Caroline (Carrie) Roberts presented an interesting talk about a multi-year collaborative project that demonstrates the real impact that surveys and technical studies can have on collections. In less than two years, the authors were able to survey a collection of 200 limestone stelae, assign treatment priorities, identify the agents of deterioration, suggest environmental guidelines, carry out treatments, and develop an informed treatment protocol.
The project began with the survey of the collection of limestone stelae by then 3rd year intern LeeAnn Barnes Gordon and continued as part of Carrie Roberts’ fellowship project at the Kelsey Museum. LeeAnn and Carrie collaborated with scientists in analytical laboratories at the University of Michigan and at the Detroit institute of Arts, including co-author Cathy Selvius DeRoo. Through their hard work and successful collaborations, the authors were able to accomplish an impressive amount and significantly improve the condition and long-term preservation of this invaluable collection.
Carrie first introduced the history of this collection of funerary stelae excavated in 1935 from the Roman-Egyptian site of Terenouthis. You can find some of this info on the Kelsey website here… and here:
She then spoke about the condition issues identified during the survey, which included stone delamination, surface powdering, biological staining, and peeling, darkened coatings. There were several types of salt efflorescence present including spiky salt crystals and more round gypsum like-salts. Spot tests identified chlorides and sulfates. Interestingly, research conducted into the archival holdings of the museum produced some incredibly relevant information regarding the past treatment of the pieces. A transcribed 1941 lecture by the archaeologist indicated that Duco cement was used to stabilize the stelae as they were excavated. The presence of cellulose nitrate was later confirmed using FTIR on samples of the darkened and peeling coatings.
As a result of the survey, approximately ¼ of the collection was determined to be high priority for treatment. These stelae received further study to characterize the deterioration and identify a treatment protocol. Testing was carried out using a barrage of analytical techniques including FTIR, XRF, XRD, specimen culturing and DNA analysis. The results allowed identification of soluble salts (calclacite- a calcium chloride acetate salt produced from interactions with offgasing materials + halide salts), characterization of stone properties (clay component within limestone- possibly responsible for delamination), and ID of the biological growth (black staining identified by DNA as Epicoccum nigrum of the class dothideomycetes, lichen not identified- no DNA present).
The treatment protocol that was developed through testing included:
-Consolidation of the limestone with CaLoSil (150nm particles of lime hydrate Ca(OH)2) in n-propanol. Testing was conducted using CaLoSil, Paraloid B-72, and Conservare (Ethyl silicate) consolidants. CaLoSil was most successful as it reduced powdering after 1 application without darkening stone. It is presumed to penetrate deep into the stone due to the small (nano) particle size.
-Structural stabilization using Paraloid B-72 (in 85:15 ethanol/acetone) injected into delaminating cracks. Not many of the stelae had extensive delamination but Paraloid B-72 was found to successfully stabilize cracks and areas beginning to delaminate.
-Desalination by poulticing with Arbocel paper pulp. This method was considered challenging/problematic and so the environmental controls were considered the best method of preventing future problems from soluble salts
-Coating reduction was accomplished by applying acetone followed by blotting.
-Biological staining was reduced by swabbing with ethanol; however, this was not found to be fully effective.
-Environmental parameters were set based on the equilibrium RH of the identified salts. The recommendation was to stay below 75% humidity, which is the equilibrium of halide salt and below that of calclacite (79%).
Carrie finished with some questions for future research, including: how is the CaLoSil distributed in the limestone after consolidation? What is the nature of the clay component in the limestone? What are the possibilities for reduction of the biological staining? And what is the best method for treating the stelae that had been stabilized with cyclododecane in 2009 when the collection was relocated to the current storage area.
Overall a very informative talk that hopefully will inspire similar in-depth survey and treatment projects!
Tag: Stone
AIC’s 39th Annual Meeting – Objects Session, June 1, “Deep Storage: Reburial as a Conservation Tool” by Emily Williams
During the archaeological-themed session of the Objects Specialty Group, Emily Williams spoke about her experience with a critical issue for archaeological conservators: vast quantities of objects and limited storage space. I have been tangentially involved in decisions to rebury large architectural marble columns in situ at the Etruscan site of Poggio Colla, so I was keenly interested in Emily’s approach.
Beginning in the 1970s there has been exponential growth in museum archaeological collections in America. States have been forced to close facilities to incoming finds due to lack of cataloging resources or space, and the cost of storage facilities that meet modern conservation standards can be prohibitively high. Emily made a fun and appropriate analogy: can storages move like hermit crabs? The answer: not logically.
In America there is a trend to deaccession objects that no longer fit within a collection. However, this is a risky undertaking for objects in archaeological contexts because of the interdependence of objects within a site. Deaccessioning part of a collection could compromise reliable data sets or future analysis. Disposal, sale, or transfer to another institution are equally problematic. “No one wants rusty nails.” Reburial is a tool that has been used for large-scale organics such as shipwrecks, and Emily cited the reburial of underwater material in Marstrand, Sweden.
At Colonial Williamsburg, the conservators are faced with a collection of 60 million artifacts (!), and over half of the historic area is yet to be excavated. Emily discussed a project involving the transfer of the archaeological collection to new climate-controlled storage spaces, including 50 pallets of architectural material (brick and stone fragments non-scientifically excavated from the historic area in the 1930s and 1940s). These pallets took up 5,000 cubic feet of storage and 45% of the total budget. The material was mostly non-diagnostic, not requested or accessed, and attracting animal infestation (evidenced by prolific nesting of rodents and insects). Given these concerns, the decision was made to re-bury non-diagnostic brick and stone fragments with the understanding that they could be re-excavated if necessary.
Very specific details were given about the re-burial choices. For example, the fragments were bagged and placed in their original pine crates with Tyvek tags (written in both Sharpie and pencil). They were grouped by site, only stacked 2 deep, and GPS marked. The crates were placed in an existing excavated cellar within the historic area and backfilled with sand.
I am particularly grateful when speakers present positives and negatives of a given choice, and Emily outlined both. Due to financial restraints, the original pine crates were used. If she were to do this again, HDPE would be preferred, as the pine will eventually decompose and some of the archaeological context could be lost. Individual fragments were not labeled due to time and the sheer number of small pieces, but this would have been preferred. Ideally, they would have reburied the material in a trench outside the historic area in the event that the house would be rebuilt in the future. The obvious lack of access to the collection was mentioned, and the concern that reburied collections could become “out of sight, out of mind.”
This method of reburial is not without ethical and spatial concerns, but given these limitations, there are vast preservation gains for the collection as a whole. There is no correct answer for these difficult decisions, but I agree with Emily’s approach that we need to view archaeological collections in a “holistic rather than particularistic” way.