Development of a mass spectrometry method for identification of emulsion cured leather
What is the question?
The ancient technique of emulsion (organ) curing was used by multiple cultures around the globe – from the Tungus people in Asia to the Zulus in Africa to Native Americans to Andean communities. Touching every habited continent, emulsion curing demonstrates a cross-cultural desire for a common durable, flexible, water-resistant material. Despite the abundance of emulsion-cured leather worldwide throughout history and in museum collections, there is no method to accurately identify or characterize this complex material. Current identification methods rely on a process of elimination (i.e. a lack of vegetable tannins and chrome salts or other heavy metals) and visual characteristics, such as color. This project aims to develop a method to not only definitively identify emulsion cured leather for the first time, but to characterize the poorly understood mechanism of emulsion curing on the molecular level. The results of this work would expand our knowledge of this unique material found across many cultures and collections, revealing the diversity of a truly global technique, and will be incredibly impactful to conservators, providing a robust method to not only identify these often misidentified objects but to characterize the state of preservation, better predict mechanisms of degradation, and guide treatments.
Several objects from the Met are suspected of having being emulsion tanned leather, ranging from 8th – 3rd century BC Syrian scale armor to a 19th century Crow shirt.
What is emulsion curing?
Leather has been produced since ancient times and is extensively used for its durable and malleable properties. The process involves exposing animal hides to tanning agents to crosslink the proteins (collagen, keratin), increasing the strength of the material and stabilizing against putrefaction. Emulsion curing is a traditional method using natural emulsifying agents (oils/lipids), generally from the organs (brain, liver) of skinned animals, to preserve hide and has been used by cultures globally because it is efficient, not wasteful, and does not require time-consuming sourcing of bark tannins. Ethno-historical sources provides evidence for not only traditional organ curing but also liver and urine, egg yolk, spinal fluid, fermented milk, butter, or plant emulsion alternatives.1,2 Other nuances in preparation include fermentation of the emulsion and smoking the hide.1,3 The molecular mechanism of how emulsion curing stabilizes the proteins is not clearly understood.
The process of brain emulsion curing demonstrated on a vegan bioleather model system made of laboratory-grown microbial cellulose (MC).
ARCHE is developing methods to extract lipids and proteins from a single microsample of leather, in order to reduce sampling of irreplacable cultural heritage objects. These methods are being optimized using a unique model material made of emulsion cured microbial cellulose, a vegan bioleather, which was developed by bioengineers at Columbia University and the Fashion Institute of Technology. This model material allows the methods to be created to target the curing emulsion, rather than the proteins from the leather support. Both matrix-assisted laser desorption/ionization mass spectrometry (MS) and liquid chromatography (LC)-MS methods are being used to detect the lipids and proteins from the organs in the curing emulsion. In emulsion cured leather objects, the proteins from the animal skin itself are in far greater amounts than those from the emulsion. Therefore, it is critical to have methods optimized for the emulsion itself and can characterize tissue of origin to distinguish between the leather and other organs potentially used to cure leather.