Fig. 6From: Analysis of red chalk drawings from the workshop of Giovanni Battista Piranesi using fiber optics reflectance spectroscopy a–c SEM images (SE mode) of red chalks applied wet on paper: Frankenalb (a), Theley1 (b), and synthetic (c). The chalk strokes form thick layers that coat the paper surface. Moisture-induced caking results in diminished surface roughness, which is observable to a greater extent in the natural chalks than the coarse-grained synthetic chalkBack to article page