Oil paintings yellow due to a complex combination of factors, primarily related to the oil itself and environmental conditions.
Here's a breakdown:
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The Oil Itself: The primary culprit is the oil binder, typically linseed oil.
- Oxidation: Over time, linseed oil undergoes oxidation. This process causes the oil molecules to cross-link, leading to the formation of a yellowish film.
- Dark Yellowing/Reversibility: In the absence of light, linseed oil tends to yellow significantly. This is often referred to as "dark yellowing." However, this yellowing is usually reversible; exposing the painting to light will often bleach the yellowing.
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Environmental Factors: These external conditions can accelerate or exacerbate the yellowing process.
- Light Exposure: The amount and type of light are crucial. Ultraviolet (UV) light can damage the oil, potentially contributing to yellowing in the long run. Paradoxically, periods of darkness (as mentioned above) can initially increase yellowing, which can then be reversed with exposure to light.
- Humidity and Temperature: High humidity and fluctuating temperatures can accelerate chemical reactions within the paint film, including those that lead to yellowing.
- Air Pollution and Chemicals: Exposure to air pollutants and certain chemicals can react with the oil paint, contributing to discoloration.
- Varnish: Over time, varnish layers can also yellow, impacting the overall appearance of the painting.
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Pigment Interaction:
- Some pigments can accelerate the yellowing of oil. The choice of pigments plays a role, as some may interact with the oil and promote yellowing more than others.
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Other Factors:
- Oil Processing: The method of processing the oil (e.g., refining, bleaching) can affect its yellowing properties.
- Additives: The use of driers, mediums, and other additives can influence yellowing. The formulation of the paint itself is therefore crucial.
- Paint Thickness: Thicker layers of paint may yellow more noticeably.
In summary, yellowing in oil paintings is a multifaceted issue resulting from a complex interplay between the oil binder, environmental conditions, pigment interaction, and the specific materials and methods used in creating the painting. Minimizing exposure to drastic temperature and humidity fluctuations, protecting the painting from UV light, and carefully choosing painting materials can help mitigate yellowing.
