Examination of the composition and mechanism of discoloration of the fugitive pigment copper resinate

Abstract

Copper resinate was a green pigment used through the Renaissance when it was largely discontinued due to its tendency to discolor to brown over time. Historically, this pigment was made by grinding copper(II) acetate, or verdigris, in a tree resin rich in abietic acid. Commercial copper resinate and copper(II) abietate mixtures were analyzed by electrospray ionization mass spectrometry, infrared and visible spectroscopy. The pigment consists of mixed organic copper salts with bridged acetate (Ac) and abietate (Ab) ligands (i.e, [Cu2Ab4], [Cu2Ab3Ac], and [Cu2Ab2Ac2]) based upon sodiated ions detected in mass spectra. The lipophilic abietate groups allow the molecules to dissolve in the resin and oil binders for a transparent glaze free of pigment particles. A paddlewheel bridged-dimer structure of [Cu2Ab4] modeled by density functional theory has similar bond distances to copper(II) acetate. Experimental results suggest that copper(II) abietate lacks the axial water ligands found in copper(II) acetate. The green color of copper resinate originates from a combination of the blue typical for organic copper salts and the yellow of free abietate/abietic acid. Solutions of both verdigris and copper resinate quickly discolor to brown upon the addition of a dilute basic solution. Slow conversion of organic copper salts in the paint layer to brown copper(II) oxide, possibly enhanced by environmental conditions or cleaning with alkaline materials, may be an alternate discoloration mechanism in cultural artifacts. Copper detected by portable energy dispersive X-ray fluorescence (XRF) spectroscopy and scanning electron microscopy with energy dispersive X-ray (SEM-EDX) in discolored paint layers of an Italian Renaissance painting by Tintoretto is consistent with this artist’s established use of copper resinate.