to use on this object to remove the tarnish, but not the gold layer. The two methods we will consider are mechanical cleaning with a cosmetic sponge and chemical cleaning with an acidified thiourea solution. The first week together was spent developing an experimental plan.

Our first step is to investigate common mechanical cleaning methods. Usually, conservators soak a cosmetic sponge in solvent (water, ethanol, or a mix of the two) and then rub it on the tarnished surface, removing the black Ag 2 S. Yet, as stated before, this method removes the gold on the nargile as well. To understand why this is happening, we will take several types of cosmetic sponges to University of Maryland Baltimore County’s (UMBC) Scanning Electron Microscope (SEM) Lab. This will allow us to better understand what the surface of these sponges look like on a nanometer scale. We expect to find subtle differences that may indicate why some sponges are more effective than others at scraping off the silver sulfide complexes and the gold. We will then use Gas Chromatography Mass Spectroscopy (GC:MS) to analyze extracts after letting the sponges soak in solvent. There is concern that residual sponge extracts may be making the nargile more vulnerable to tarnishing. Another aspect we will consider is looking at the sponges themselves after cleaning. We can visually tell that some material has imparted itself upon the sponge after use. If gold is being removed, we can collect the particles and see exactly how much was removed.

The next part of this study involves the chemical cleaning. This is where the most variety comes into play. We know that thiourea solution can be made with many different acids. We choose to test four commonly used acids, listed from weakest to strongest: formic, citric, phosphoric, and sulfuric acid. We know that once the thiourea solution is applied, and the

tarnish is removed, some of the solution remains on the silver. This remaining liquid, while most is rinsed off, forms complexes on the silver surface. Often these complexes seem to increase the rate at which the object tarnishes again. We aim to determine if different acid solutions create different complexes, and if so, is one less harmful than another. This will be characterized using Surface Enhanced Raman Spectroscopy (SERS) and GC:MS.

We began our laboratory work with Dr. Chris Geddes from the Institute of Fluorescence (IoF). There, we made very pure samples of metallic layers of gold, silver, and copper on glass slides. To achieve a very uniform deposition of these metals on the glass slides, we used Thermal Vapor Deposition (TVD). This instrument was the preferred method since it allowed us to create ultra thin layers of these metals. In our case, we produced variations of a silver layer of two micrometers with a thin, 100 nanometer layer of gold on top. The TVD also allowed us to vary the texture of our metal layers by adjusting the rate of deposition onto the glass slides. A fast deposition creates a rougher surface, and a slow deposition creates a smoother surface.

Through this process, we were ultimately able to make rough slides of pure gold, smooth slides of silver with a layer of gold on top, smooth slides of a silver-copper alloy, and finally smooth slides of this alloy with a thin layer of gold on top. The slides we prepared will serve as the pure samples in our experiment. The slides of the copper and silver alloy with gold on top are most characteristic of our actual nargile.

However, all the variations were created to isolate the different metals involved in the tarnishing process we are studying. By storing these pure samples under vacuum, we are preventing them from outside contaminants and isolate their interaction with the tarnish we will later artificially create. Although the nargile itself interacts with environmental contaminants, for the

purposes of our experimental procedure we needed to reduce the variables we are observing. Moving forward, tests of different cleaning methods will be applied for comparison in a controlled way.

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With the help of the Walters Art Museum conservators and conservation scientist, Our team is studying a conservation problem presented by a nineteenth-century Turkish nargile (commonly referred to...

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