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!