Abstract
This paper reports on the mineralogical characterisation of samples of wall paintings from various Roman sites in Lombardy (Italy), revealing recurrent types of stratigraphy. One of the stratigraphic samples analysed was found to be a particular kind of plaster: a three-coat work featuring two coats made of clay mud, found in the site of Santa Maria alla Porta (area of the Imperial Palace of Milan—first century CE). The fragments were analysed using optical microscopy on thin sections, X-ray diffraction, scanning electron microscopy with an energy-dispersive spectrometer and infrared spectroscopy, also in non-invasive external reflection mode (7500–375 cm−1). The most interesting feature found was the finish coat made of clay mud (illite, chlorite, kaolinite and fine quartz) with a few coarse clasts and linear cavities. This clay coat was the first example ever detected in Roman Lombardy and was used in combination with a thin painted coat made of clay mud with coarse clasts together with a blue pigment (Egyptian blue) and a render coat made of lime associated with lithic clasts (sand). Our findings brought to light a particular construction technique, since in the historical sources clay is only recommended for daubing on reeds and as a render coat.
Highlights
The plaster found at the site of Santa Maria all Porta s hows the first detection in Roman Lombardy of a three-coat work (Figure 3), combining a finish coat and an intermediate coat, both containing clay with quartz and different proportions of coarse clasts and the proportion and shape of the cavities; these coats are lying on a render coat made, as usual, of lime and sandy aggregate
The wall painting fragments excavated in the Palatium of Mediolanum show a stratigraphy combining a painted and a finish coat both containing clay mud and fine quartz, lying on a render coat made of lime and silicate sand
A similar painted coat was found in the wall plasters of the Republican Sanctuary of Brixia; a finish coat made of clay was detected here for the first time in terms of the plasters of Roman Lombardy
Summary
Roman painted plasters made up of overlying coats are reported by Vitruvius and by Pliny in a sequence containing: a render coat, made of lime as binder together with coarse sand as aggregate (sand from river deposits—harenatum); a finish coat, made of lime as the binder with fine marble powder as the aggregate (crushed calcite crystals—marmoratum); and a painted coat as the top layer [1,2,3]. The difference in the composition between the render coats (silicate sand) and the finishing coats (calcite powder) was very often in accordance with the guidelines; the number of coats was significantly reduced (two or, rarely, three coats) [5]. Other kinds of aggregate were employed (i.e., a marmoratum made of crystals of quartz instead of crystals of calcite)
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