The use of vibrational spectroscopy techniques as a tool for the discrimination and identification of the natural and synthetic organic compounds used in conservation

Abstract

Early restoration work on archaeological objects generally made use of glues, fillers, consolidants or varnishes composed of natural or synthetic organic compounds. They were chosen for their chemical and physical properties as well as their compatibility with the constitutive material of the object in question. This paper aims to develop a methodology based on a discrimination tool to achieve step-by-step identification of the organic compounds according to their chemical characteristics, by a complementary use of infrared and Raman spectroscopy techniques. 23 representative references of commonly used natural and synthetic products were analyzed. A flowchart containing all the references was constructed based on the interpretation of their respective infrared and Raman spectra. 4 distinct families were differentiated, namely those consisting of long carbon chain molecules (e.g. alkanes), molecules with a CO bond (e.g. esters, ketones, carboxylic acids), ethers and amines. Within each family, organic compounds were identified by the presence or absence of intense and medium peaks in the infrared and Raman spectra. Only the distinction between different types of mineral wax or proteinaceous compounds proved inconclusive. Case studies on archaeological samples highlight the strengths and weaknesses of this tool. The flowchart was effective in identifying organic compounds.