Water‑Sensitive Oil Paint: Why Some Modern Oil Paints Become Vulnerable to Water

Water‑Sensitive Oil Paint: Why This Research Matters for Artists

Water‑sensitive oil paint has become a significant conservation concern in modern painting. In some paintings made since the late twentieth century, even light contact with water can remove pigment, disturb gloss, or disrupt the paint surface. Conservators encounter this problem most often when attempting routine cleaning of unvarnished oil paintings, where a water‑sensitive oil paint film can lose pigment or binder during cleaning.

Jacopo La Nasa and colleagues investigated this issue by comparing two naturally aged Winsor & Newton Winsor Green oil paint swatches manufactured about a decade apart. One swatch (1993) showed no water sensitivity in a standardized cotton‑swab test. The other (2003) showed pigment pickup after only a few swab rolls (La Nasa et al. 2019, 2).

At first glance, the two paints were similar. Both contained phthalocyanine green pigment (PG7) with similar extenders and oil components. Yet their behavior during cleaning was dramatically different.

The research, therefore, asks a practical question that matters to artists: why can two paints with similar ingredients age so differently?

The authors systematically tested several explanations—including epsomite formation, free fatty acids, and metal soaps. Among the factors examined, the strongest correlation with water sensitivity was the structure of the cured oil network itself. The water‑resistant paint contained a more extensively cross‑linked polymer network, whereas the water‑sensitive paint exhibited a less polymerized, more polar network structure (La Nasa et al. 2019, 7–8).

Because the study compares only two samples, it cannot establish universal rules for all modern oil paints. However, it provides valuable evidence that the quality of the polymerized oil network may play a decisive role in whether a paint film becomes vulnerable to water during aging.