Prior to the conservation and restoration of both paintings was the analytical study of the pigments and binding media used in both works. Of special interest was the exact understanding of their specific ageing processes, which became apparent on both paintings but in different ways. Discolorations or blanching impair the legibility of the paintings. This became very apparent on the formerly blue garment of St. Peter in ter Brugghen’s painting, which today appears to have altered to a greyish tone. Through non-destructive pigment analyses with the aid of a mobile micro X-ray spectrometer it was possible to discern a discolouration of the blue pigment smalt, which was very frequently used in this time period.
