Contributed by John Bergman-McCool
In January, the HVAC system in one of our collection storage areas malfunctioned. Repair work required that the system was turned off for several days. During this time, we monitored the objects for any changes. One vessel caught our eye.
Thanks to Marla’s experience with the collection, she noticed that previously documented spalling due to salt efflorescence was likely developing further (see figure). A quick look at older photographs confirmed that the damage had indeed progressed. The vessel was stored on open shelving and an inspection of the area around the object determined that no fragments had fallen completely off. We decided to rehouse the vessel in a box to buffer it against changes in environment during the current or future failure of the HVAC system.
Since I’ve encountered salt efflorescence a few times, I thought I’d add a bit more information. Porous materials, like bone, ceramic and stone, can absorb salt from various sources. Once inside, salts can be dissolved by moisture in the air through a process called deliquescence. Eventually, the water evaporates and the salt recrystallizes. In very porous objects, the salt crystals form on the surface. In objects where the surface is less porous than the underlying body, recrystallized salt can generate massive forces than can spall or pit the surface (Source: NPS Conserv O Gram 06/05 page 1). In worst case scenarios objects can disintegrate.
As I mentioned in an earlier blog, salts can enter porous objects through groundwater or seawater in buried or submerged contexts (Source: NPS Conserv O Gram 06/05 page 1). They are a major source of salt in archaeological collections such as ours. In the case of ceramics, food and water stored in objects during their pre-burial use life can also leave salt residues (Source: Minnesota Historical Society Page 2). Salts can be introduced to ceramics during manufacture through additives that modify the clay body and through water (Source: Minnesota Historical Society Page 2, Source: Digital Fire). Even clay itself can be salty. When I lived in Arizona, I can remember hearing a potter discuss that they would check their clay by tasting it to make sure it wasn’t too salty.
After ceramic objects are recovered during excavation, salts can continue to be added in archaeological labs and museums. Hydrochloric acid has been used to remove calcium carbonate, an insoluble salt that adheres to ceramics during burial that impedes analysis. An unintended result of this process creates calcium chloride, a soluble salt, which is absorbed into the ceramic matrix (Source: The International Institute for Conservation of Historic and Artistic Works- Studies in Conservation Page 172, Source: NPS Conserv O Gram 06/05 Page 2). I would be highly doubtful of repairs that were done years ago. Without detailed treatment records, who knows what glues were used and what contaminants they might introduce.
Deliquescence and evaporation of soluble salts can be greatly diminished by keeping the storage environment below 60% relative humidity and by reducing humidity and temperature fluctuations (Source: NPS Conserv O Gram 05/06 Page 3). However, there is a continued danger of efflorescence. Display cases and storage shelving made from wood have the potential to release acetic acids. This volatile organic compound has the potential to interact with soluble salts leading to precipitation even in controlled storage environments (Source: ICOM Committee for Conservation Page 640).
There may not be quick or inexpensive solutions to mitigate efflorescence. Our current plans for renovation of Peabody collections spaces call for the replacement of wood drawers and cabinets, but this is expensive. In regards to removing salt from objects, the traditional method is through a desalination wash or soak, wherein the object is immersed in distilled or deionized water until the salt level is reduced. This is a complicated process and shouldn’t be done without involving a conservator. Desalination risks removing important residues and compounds that can reduce the usefulness of the objects for future analysis and weaken the object (Source: NPS Conserv O Gram 05/06 Page 3).
Here at the Peabody we’ve taken steps to remove salt through dry brushing, environmental controls, and monitoring. In the future, we have plans to improve our storage space so that these issues will no longer be a concern.