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Conservation Highlights: Get Brassy With It

Wednesday, April 18, 2018
Conservation Highlights
Author: 
Arianna DiMucci, QAR Conservator

What might dividers, trigger guards, nesting weights, an apothecary mortar, buttons, a powder scoop, and the inner workings of a pocket watch have in common? While used for a myriad of different purposes one thing these artifacts share is that they’re all cupreous, or copper alloys.

Cupreous metals are those that are primarily copper. The term also includes bronze (an alloy of copper and tin) and brass (an alloy of copper and zinc). The worm gear from the pocket watch, for example, has been identified as brass and is mainly composed of copper and zinc with trace amounts of lead, tin, and nickel, whereas the apothecary mortar is predominately copper and lead.

As you probably know by now from reading our blog, iron corrosion underwater encourages the formation of concretions, a concrete-like material that routinely encapsulates nearby artifacts, including the copper objects pictured above. Trapped in concretions alongside iron artifacts, these must be removed mechanically using an air-scribe and additionally de-concreted, often under a microscope depending on their size and fragility. Certain artifacts, like the copper alloy buttons, need to be cleaned under a microscope because of their construction; concretion can get into the holes in the back of these two-piece buttons or adhere along the sides where the two halves attached. Scalpels, dental picks, and even toothpicks are routinely used to carefully remove this concretion.

While salts, our persistent nemesis in the long-term preservation of artifacts, are typically front and center in the formation of corrosion products on materials recovered from an underwater environment, the copper artifacts recovered from Queen Anne’s Revenge are in remarkable condition and do not require the extensive desalination associated with many of our other artifacts. Cupreous artifacts are routinely soaked in a dilute solution of citric acid to clean concretion. This is done alongside mechanical cleaning to help remove particularly resistant patches of concretion. Artifacts are then rinsed in RO (reverse osmosis) water and cleaned with a sodium bicarbonate paste before being dehydrated and immersed in benzotriazole (BTA) to inhibit corrosion. This creates a barrier between the artifact and moisture to prevent the pitting, surface damage, and other deleterious effects so often seen with artifacts recovered from a marine environment. A final protective coating of an acrylic resin is the last step in the treatment of copper alloys.

Sources:
-Cronyn, J.M. The Elements of Archaeological Conservation. London: Routledge, 1990.
-Hamilton, D.L. Basic Methods of Conserving Underwater Archaeological Material Culture. Washington, D.C: Legacy Resource Management Program, U.S. Department of Defense, 1996.
-Scott, D. Copper and Bronze in Art: Corrosion, Colorants, Conservation. Los Angeles: Getty Publications, 2002.

Images:
-Worm gear from QAR site. Image by NC Department of Natural and Cultural Resources.
-Concreted button from QAR site. Image by NC Department of Natural and Cultural Resources.
-Compass from QAR site. Image by NC Department of Natural and Cultural Resources.