The following is a guest post by Andrew Davis, a chemist in the Preservation Research and Testing Division (PRTD). Andrew works to understand polymeric materials in the Library’s collection, such as paper, adhesives, and audiovisual materials and also researches the effects of light and the environment on collections objects.
The Library has been expanding its visitor hours over the past few months, providing the widest reopening of its doors to the public in over two years and allowing more in-person exploration of its exhibits and reading rooms.
When it comes to seeing items on exhibit, we recently wrote about some of the testing that happens to evaluate how best to balance exhibition lighting and preservation of collection objects. Testing for light sensitivity is part of the work, however the amount of time an object spends on display, and hence its exposure to light, is also a major consideration. Some collection items are so iconic that it is assumed they will ALWAYS have light on them, as permanent displays. For example, the Library has two striking maps with this concern: the 1507 Waldseemüller world map and Abel Buell’s A New and Correct Map of the United States of North America. Other institutions think about this too. The National Archives has the Charters of Freedom. And the Smithsonian’s National Museum of African American History and Culture (NMAAHC) has the 13th Amendment and Emancipation Proclamation.
I highlight these specific objects for a reason: they all are currently being displayed in “anoxic”, or low-oxygen, display cases. You can see the 1507 Waldseemüller world map in its anoxic display here.
I also mention these items and institutions because scientists in PRTD regularly collaborate with our counterparts at the National Archives and the Smithsonian to build knowledge and serve as technical leaders on anoxic display science. We have an ongoing working group of these DC-area institutions for exchanging knowledge, and we have been working on a series of white papers about encasements, the first can be read here. It also doesn’t hurt to have friendly faces nearby to ask for a reminder on how to flush a gas manifold, or for borrowing a helium leak detector when ours is on the fritz.
Which goes to say: If you are visiting or working around the Washington, DC National Mall, you could take a walk around for a self-guided exploration of anoxic display enclosures.
Why do we use anoxic displays? To answer this question, it is useful to start with a different question: why do materials discolor in the first place? Many fading or color-change concerns are the result of photo-oxidative chemistry, where light and oxygen combine to cause visible changes in a material. Most of us enjoy the light and oxygen all around us! They are so integral to daily life that it’s easy to forget that light and oxygen are two of the most potent chemical reactants available.
The easiest way to prevent photo-oxidation is to remove one of those reactants: light or oxygen. There’s a third reactant too. The object on display. But we’d like to keep that one.
When thinking about exhibiting collection items, we can’t very well remove the lights if we want to see anything. We might dim them or put sensitive objects on display for short periods of time, but we can’t remove lights entirely. So if iconic items are planned to be on illuminated display for a long time, we’re left with one option. Remove the oxygen. To do this, the object is enclosed in a sealed display case, the air inside the case is flushed and replaced with inert and humidified Argon gas, and this atmosphere is hermetically sealed to provide an oxygen-free, air-tight, environment. At the Library, anoxic encasements represent much collaborative work between the conservators and scientists in the Preservation Directorate, as well as the experts at the National Institute for Standards and Technology. Removing oxygen is easier said than done. The sealed display encasements are a work of impressive science and engineering. There is plenty to distract the installation team away from the unique world map in the window.
Most days, however, all of this engineering is tucked away behind the gallery display cabinets, where the inside looks more like this.
The above images all show the anoxic case currently displaying the 1507 Waldseemüller world map. This map was produced by Martin Waldseemüller, highlighting the recent explorations of Amerigo Vespucci and being noteworthy for its first ever presentation of the separate continents of the western hemisphere and with a new label on them: “America”. The encasement for this map was developed and tested over the course of a few years and installed in the Library’s Jefferson Building in 2007. To date, this encasement at the Library remains the largest anoxic display encasement in the world. You might better appreciate its size and ambition with this picture from when it was installed, showing the encasement being hoisted through a window of the Jefferson Building.
The second low-oxygen encasement currently in use at the Library is exhibiting the Abel Buell’s A New and Correct Map of the United States of North America, the first map of the United States made and published in America by an American. This encasement was installed in 2013 and incorporates some changes from the Waldseemüller map case. One notable difference is that this case’s atmosphere is hypoxic rather anoxic, meaning it is not truly oxygen-free. Rather it has a small concentration of oxygen, approximately 5% oxygen, which is still much less than 21% oxygen in air. This map case holds its desired atmosphere just as well as the Waldseemüller map case, but a small amount of oxygen was intentionally included in its sealed atmosphere. The reason for this was the presence of the pigment Prussian blue, prominently visible on the map’s flag. Prussian blue is one of a few curious pigments with an unusual response to light and oxygen. Unlike most materials where removal of oxygen inhibits discoloration, Prussian blue benefits from the presence of oxygen. The exact mechanism is fairly complex and still a topic of scientific research, but it has been observed that oxygen actually helps Prussian blue revert close to its original color after light exposure. The small amount of oxygen was included in the Abel Buell map case to help keep the map’s flag blue, while still being low enough to preserve the color of other materials on the map
Each of these map encasements has its own gas manifold running across their back, where the sealed atmosphere can diffuse to reach a variety of sensors for reading oxygen content, as well as other properties inside the case such as temperature, relative humidity, and pressure relative to outside conditions. This last point is important. Barometric pressure changes all the time, but the gas is only sealed inside the case at one moment in time. As the atmospheric pressure changes with weather, the sealed gas inside the case exerts a relative positive or negative pressure difference. If the pressure changes dramatically, it may be enough to break the display window and cause damage. Fortunately, that has never happened, as the encasements (and our contingency plans in emergencies) were designed with typical DC-area conditions in mind. But we monitor them just in case.
In the years between the two map cases at the Library, the science of encasements did not change much but measurement instrumentation in these sensor manifolds did. The sensors on the Abel Buell encasement, being 6 years newer, integrated much more easily into online access tools than the original equipment on the Waldseemüller encasement. This allows us to easily get live browser-based tracking of the conditions in the case at any moment or previous time period. The Waldseemüller encasement has slowly been upgraded to similar functionality over the past years.
These live data readouts are useful in ensuring stability of the encasement. For example, particularly stormy conditions are frequently accompanied by rapid changes in barometric pressure that might wreak havoc on the case integrity and seal. The plots above include a particularly nasty day of storms in February 2020, leading to one of the greatest differential pressure spikes seen between the case and its surrounding environment since its installation. Yet the oxygen reading (top left) remained steady, indicating that a good seal was maintained throughout the storms. This live accessibility provides us preservation peace-of-mind, and easy access to historical data allows us to trace trends and see if conditions (or more likely: sensor calibrations) are drifting. In the years since their installation, these cases continually maintained their desired atmospheres and seals.
I do like to pause at the above picture at the back of the Waldseemüller map case. With the gallery display cabinet usually in place around the encasement, you rarely get to see the combination of the highly engineered encasement for a 16th century map against the backdrop of the Jefferson Building’s domed ceiling in the very appropriately named Pavilion of Art and Science. It is a lovely demonstration of STEAM (Science, Technology, Engineering, Arts, and Math) at the Library.
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