Carbon-based Ink and Paint

Stylianos Aspiotis, Olivier Bonnerot, Claudia Colini

Carbon inks are dispersion inks, characterised by a suspension of carbonaceous material (charcoal or soot) in a water-soluble binder dissolved in a water-based medium. Their colour can vary from dark black to grey, and they do not easily penetrate the writing support. This is the oldest known ink type and the most widespread, as it was used all over the world. A variety of binders are known to be used and they often reflect regional practices: for instance gum arabic was the preferred binder in the West, Africa and the Middle East, while proteinaceous glues were favoured in the Far East. Egg white, and to a lesser extent egg yolk, were also occasionally used.

From Pliny the Elder’s detailed account of the manufacture of various soot-based inks in the Historia naturalis, we learn that, despite its seeming simplicity, producing pure soot of high quality was not an easy task in Antiquity (see Christiansen 2017, 172). Specific qualities of soot mentioned in recipes from the 3rd and 4th centuries CE include ‘goldsmith soot’ (αἰθάλη χρυσοχοϊκή) and ‘coppersmith soot’ (αἰθάλη χαλκέως) probably collected on metallic vessels (Christiansen 2017; Nehring et al. 2021). Another type of soot of apparently particularly good quality is the ‘painter’s soot’ (ἀσβόλη ζωγραφική) mentioned by Dioscorides in De Materia Medica (V.161). This soot is said to be collected from glassworks. Therefore, we expect to find various detectable additives that might be indicative of the time and place of production. Copper was found in inks from Hellenistic papyri by Delange et al. 1990, but the authors did not check for the presence of carbon. More recently, several examples of carbon ink from Hellenistic, Roman and early-Byzantine Egypt, and which contain a significant amount of copper were published (Rabin et al. 2019; Ghigo 2020; Nehring et al. 2021). Other metals like iron or lead have also been detected on carbon inks from Antiquity (Bonnerot et al. 2020; Brun et al. 2016; Christiansen et al. 2020). In some cases, it is not clear whether such inks should still be considered as carbon inks or as mixed inks.

Carbon-based paints are made similarly to carbon inks from a suspension of carbonaceous material (charcoal or soot). Unlike inks, these can be mixed with different types of binders (egg yolk, egg white, various oils or resins, animal glue etc.), and not only with water-soluble binders. The origin of the carbonaceous material used is sometimes reflected on the name of the pigment: vine black made from carbonised vine twigs or vine lees, ivory black from charred ivory or bone, lamp black using soot collected from burning oil. These can sometimes be differentiated by the presence of minor or trace elements reflecting the origin of the charred material (e.g. phosphorus and calcium in ivory black). The same sources of carbon can be used for inks.

Suitable analytical techniques are the same as for carbonaceous materials in general:

• Raman spectroscopy can be used as a non-destructive and non-invasive method to unequivocally identify carbon in inks. Furthermore, spectra characteristics such as Raman peak position, full width at half maximum (FWHM), integrated intensity and intensity ratios between the two peaks of carbon can theoretically allow the discrimination of different C-based inks. However this is in practice rather difficult, as some writing supports and ink components can lead to fluorescence background, and handmade inks tend to be heterogeneous materials (varying particle sizes, etc…). 
• Infrared reflectography (IRR) can be used to securely identify carbon, increase the legibility of text written with carbon ink, or see through a thin layer of obstructive material (e.g. dirt, paper). 
• XRF can be used to identify some contaminants, in particular metals, sometimes present in the inks (Bonnerot et al. 2020; Brun et al 2016; Delange et al. 1990; Nehring et al. 2021).