Formation of zinc oxalate from zinc white in various oil binding media: the influence of atmospheric carbon dioxide by reaction with 13CO2

Kim Pilkjær Simonsen,Jana Sanyova,Francisco Mederos-Henry,Koen Janssens,Morten Ryhl-Svendsen,Jesper Bendix,Frederik Vanmeert,Josephine Niemann Poulsen

doi:10.1186/s40494-020-00467-z

Abstract

The formation of metal oxalates in paintings has recently gained a great deal of interest within the field of heritage science as several types of oxalate compounds have been identified in oil paintings. The present work investigates the formation of metal oxalates in linseed oil in the presence of the artists’ pigments zinc white, calcite, lead white, zinc yellow, chrome yellow, cadmium yellow, cobalt violet, and verdigris. The oil paint films were artificially photo-aged by exposure to UVA light at low and high relative humidity, and afterwards analysed by attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). The results showed that, compared to the other pigments investigated, zinc white is especially prone to metal oxalate formation and that high humidity is a crucial factor in this process. Consequently, the reactivity and photo-aging of ZnO in various oil binding media was investigated further under simulated solar radiation and at high relative humidity levels. ATR-FTIR showed that zinc oxalate is formed in all oil binding media while X-ray powder diffraction (PXRD) revealed it was mainly present in an amorphous state. To examine whether atmospheric CO2(g) has any influence on the formation of zinc oxalate, experiments with isotopically enriched 13CO2(g) were performed. Based on ATR-FTIR measurements, neither Zn13C2O4 nor Zn13CO3 were formed which suggests that the carbon source for the oxalate formation is most likely the paint itself (and its oil component) and not the surrounding atmosphere.

Highlights

Metal oxalates in oil paintings have recently attracted the attention of heritage scientists as their origin and mechanism of formation has not yet been fully elucidated [1,2,3,4,5,6]
Considering the above, the present paper investigates the formation of metal oxalates in linseed oil paint films containing pigments such as zinc white (ZnO), calcite (CaCO3), lead white, (Pb3(CO3)2(OH)2), zinc yellow (KZn2(CrO4)2(H2O)(OH)), chrome yellow (PbCrO4), cadmium yellow (CdS), cobalt violet (Co3(PO4)2) and verdigris, (Cu(CH3COO)2·H2O)
UVA light ageing of various pigments ATR-FTIR measurement of the surface of the various pigment samples exposed to UVA light showed no sign of oxalate formation after 50 and 224 h

Summary

Introduction

Metal oxalates in oil paintings have recently attracted the attention of heritage scientists as their origin and mechanism of formation has not yet been fully elucidated [1,2,3,4,5,6]. Considering the above, the present paper investigates the formation of metal oxalates in linseed oil paint films containing pigments such as zinc white (ZnO), calcite (CaCO3), lead white, (Pb3(CO3)2(OH)2), zinc yellow (KZn2(CrO4)2(H2O)(OH)), chrome yellow (PbCrO4), cadmium yellow (CdS), cobalt violet (Co3(PO4)2) and verdigris, (Cu(CH3COO)2·H2O).

Results

Conclusion