Experimental investigation of environmental and material factors in darkening of historic and modern lead white

The progressive darkening of lead white represents a critical challenge in the preservation of artworks on paper. The National Gallery of Denmark has witnessed an alarming acceleration of this phenomenon since the early 2000s, with lead white highlights in numerous old master drawings undergoing rapid discolouration. This study investigated the factors contributing to the darkening of lead white, specifically comparing industrially manufactured pigments to historically reproduced ones. The research aimed to determine if modern lead white, typically characterised by a higher proportion of hydrocerussite to cerussite in its composition, is more susceptible to darkening. Lead white paint mock-ups were subjected to hydrogen sulfide exposure under varying environmental (such as temperature and relative humidity levels) and material (such as pigment and binder properties) conditions to examine their impact on pigment stability. Due to the extensive number of variables involved, the investigation focused on a subset of the possible experimental conditions. The results indicated that modern lead white is more prone to rapid darkening, likely due to increased lead hydroxide content and smaller particle size, leading to higher reactivity with hydrogen sulfide. Additionally, the pigment-to-binder ratio significantly influenced darkening, with high ratios exacerbating the issue for both industrially made and historically reproduced pigments. Although temperature and relative humidity had minimal direct effects, elevated temperature and humidity conditions could potentially accelerate deterioration by hastening chemical reactions and promoting hydrogen sulfide dissociation into more reactive ionic species, respectively, thus increasing lead white vulnerability to darkening. However, given the observed greater influence of chemical and structural properties over environmental physical factors in the darkening of lead white, these findings emphasise the crucial role of pollution control and suggest that effective preservation of artworks and historical artefacts containing lead white highlights should prioritise strategies to reduce hydrogen sulfide exposure, rather than solely relying on climate control measures.