Carbonaceous materials (Charcoal, soot, etc.)

Stylianos Aspiotis

In the field of Cultural Heritage, carbonaceous matter can be mainly found as an environmental pollutant in terms of airborne solid particles, forming a superficial layer on rock-based artefacts, and as carbon-based black pigments. Starting from the first category, a generic term to describe a range of carbonaceous particles interacting with the atmosphere is black carbon, which is predominantly the result of fossil fuel combustion. Charcoal and ash are the main two organic components of black carbon (see Ismail-Meyer et al. 2018), being the product of combustion of organic matter, like wood, at the Earth’s surface (for instance a forest fire). Based on the crystal structure of the black carbon particles, we can further categorise them as for instance soot has a characteristic and well-defined crystal structure consisting of graphene-like layers wrapped into spherules and originating from electric-power generation, traffic and oil- and wood-fires heating, industrial manufacturing processes, and fossil fuel combustion (see Wentzel et al. 2003; Engling and Gelencsér 2010; Grobéty et al. 2010). On the other hand, amorphous carbon (a-C) has mostly a poorly crystalline carbon system which derives from charcoal combustion.

Concerning the second category of carbonaceous materials, carbon-based black pigments belong to a wide group of dark-coloured materials, which are separated from each other based on the composition of the starting material and the procedure followed for the manufacture of the end product. 

Raman spectroscopy is the ideal non-destructive analytical technique to study carbonaceous matter as it is sensitive to crystalline and amorphous phases and can provide information of the molecular composition of C-based compounds. In particular, carbonaceous materials belonging to both categories have two characteristic Raman peaks, however clear differences in Raman peak position, full width at half maximum (FWHM), integrated intensity and intensity ratios between those peaks allow the discrimination of different C-based black pigments, soot-like carbon and various forms of environmental carbon matter (see Ferrari and Robertson 2004; Tomasini et al. 2012; Ferrugiari et al. 2015).

Infrared reflectography (IRR) can be used to increase legibility or see through a thin layer of obstructive material (e.g. dirt, paper).