Artifact investigation

CAROLINE ROBERTS, Conservator

I love a good mystery, and nothing (save a really good crime novel) is better than an artifact mystery. I love the thrill of investigating an object, identifying its agents of deterioration, and nabbing those culprits one by one. I also really enjoy teaching new conservators how to use investigative tools to make their own observations. I recently spent a day looking at an object with Ellen Seidell, a U of M junior who is interning in our lab. The ceramic bowl — excavated at Karanis in 1929 — was covered with feathery white crystals, as well as a drippy, peeling surface coating. I had my suspicions as to what these were, but wanted Ellen to learn for herself how to identify unknown materials.

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Left: Ceramic bowl before treatment. Right: bowl under longwave ultraviolet light.

To do this, we examined the bowl under longwave ultraviolet light. This is a useful tool not only for crime-scene investigation, but also for identifying varnishes and coatings. Ellen and I could immediately see a bright yellow luminescence on the surface. We then performed a chemical test to determine that the coating was cellulose nitrate — a material used to treat newly excavated artifacts in the 1920s and ’30s. Finally, we determined that the white crystals were salts. Water-soluble salts like these can be absorbed into artifacts during burial. Fluctuations in humidity can cause salts to crystallize and re-crystallize inside the object, which can cause damage to artifact surfaces.

So what did we do with this evidence? First, we decided to remove the salts. I felt that this would be a good experience for Ellen, since not all salty bowls have the advantage of being in a climate-controlled museum, and since monitored desalination is an important conservation skill. Next we addressed the coating, whose identity allowed us to choose an appropriate solvent for its removal — which Ellen did herself. The treatment is complete, bringing the case of the salty, peeling bowl to a close (for now).

Conservation for Seleucia Show, Part 3

BY BRITTANY DOLPH, Graduate Conservation Intern, UCLA/Getty Conservation Program

It may come as a surprise that sometimes conservators decide to not treat an object, after consulting with archaeologists and curators. One reason for this might be that the treatment would destroy information that’s important to scholars. This always has to be weighed against the risk that the artifact could fall apart or change irreversibly. An instance in which we might not treat an object would be the case of this figurine that has a black resin or sticky substance on it.

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A ceramic figurine, shown from the rear, showing a thick, black accretion along a break edge.

Though the resin might be considered an eyesore, we did not remove it in case it is actually an ancient repair. In fact, there are references to repair materials and practices in classical sources, one of which is pitch. On the other hand, a more modern mounting material, such as displayed by this figurine, was indeed removed, by placement in a vapor chamber. The solvent vapors penetrated the adhesive attaching the mounting material, allowing for quick and gentle removal.

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Left: stone head fragment “before treatment” (IL2012.04, TMA 1931.436), to which has been adhered a modern mounting material. Right: “after treatment” shot, in which the mounting material was removed.

Another way we use information is to decide which objects are in greatest need of treatment. Given a limited amount of time, objects that could just use a surface cleaning to spruce up their appearance may have to wait three months if there’s another object in danger of falling apart! One of the more major treatments undertaken for the Seleucia exhibition was the desalination of this vessel, which showed a frightening patch of salt crystals around the rim and handle.

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Left: detail image of salt crystals on the surface of a ceramic vessel (IL2012.04, TMA 1931.144). Right: the ceramic vessel in a desalination bath, with the probe of a conductivity meter immersed to measure the concentration of the remaining salts.

Archaeological objects are exposed to numerous substances during burial, the source of which can be the soil, materials dissolved in liquid water moving through the site, or even the object itself, perhaps from manufacture. Salts of the water-soluble variety can be dangerous when they enter the body of a ceramic or other porous object, simply because the crystals can expand and contract with changes in the humidity of the air. Expansion, as you can imagine, places a lot of stress on something, especially if it’s expanding from the pores within. To combat the salts, conservators perform what we call desalination: in these cases where the vessels are well fired with no surface decoration, we immerse the vessel in a bath of specially purified water. The idea is to flush the salts out of the ceramic, while tracking the progress with a meter that measures the presence of the dissolved salts in the water. If the vessel had had surface decoration, the process would have been trickier; likewise, if it had been poorly fired, or even fired at a low temperature, it would have run the risk of disintegrating completely in the water. Thankfully, we determined that this vessel was good to go for the desalination bath — and by all counts, was successfully rid of its soluble salts.

As you can see, we do keep busy in the conservation lab — and the Seleucia exhibition was no exception!