State of oxidation of diterpenoid Pinaceae resins in varnish, wax lining material, 18th century resin oil paint, and a recent copper resinate glaze

The state of oxidation of diterpenoid resins in complex and minute painting samples was determined by gas chromatography/mass spectrometry (GCMS) using a direct thermally-assisted methylation method that requires no chemical workup. Highly oxidized diterpenoid resins as well as fatty acids from oxidised oil and wax are determined in one analytical step. A very high degree of oxidation is demonstrated for 100 year-old varnish, an 18th century resin containing oil paint and aged wax/resin lining material. The state of oxidation was measured by the relative ratios of the abietane acids: abietic acid, dehydroabietic acid, 7-oxodehydroabietic acid and 15-hydroxy-7-oxodehydraobietic acid. A ratio of the order of 0:20:40:40 was found for the varnish and resin oil paint. No acids with a pimarane skeleton were detected. Wax/resin lining material had a lower state of oxidation. A copper resinate glaze was found to have become considerably more oxidised than the pigment itself in less than two years.