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The industrial developments of the 19th century included the production of a variety of synthetic pigments and dyes, which were often used by artists who were not concerned with their stability over time. Among these pigments, cadmium yellow, based on cadmium sulfide (CdS), was popular with artists beginning in the mid-19th century. This pigment may discolour or darken and/or exhibit loss of adhesion and formation of white globules on the painting surface [1-2]. Its degradation involves photooxidation of CdS and the formation of degradation products (sulfates, oxalates, and carbonates) [1-4].
However, not all historic paints containing cadmium yellow degrade. It appears that CdS paints produced between the late 19th and early 20th centuries are particularly prone to degradation, as has been documented in paintings dating from 1880 to 1920 [4]. Because these paintings have been exposed to different environmental conditions, it has been speculated that the degradation of cadmium yellow may be related to imperfect syntheses, resulting in the formation of a more reactive form of CdS. However, the link between the presence of these reactive pigments and the tendency of the paint to deteriorate is not well understood.
CdS is a IIb-IVa semiconductor. When excited by light of an appropriate energy, the semiconductor exhibits a characteristic near band edge (NBE) emission, closely related to the CdS energy bandgap, and emission from intra-bandgap trap states (TS) related to crystal defects [5-6]. Even though little considered when dialing with luminescent paints, the TS emission can provide useful information about surface states in nanocrystalline semiconductors [7]. Moving from nanoscience to conservation studies, the detailed investigation of TS emission in CdS paints is proposed here to probe changes in the density and energy of surface defects and ultimately in the surface reactivity of CdS following degradation.
Photoluminescence imaging, microscopy, and spectroscopy – complemented by other X-ray based spectroscopies and microscopies – have been used to examine historical and modern manufactured CdS-based paints following artificial aging. Lastly we investigated the degradation of cadmium yellow paints in the painting Femme (Époque des “Demoiselles d’Avignon”) (1907) by Pablo Picasso.
This study found that in degraded CdS paints the TS emission is much higher in intensity (with respect to NBE emission) and shifted to shorter wavelengths with respect to preserved paint layers. This observation indicates a higher density of TS in the degraded paint, which promotes the surface reactivity of CdS particles and the subsequent paint degradation. In general, results indicated that the analysis of TS emission is a highly sensitive method for identifying early signs of degradation in CdS paints.
[1] Van der Snickt, G. et al., Anal. Chem. 2009, 81 (7): 2600–2610
[2] Mass, J. et al., Analyst 2013, 138: 6032–6043
[3] Anaf, W. et al., Dyes and Pigments 2015, 113: 409-415
[4] Monico, L. et al., Chemistry–A European Journal 2018, 24.45
[5] Cesaratto, A. et al., Analytical Methods 2014, 6.1: 130-138
[6] Rosi, F. et al., Microchem. J. 2016, 124: 856-867
[7] Krause, M. M. et al., Phys Chem Chem Phys 2015, 17:18882-18894.
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