Fig. 19 aFrom: Analysis of red chalk drawings from the workshop of Giovanni Battista Piranesi using fiber optics reflectance spectroscopy Representation of selected original drawings in a PCA score plot matrix that differentiates graduations of red chalk from higher to lesser chromaticity (PC-1) and high to low coverage (PC-2). Drawing 36-18-4 is an outlier, executed on oiled paper. Selected photo macrographs illustrate the appearance of the chalk lines; photographs: Staatliche Kunsthalle Karlsruhe, Maria Krämer. b–c (b): FORS reflectance spectra of selected original drawings and drawing mock-ups assigned to different clusters via PCA, and (c) their first derivativeBack to article page