Coloured Dispersion Ink and Paint
Stylianos Aspiotis, Olivier Bonnerot
These inks are based on inorganic pigments mainly composed of certain minerals, which in powder form, have a characteristic colour (streak), that is responsible for the final colour of the pigment itself. For instance, hematite (Fe2O3) can appear macroscopically from black to red, but regardless of its different appearance, all varieties of hematite are characterised by a distinctive reddish-brown streak, which is the well-known red pigment produced by hematite. Similarly, orpiment (As2S3) powder will always have a golden-yellow colour, while malachite (Cu2(OH)2CO3) and azurite (Cu3(CO3)2(OH)2) a typical green and blue colour, respectively. Some pigments, such as hematite and cinnabar (HgS), were used since the Neolithic period in South America and the mid-9th millennium BCE in the Mediterranean (Prieto et al. 2016; Gliozzo 2021), whereas others, such as a white pigment composed of anatase (TiO2) and barite (BaSO4), were introduced in the 20th century (Fitzhugh 1997). Besides natural (mostly mineral) pigments, artificial pigments have been also synthesised already from the Early to Middle Bronze Age, with the most characteristic example being the synthesis of a blue pigment called Egyptian blue (first occurrence c.2600 BCE; Pagès-Camagna et al. 1999), which is the synthetic equivalent of the naturally occurring mineral cuprorivaite (CaCuSi4O10).Both varieties (i.e. natural and synthetic pigments) are finely ground and dispersed in a binding medium. A variety of water-soluble binders (such as gum arabic, proteinaceous glues, egg yolk or egg white) were used, but pigments can also be dispersed in an oil-based medium (linseed oil, for instance). The binders, as well as the refinement of the grounding, may affect the resulting colour of the ink.
Analytic methods:
- VIS-NIR Spectrophotometry / FORS can allow the identification of some pigments.
- Elemental analysis can be performed with XRF to identify the main elements present. In some cases, this is enough to identify the pigments used (for example the presence of mercury is characteristic of cinnabar/vermilion).
- Vibrational spectroscopies, particularly Raman spectroscopy and FTIR spectroscopy are well-suited for the non-invasive identification of mineral pigments.