ATR-FTIR spectroscopic imaging of white crusts in cross sections from oil cartoons by Edward Poynter in the Heritage Collections at UK Parliament

The formation of white crusts on the surface of oil paintings can affect both the physical and aesthetic values of the artwork. Characterisation of these white crusts enables better identification of methodologies and solutions when performing conservation treatments of oil paintings. In this paper, micro Attenuated Total Reflection-Fourier Transform Infrared (ATR-FTIR) spectroscopic imaging was used to investigate the white crusts and metal carboxylates and its relationship to the migration of lead elements, with analysis focused on two cartoons by Edward Poynter from the Heritage Collections at UK Parliament. The white crusts may be linked to the metal ion migration via metal carboxylates to the surface of the painting, resulting in the interaction between the metal ions and air pollutants. The results show that the white crusts on the Poynter cartoons are heterogenous, containing three different components (sulphates, oxalates and carbonates). This information has offered conservator guidance for cleaning treatments of the white crusts. Furthermore, metal carboxylates with different crystallinity (crystallised and amorphous) and with different metal ions (lead and zinc) were successfully characterised and their distribution were revealed in ATR-FTIR chemical images. These chemical images shed light on possible degradation process associated with relocation of lead and calcium from ground layers, and invasion of calcium from the air, to form the white crusts. In the ATR-FTIR spectra, qualitative analysis of one of the degradation products (CaCO3) was combined with quantitative analysis, conducted by evaluating the peak shifts calculating the integrated absorbance. This enabled the authors to evaluate the crystallinity and formation directions of CaCO3 in the white crusts. It is hoped that these results of the case study and advancement of knowledge in this paper can aid the development of scientifically supported and improved conservation strategies. These results can be used as a trigger for further investigation into the degradation mechanisms related to white crusts and pigments/binding media used in the paintings.