Based on recent research on mural paintings from a number of ancient Roman sites, i.e. Ostia, Ephesos, Carnuntum and Saalfelden,
the contribution presents the results obtained by microscopic techniques used to study similar paints from different sites
as well as paints of different artistic quality, style and date from one and the same site.
It is shown that careful microscopic investigations are capable of detecting the stratigraphic sequence of plaster layers,
the way of preparing and applying the mortars, the raw materials used, and the means to produce the desired surface in terms
of colour and texture. The analyses focus on petrographic thin-sections observed in different modes of illumination. Pseudocoloured
images produced from characteristic sample micrographs are presented in order to better illustrate the relevant features.
The preparatory – arriccio – plaster layers always contain aggregate from local sources, preferentially unwashed river sands
with enhanced amounts of silt. For the finish – intonaco – plaster layers, the aggregate was produced by crushing selected
stone material of light colour; when available or appropriate, marble was employed, but other stones can be found as well.
For high quality paints, a number of intonaco layers were applied always using the same raw materials at reduced grain sizes
and eventually admixing pigments. While this was the usual way to produce a smooth surface, a few examples were found which
indicate grinding of the hardened intonaco prior to painting.
The microscopic aspect of the paint layers clearly point to the application of a lime-pigment colour onto a dry or semi-dry
surface, i.e. no true fresco technique was used.