Weathering of historical paints based on linseed oil and plant resins

Abstract

The degradation of wood in outdoor applications, which affects its physical and mechanical properties, is a significant issue in the context of cultural heritage. Traditional oil paints applied to exterior wood surfaces offer certain benefits, including moisture resistance and excellent coverage, though one of the negative effects is surface oxidation. Historically, oil paints have been shown to be durable, lasting 50–100 years if maintained regularly. However, there are no long-term data on the durability of linseed oil-based coatings under natural weathering conditions. To fill this gap in the research literature, our study aimed to compare the performance of these coatings on wooden surfaces and assess their protective properties over time. We tested four paint formulations containing chromium oxide green, linseed oil, and different plant resins for comparison (triterpenoid resins—dammar and copal, diterpenoid resin mastic). We looked at two forms of each paint: semi-transparent and opaque. The prepared specimens were exposed to weather conditions typical of the Central European climate for 6 years. The specimens were then analyzed for chemical, contact angle and color changes. After 6 years, the wood painted with opaque coatings showed the least color change, demonstrating their superior durability. In contrast, semi-transparent paint displayed more significant variations in all measured parameters when compared to opaque paint. Nevertheless, semi-transparent paint still performed adequately after 6 years of exposure to the elements, fulfilling its intended purpose. The need for research arises from the lack of long-term data on the durability of linseed oil-based coatings under natural weathering conditions, and the study aims to compare the performance of these coatings on wooden surfaces and assess their protective properties.