For centuries, linseed oil paint has been the binder of choice for oil painting, prized for its flexibility, gloss, and ability to form durable paint films. The durability of linseed oil paints depends on the oil’s natural properties and how it is processed before mixing with pigments. A recent study by Ilaria Bonaduce and colleagues sheds new light on how different 19th-century pre-treatments of linseed oil affect the drying, aging, and performance of oil paint over time (Bonaduce et al., 2012).
This article reviews the study’s key findings and explores how traditional pre-treatment methods, such as washing, heating, and adding metal-based driers, influence pigment wetting, drying rates, and the stability of paint films. Importantly, it also highlights the beneficial effect of lead white in stabilizing the oil binder and reducing the evaporation of free fatty acids. Understanding these insights can help artists make more informed choices about their materials and techniques.
The featured image of this article is the Slave Ship (originally titled Slavers Throwing overboard the Dead and Dying—Typhon coming on), an 1840 painting by British artist J. M. W. Turner. This oil on canvas painting depicts a scene of a slave ship during a storm, throwing enslaved people overboard. The painting is currently located at the Museum of Fine Arts, Boston. Turner made extensive use of pre-treated oils and mediums in the 19th century.
Nineteenth-Century Pre-Treatments of Linseed Oil
Bonaduce’s study investigated linseed oil that underwent various pre-treatments common in 19th-century painting practices. The treatments included:
1. Untreated Oil
- Mechanically extracted from linseed without additional processing.
- Slow drying and prone to oxidation and yellowing over time.
- Limited ability to wet pigments evenly, leading to inconsistent pigment dispersion and texture.
2. Water Washing
- The oil was washed in purified water to remove mucilage and impurities.
- Improved drying time and clarity but did not significantly alter aging patterns.
- Enhanced pigment wetting, resulting in better pigment dispersion and more uniform paint.
3. Heat Treatment (150°C and 300°C)
- Oil heated to 150°C or 300°C before mixing with pigment.
- Promoted cross-linking and increased viscosity.
- Heat-treated oils increased viscosity and darkening due to the heat treatment but showed little overall effect on oxidation and aging.
4. Addition of Lead-Based Driers
- Litharge (Lead(II) oxide) at room temperature and heated to 150°C
- Lead Acetate at room temperature and heated to 150°C
- Lead Subacetate at room temperature and heated to 150°C
- Lead-based driers accelerate drying and foster oxidative processes. These treatments form lead soaps that slow the evaporation of free fatty acids, thereby preserving some components of the binder.
How Pre-Treatments Affect Pigment Wetting and Drying
Improved Pigment Dispersion in Linseed Oil Paint
Pigment wetting is essential in paint making because it determines how evenly pigment particles disperse in the binder, which affects the paint’s consistency, color intensity, and ease of application. Heat-treated and lead-treated oils produced smoother dispersions, especially with reactive pigments like lead white (Bonaduce et al., 2012, p. 5). Water-washed oil also showed improved pigment dispersion, but untreated oil was less effective.
In a previous study, the authors showed that most treatments were aimed, from a chemical point of view, at consuming the antioxidants naturally present and part of the double bonds. This produced a partially cross-linked and oxidized oil, which was more polar than the untreated oil.
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