43rd Annual Meeting- Research and Technical Studies Session, May 15th, “Back to Natural Processes: Controlled Carbonation for Recalcifying Malacological Artifacts”, Presented by Edgar Casanova-González, Jocelyn Alcántara-García & Nora Ariadna Pérez-Castellanos

Seashells were considered to be very significant items in the pre-Hispanic world. For certain cultures they were as valuable as precious stones. They were used as jewelry, decoration for textiles, musical instruments, currency etc. In the Tlalocan-Tepantitla temple, located at the famous Mexican pyramids site “Teotihuacan”, as in many other sites, seashells were discovered in hundreds, probably serving as sacred offerings. Some were also decorated.
The seashells from Teotihuacan were buried for hundreds of years in a damp acidic soil. Therefore, the protein matrix that is embedded in the CaCO3 layered structure has solubilized almost completely. Moreover, CaCO3 structure itself has greatly degraded, as it naturally reacts with acids. Alcántara-García and Casanova-González indicated in their presentation that the shells “would crumble by the touch of a hand”. Mechanical cleaning was not a viable option in their state.
SEM image of an Archeological shell showing two mineral layers (aragonite) about to delaminate (photograph courtesy by the presenters. Has not been published yet)
The two researchers presented their initial trials in establishing a mass treatment procedure for the degraded, non-painted seashells. The procedure should be inexpensive, time efficient and on site, via controlled carbonation. They tested both artificially aged seashells and actual samples from the site. Firstly the shells were stabilized in a humidity chamber, then they were submerged into a limewater (Ca(OH)2) solution and were kept inside a sealed CO2 saturated atmosphere for controlled carbonation.
Before and after treatment, the samples were examined for changes in their properties: hardness and water absorption tests were performed as well as colour change assessment. The mineral structure was also analyzed by XRD and SEM. The preliminary results were encouraging. The hardness and porosity properties were improved and the colour change was minor. However, on the archeological samples, the CaCO3 which was formed by this process, accumulated as a superficial layer on top of the natural ones, and initially did not penetrate well. This new layer did not render enough stability to withstand handling and to hold the shells without them disintegrating.
My thoughts as a listener object conservator: Shells, much like bone or ivory, are created in biological processes but their composition is mainly inorganic. In cases of structural degradation of these materials, I am more familiar with consolidation treatments which involve diluted synthetic resins of various types. What I liked in this presentation was the shift in approach towards shells. Alcántara-García and Casanova-González presented an interesting new approach to restore the degraded shells which mostly lack protein matrix. As a mineralized, non-organic substance, they applied a treatment that is more common in stone conservation. This diffusion in materials, methods and thoughts between the different fields was very interesting for me.
(The presenters requested that I mention that the results of their research have not been published yet)
Hadas Seri, Object Conservator, Chemistry Conservation Laboratory for Organic and Metal Artifacts, The Israel Museum