Crack morphology and its correlation with ground materials used in paintings by Danish portrait painter Jens Juel

Jens Juel (1745–1802) was a prominent Danish portrait painter, mainly known for creating over 600 portraits during his prolific four-decade career. This study investigated 22 canvas paintings by the artist, the majority of which are well preserved despite displaying typical signs of ageing like cracking. Crack formation in paintings results from a combination of external factors, such as environmental conditions, and inherent properties, including the composition of painting materials. Different materials used in the layers of a painting contribute to distinctive crack patterns, and this work focused on understanding how the composition of the ground layer and the density of the canvas influence these patterns. By employing a multi-analytical approach involving imaging, elemental and molecular analysis, along with data-driven methods such as dimensionality reduction, cluster analysis and statistical tests, the research established relationships between crack patterns and ground layer compositions in Juel's paintings. The findings unveiled significant physical and chemical differences between artworks created in Denmark (and partly Germany) and those from France and Switzerland, which can be attributed to different canvas preparation traditions. Crack patterns in Danish paintings are primarily characterised by orthogonal patterns with jagged lines, and these artworks have ground layers rich in calcium carbonate with minimal lead white and denser canvases. In contrast, French and Swiss paintings exhibit diagonal cracks in larger islands, and predominantly contain lead white with little to no presence of calcium carbonate in their ground layers. By integrating visual and chemical data, the investigation revealed four distinct crack pattern groups aligned with four ground types, underscoring the resilience of lead white-rich grounds, which are likely to be less susceptible to canvas-induced cracking compared to calcium-based grounds. Hence, the research uncovered a correlation between the proportion of these two components in the ground and the observed variations in crack patterns across different geographical and temporal contexts. These insights are crucial for conservators aiming to preserve not only Juel's artworks but also those of his contemporaries. The study highlights the significance of linking material choices to natural ageing patterns, and emphasises the importance of material analysis in predicting responses to ambient conditions and guiding conservation strategies for long-term stability of paintings.