Yellowing

“The discoloration or shift toward a yellow hue observed in oil films, varnishes, or painting media over time due to oxidative, photochemical, or environmental factors. In the context of oil painting, yellowing may affect the clarity of paint layers, alter perceived color relationships, and compromise the visual integrity of a work—particularly in areas of light value or in the presence of clear media.

Yellowing in drying oils (especially linseed oil) results from the oxidative polymerization of unsaturated fatty acids (primarily linoleic and linolenic acids) as they react with atmospheric oxygen to form linoxyn, a cross-linked polymer that binds pigment particles into a durable film. However, in the absence of light—particularly ultraviolet light—linolenic acid-based glycerides may undergo unwanted reactions that promote the formation of yellow chromophores (molecular structures that absorb specific wavelengths of light, producing visible color), leading to visible discoloration.

Linseed oil, prized for its durability and strong film formation, contains higher levels of these reactive fatty acids than alternatives like poppy or safflower oil, which dry more slowly but yellow less.

Numerous extrinsic factors modulate yellowing including: light exposure: prolonged absence of diffuse daylight—especially when paintings are stored in darkness—exacerbates yellowing. This effect is largely reversible with exposure to natural light, which photochemically reverses certain yellow compounds​, air pollution: urban environments with high sulfur dioxide, ozone, and soot content can deposit reactive compounds on surfaces or accelerate oxidative degradation, contributing to yellowing and embrittlement​, and humidity & temperature: excess humidity can slow drying and promote water-mediated hydrolysis of oil components, while excess heat can hasten oxidative polymerization, leading to brittleness and discoloration. Extreme or fluctuating environments pose the greatest risk.

Material factors include: oil purity & processing: poorly refined oils (especially those with residual chlorophyll or protein impurities), yellow more. Mayer notes that cold-pressed and sun-refined linseed oils may resist yellowing better if applied properly. Another is pigment dispersion: pigment-rich paint films mask oil yellowing, especially when the oil content is minimized. Yellowing is most visible in clear glazes, impure media, or overbound paint. Additionally, surface layers: varnishes or unpigmented media exposed at the surface, are more prone to yellowing than embedded paint films. Dirt and aged varnish can often be mistaken for intrinsic yellowing.

Despite marketing claims, no drying oil is truly non-yellowing. All oil-based media undergo some color change due to their organic chemistry and environmental reactivity. Modern synthetic resins or acrylics may yellow less, but they introduce other trade-offs in film behavior, optical quality, and reversibility.

Some ‘best practices’ to mimize the magnitude or speed of yellowing include: using well-refined oils in minimal quantities, avoiding clear mediums or excessive oil additions in light passages, expose fresh paintings to normal daylight but avoid direct sun, maintain stable indoor humidity (~50%) and moderate temperatures, and store artworks upright, uncovered, and not in darkness during drying.

Ralph Mayer, in The Artist’s Handbook of Materials and Techniques, argues that yellowing in oil paintings is not an inevitability but largely a consequence of chemical formulation, environmental conditions, and technical execution. He asserts that ‘correctly executed normal oil paintings do not turn yellow‘ when sound materials and procedures are used. However, this position reflects ideal conditions and practices, and many conservation professionals still observe yellowing as a common aging behavior—especially in works exposed to poor storage or inferior materials.

Yellowing in Other Materials: While yellowing is most commonly associated with oil-based paints, it also occurs in papers, acrylics, drawing supports, and certain dry media, though often through different mechanisms:

Paper & Drawing Surfaces : Acidic degradation of lignin-containing wood pulp is a primary cause of yellowing in low-quality paper. This oxidative breakdown generates chromophores (yellow-brown compounds) that accumulate over time. Light exposure, humidity fluctuations, and airborne pollutants accelerate the process, especially in newsprint, cardboard, or unbuffered sketch paper. Even archival-quality papers can yellow if sizing agents, residual processing chemicals, or environmental exposure introduce instability. Mayer confirms that non-rag, low-pH materials are inherently prone to yellowing unless buffered or sealed​.

Acrylic Paint: Acrylic polymers yellow far less than drying oils, but do not remain perfectly colorless over time. Yellowing can occur in: unpigmented gel mediums, especially when applied thickly or exposed to low light, lower-quality formulations or those containing certain plasticizers or additives that degrade, or with environmental exposure (UV, ozone, or prolonged darkness) can cause subtle shifts in tone or gloss. Mayer notes that acrylic films are more stable in light but less reversible than oil films​.

Pastel, Charcoal, and Other Dry Media: The pigments themselves in high-quality dry media are generally lightfast and chemically stable. However, fixatives and surface coatings may yellow with age, especially if made with shellac or resin-based binders. Paper choice is again crucial—low-quality supports will yellow underneath or around otherwise stable pastel or charcoal marks.

Varnishes and Coatings: Both natural resin varnishes (like dammar) and some synthetic coatings can yellow over time. Surface-positioned varnishes are more prone to yellowing than those bound within a paint layer. Light exposure often mitigates yellowing in resin films, while dark storage accelerates it—a phenomenon similar to drying oils.

In summary, yellowing affects many materials, but the causes differ: in oils: oxidation of fatty acids, in paper: acid hydrolysis and lignin breakdown, in acrylics: polymer degradation or additive instability. and in coatings: photo-reactive resin oxidation. True ‘non-yellowing’ materials are rare as almost all materials will exhibit some shift in tone over time due to molecular aging, environmental exposure, and light behavior.”