Mass spectrometry as a micro-analytical method to study kassel earth (vandyke brown) pigments in oil paint

Kassel earth or Vandyke brown pigments have been used in paintings since the 16^th century and appear to be a cause of defects (e.g. slow drying) in 19^th century paintings. This behaviour is thought to be related to the organic “bituminous” nature of Kassel earth and could be aggravated by the frequent substitutions and adulterations of the pigment with various other materials. Mass spectrometric techniques facilitated the investigation of a 25 years natural aged Kassel earth oil paint (CCI), the original pigment and a 19th^th century Kassel earth sample (Hafkenscheid Collection). Unexpectedly, the Kassel earth pigment, thought to slow down the drying of oil shows a totally opposite activity. Specific markers for the pigment itself were identified in all three samples, the two pigments and the oil paint.