This interview is part of the Preventive Care Network's blog series, which features interviews with conservators and collection care professionals. The stories and insights shared in these interviews highlight the many aspects of collection care and its cross-disciplinary nature. If you have a project or story you'd like to share or know someone we should feature in this series, please contact us at firstname.lastname@example.org.
This edition of the blog is an interview that PCN Secretary/Treasurer Tara Hornung and Editor Wendi Field Murray conducted with Lisa Young, Supervisory Conservator at the Smithsonian Institution's National Air and Space Museum in Washington, DC. It coincides with the PCN column in the November 2023 edition of AIC News, which focuses on the care of tin and aluminum objects to celebrate the 10th anniversary of the Preventive Care Network.
Can you introduce yourself and tell us a bit about your background? What is your primary role at the Smithsonian’s National Air and Space Museum?
I am an objects conservator by training who started in the field of archaeological conservation after receiving my degree from the University of Wales, Cardiff. After my summer internship at NASM, I quickly became interested in modern materials. I began working with the spacesuit collection shortly after that and conducted material research on polymers and aluminum alloys along with other materials associated with the suits. Although conservators do not traditionally study modern materials as part of their training, experience with traditional metals such as copper alloys, iron, or even lead and zinc have similar corrosion to modern metals and require the same specialized care for their long-term preservation. Currently I am serving as conservation supervisor to a team of conservators at NASM supporting the renovation project of our Washington, D.C. Museum, where we are spending most of our time treating objects scheduled for reinstallation. The museum is being completely redesigned and renovated, and there are over 3000 objects (large and small) scheduled to be installed in 23 galleries.
Historic aircraft and spacecraft highlight the NASM collection. What are the unique challenges of a preventive care program for technological artifacts?
There are several challenges to a technological collection. Most people are unaware that technological objects and materials appear robust, strong and are even composed of “new” materials compared to other museum artifacts, yet they can be just as fragile and sensitive to the museum environment and often require specialized care to support their long-term preservation. The materials may look a bit different, often composite and represent functional artifacts, but can be more challenging to preserve. With a larger object displayed outside a museum exhibition case, the building envelope becomes your display environment. Controlling that space can be a bit different, and you rely on the building engineers or maintenance program for the entire gallery rather than having more direct control over a micro-environment within a case. Temperature, relative humidity, and lighting are generally what people think about with preservation of objects. But with the large artifacts, concerns about transport, handling and display can impact the preservation too. Technological objects oftentimes require the input of people outside the museum and conservation community. Collaboration with engineers, for instance, helps inform the treatment methodology and brings a necessary expertise to projects.
Metals are often assumed to be robust materials. Can you talk about the way you educate your colleagues and the public about the unique vulnerabilities of metal objects?
Metal objects are structurally and physically robust but are just as sensitive to environmental agents of decay like other museum artifact materials. Corrosion is primarily the largest issue with metal objects, when they are exposed to high humidity and salts, for example, which can form a corrosion cell within a metal alloy. Most people are familiar with corrosion if you approach it in a familiar way. Rust on iron, green surfaces of a copper penny or tarnish on silver are recognized as corrosion. What is important is that once an object is treated or removed from the environment that may have caused the corrosion, its important to emphasize how to prevent it from reoccurring. Providing a stable environment free of the agents of decay that introduce corrosion is key, particularly after a treatment to remove the corrosion and coat an object. Corrosion can return quite rapidly on materials such as aluminum or silver where they are sensitive to oxidation and pollutants.
What kinds of objects in the NASM collection are made of aluminum alloys?
Almost everything! NASM has a large collection of aircraft entirely constructed from aluminum alloys or have some part (engine, instruments, propellers, etc.) constructed of these materials. Some of them are obvious- you look at the Douglass DC-3 for instance and the entire body is polished aluminum. Others are not as immediately noticeable--the engine of the 1903 Wright Flyer is composed of aluminum alloy. Some have surface treatments such as anodizing, paint, coatings or even fabric that mask the aluminum alloy metal and may not be as recognizable. There are smaller objects in the collection as well- engines, satellites, space suit components, equipment, and even artwork.
What specific challenges do aluminum alloys present to your preventive program? Do you have different priorities for objects on exhibit versus objects in storage?
Aluminum alloys are not treated differently from other metals on display and in storage. We keep the environment stable, and because many of our objects are composite, we use general, slightly dry environmental ranges from 40-50% relative humidity. For specialized collections such as the spacesuit collection which has aluminum alloy hardware, we keep the relative humidity at 35%, due to the issues we’ve seen with corrosion on some of the components. Typically, the original manufacturer will have taken measures to prevent corrosion, such as specialized coatings and surface treatments. If those treatments are still intact and the environment is stable, there’s not much to be concerned about. For display verses storage, there is really no difference apart from ensuring that surfaces are kept clean and out of reach from visitors. For objects that have failing surface treatments and had previous corrosion it’s important to conduct periodic condition assessments to ensure that corrosion has not become active again. In all cases, identifying the aluminum alloy and surface treatment is key- for example, most aircraft and space materials are composed of Aluminum alloy 2024 which has the highest percentage of copper, giving it optimal performance characteristics but making it more susceptible to corrosion. The preventive program for the materials should consider the type of aluminum alloy the object is composed of, whether it is combined with other materials or metals, and the condition of the treatments. Of course, the history, previous use and condition of each artifact is considered as well.
What is anodization and how does it affect the long-term care of aluminum objects?
Anodizing is a common type of treatment found on aluminum alloys. Aluminum is known for being corrosion resistant primarily because it quickly develops a protective oxide layer. Anodizing takes this process further by creating a thicker passive layer of oxidation on the surface to protect it from corrosion after manufacturing. By changing the chemical bath of the anodizing, or adding dyes, this can produce a wide range of colors. Anodizing can be detected through analysis of the metal, and is a process used with all types of objects. Sometimes this is a clear layer and its not as obvious, or it can be dyed to be red and blue, like the hardware on spacesuits. Commercial products such as the Apple iPod originally were composed of anodized aluminum alloys and were sold in brilliant colors. The primary benefit of anodizing is that it imparts a hard protective layer that protects it from corrosion. When the anodized layer is compromised such as a scratch, this exposes the aluminum to the elements and if the aluminum is exposed to high humidity or salts, corrosion can form under the anodizing and become disruptive to the surface, causing extensive corrosion. For susceptible alloys this process can be difficult to stop or slow down.
Can you tell us about your research into spacesuits and the preservation of composite artifacts containing aluminum alloys? Based on your research, how are composite artifacts containing aluminum alloys factored into the preventive care plan?
The anodized aluminum alloys used on spacesuit hardware, particularly the glove disconnects (metal hardware that connects the glove to the arm of the spacesuit) have shown varying degrees of corrosion for years. Each glove is constructed of layers of synthetic and natural materials with an interior pressure bladder adhered and fastened to an anodized aluminum wrist disconnect. A survey of over 300 gloves in the NASM collection showed that approximately half of them are suffering from advanced forms of aluminum corrosion at this mechanical disconnect. The corrosion is more prevalent on the interior surfaces and at the interface where the soft materials are joined to the aluminum. Analysis was performed and identified the two main contaminants in the corrosion layers- chlorides and acetates. Testing confirmed that soft goods, and in particular the rubber bladder, are continuing to off-gas even after 45 years and are contributing to the corrosion products on the gloves.
As on-going research on the interventive treatment of aluminum corrosion is still being conducted, preventive conservation is critical to their long-term care. Due to the extensive corrosion seen on the aluminum alloys and their attachment to chloride-contaminated rubber, research shows that two primary factors will assist in slowing down this corrosion in the future. Ventilation is key both in storage and on display, so the off-gassing of harmful vapors from the nearby rubber and other soft materials are moved away from the aluminum and other suit materials. A dry environment will also help slow down future corrosion. For space suits on display we also use pollutant scavengers to help capture any remaining vapors contained within the display case. We often get asked if cold temperatures and a non-oxygenated environment would help slow down or stop corrosion and degradation of spacesuit materials. We do keep our storage environment cool (60 degrees F) not cold. Purging a storage area or display case with nitrogen or another inert gas is costly to maintain and has not been implemented so far as a viable option at NASM.
Aluminum alloys are so numerous that they are identified by a numbering system that represents the various series of wrought a cast alloy. How does the series number of the aluminum alloy affect the long-term preservation plan of the work? How do you integrate aluminum alloy series into your preventive conservation plan?
Familiarizing yourself with the alloy you have on your object and within the museum collection is important for establishing a treatment methodology and long-term preservation plan. Aluminum alloys and their treatments are numbered in a series based on their classification by the aluminum industry. For instance, 2024 is part of the 2000 series, and is an alloy often used in the aircraft industry because of its strength and working properties. Yet it is also unstable as its high copper content makes it more susceptible to corrosion. The numbering system assigned to alloys is further divided between how they are manufactured, cast, or wrought, plus tempering and annealing processes, which can also influence a treatment plan for artifacts. The surface treatment of the materials will also need to be factored into the preservation planning of aluminum alloys- anodizing, painting, coatings and the combination with other metal alloys or materials will need to be considered when planning for their long-term preservation. The good news is using an XRF (X-ray Florescence) instrument you can easily identify the alloy and surface treatment of aluminum alloys. Conservators use this information to better determine the potential for future corrosion, eliminating agents of decay from storage and planning for the display of objects in the museum.
If you enjoyed this post, check out PCN's other most recent blog, an interview with Shelley Smith on Preventive Care for Aluminum Alloys at The Chinati Foundation.