Implementation of Red Ochre in Sinopia: Non-invasive characterization of the invisible frescos beneath Roman and Byzantine mosaics in Judea

Abstract

Preparatory drawings in mortars have guided mosaicists in the placement of colored tesserae, as a form of blueprint for the style and content of the mosaic. These drawings are made by implementing pigments in the substrate mortars, following the fresco technique, and are commonly known as sinopia. This study analyzes the data from non-invasive techniques applied on red and pink sinopia that was found below Roman and Byzantine mosaics in Judea. The mineralogical and chemical characteristics show hematite as the main chromophore, with different implementation of the red ochre in lime to achieve pink colors. The shift from red to pink colors is quantified using non-invasive portable reflectance spectroscopy, showing a reduction in the red colorimetric values, which is correlated to lower Fe/Ca values as determined by a portable X-ray fluorescence spectrometer. Diffraction and electron microscopy support the notion that the artisans added an additional layer of lime plaster on top of the red ochre, about 200 μm thick, to lighten the hue from red to pink. This research underscores the importance of studying sinopia beneath floor mosaics to understand pigments and techniques, contributing to the understanding of how red ochre attenuates through lime mortars.