BY CARRIE ROBERTS, Conservator, Kelsey Museum of Archaeology
When it comes to conserving works of art, how do you treat what you can’t see?
When a carved limestone lion from Karanis came to the conservation lab a few months ago, grain-sized bits of stone were coming away from the sculpture’s pitted, weathered surface. It was the type of deterioration conservators often associate with salt activity. Egypt has salty burial conditions, and objects that are porous — like ceramics and stone — absorb salty water into their bodies.
Sodium chloride, or table salt, is a common type of water-soluble salt found in archaeological artifacts. When moisture levels in the air fluctuate, sodium chloride crystallizes and liquefies. This action is called “cycling,” as the salt goes through cycles of change from liquid, to solid, to liquid phases. With these phase changes come changes in volume — an expansion and contraction of the salt. In the crystalline phase, salts are sometimes visible on an artifact’s surface; this is called “efflorescence.” Crystallizing salts can grow right through the surface of an artifact, sometimes even pushing it off. Imagine salts cycling inside an artifact, and, well, you can see why so many ancient stone sculptures are in such bad shape!
A close look at the Karanis lion revealed no salt crystals visible on the surface. However, salts can be present even if visible efflorescence is not. I had a hunch. I took a small sample of stone powder that had detached from the surface, and tested it for chloride salts. Sure enough, the test came back positive.
Since salts are so sensitive to fluctuating humidity levels, and since the lion was slated for display outside its usual climate-controlled storage environment, I decided it was time for the lion to take a bath. Soaking artifacts in a water bath is an often-used method for extracting potentially harmful salts, and by measuring the amount of salt in the water bath over time, we could monitor this extraction in a quantitative way. After a few hours in a bath of purified, or deionized, water, quite a bit of salt was removed from the lion, and with it much of the risk of potential damage from salt activity.
