Understanding Oil Paint Aging: Shrinkage and Cracking

The longevity and preservation of oil paintings are crucial for artists and conservators. Two pivotal studies—one by Janas et al. in Heritage Science and another by Marion Mecklenburg and colleagues—provide critical insights into how oil paints age and respond to environmental conditions. These findings are essential for artists seeking to understand the mechanical properties of oil paints and how different substrates and environmental factors can impact their work over time.

Key Findings on Oil Paint Aging and Brittleness

Oil paints undergo significant chemical and mechanical changes as they age. Janas et al.’s study highlights that oil paints become stiffer and more brittle over time due to molecular changes, which include cross-linking of molecules. This brittleness can lead to cracking, especially when the paint layer is restrained by a dimensionally stable substrate, such as a wooden panel. The study provides evidence that some oil paints can be more brittle than the glue-based ground layers beneath them, challenging the traditional understanding of the painting structure’s weakest points.

Mecklenburg’s research further explores these changes by examining the mechanical behavior of oil paints over long periods. His findings reveal that the embrittlement of oil paints does not slow down significantly, even after several decades. Factors such as temperature, relative humidity (RH), and solvents play substantial roles in altering the mechanical properties of paints. For instance, low temperatures can cause immediate brittleness in paint films, a condition that reverses when temperatures rise. In contrast, high temperatures lead to permanent changes due to the volatilization of plasticizers.

Consequences of Long-Term Shrinkage and Environmental Sensitivity

A significant observation from Janas et al.’s study is the cumulative shrinkage of oil paints over time. As paints age, they shrink due to the evaporation of low-molecular-weight components and molecular rearrangements within the paint film. This shrinkage can exceed the paint’s strain at break—the point at which it can no longer stretch without cracking—particularly when restrained by a stable substrate. X-ray microtomography vividly demonstrated this phenomenon, which showed cracking in a 17th-century panel painting’s oil paint layer while the ground layer remained intact.

Mecklenburg’s studies complement these findings by showing how different pigments and binders affect paint flexibility and brittleness. For example, paints containing zinc oxide are known to become extremely brittle relatively quickly, often within just a few years. On the other hand, paints with basic lead carbonate tend to remain flexible for longer but gradually increase in stiffness and strength over time.

Understanding Mechanical Properties: What Artists Should Know

To fully appreciate the implications of these studies, it is crucial to understand some key mechanical properties: