Using UV Light in Paintings Conservation

Using UV Light in Paintings Conservation: Benefits and Precautions

Ultraviolet (UV) light is an invaluable tool in the field of painting conservation. This non-invasive technique allows conservators to analyse, assess, and treat artworks with a depth of understanding that other methods often cannot achieve. UV light reveals subtle details and conditions hidden to the naked eye, such as underpainting, past restorations, varnish layers, and damages. However, the use of UV light in conservation demands caution, as it can have adverse effects on certain materials. In this article, we will explore the benefits and precautions of using UV light in the conservation of paintings, backed by examples from the field.

Benefits of Using UV Light in Paintings Conservation

1. Identification of Varnish and Overpaint

One of the primary uses of UV fluorescence in art conservation is to differentiate between original and non-original components of a painting. Varnishes, for instance, can fluoresce under UV light, with aged varnishes often appearing in distinct colours such as green or yellow. This property is helpful in identifying areas on the painting’s surface that have been previously restored or altered. More recently applied materials generally absorb UV light, and therefore appear black or dark purple, while older varnishes often fluoresce strongly, creating a layer that looks like a glowing green veil atop the surface.

Example: In the images below, paintings conservator Alexis Drapanas uses a 365nm UV torch to assist in the removal of a yellowed, oxidised varnish from the surface of an oil painting on wooden panel, and to inspect the numerous layers of overpaint applied in the past.

The test square shows the efficacy of varnish removal in different layers. The area marked “1” in the far right image appears bright purple under UV light because this is where the varnish has been completely removed. It appears dark because it is no longer being veiled by a thick layer of fluorescing varnish. The area marked “2” is where the varnish has been thinned. The veil of varnish is still apparent under UV light, but the oxidized layer has been removed, allowing this area to appear identical to area 1 under natural light.

Important Note: The ideal wavelength for UV inspection of paintings in conservation typically falls within the longwave UV range (320–400 nm), with the most commonly used wavelength being around 365 nm. This range is considered optimal because it minimises damage (longwave UV is less energetic and less likely to cause damage to sensitive materials compared to shorter wavelengths) and maximises fluorescence of non-original materials while still offering clear visual feedback. When considering UV torches, a 365nm wavelength is ideal, but you may also want to consider a high powered torch that doesn’t require a dark room for inspection, as you may not always have an ideal atmosphere when visiting clients or sites.

Go to UV lights and torches for art inspection

2. Detection of Retouching and Repainting

UV light can help conservators identify areas of repainting or retouching, as new pigments may fluoresce differently compared to original materials. This assists conservators in evaluating previous restorations, determining the extent of past repairs, and highlighting possible areas of damage.

Example: In the images below, Drapanas examines another detail of the same painting during various stages of varnish removal. The two images on the left are identical (center image includes digital mapping) and highlight where past areas of overpainting might be present. On the figures cape and stockings, dark areas are present above the green veil of varnish, suggesting a newer application of retouching. Less prominent are the darkened areas by the figure’s feet. These areas are visible, but are not quite as dark as the aforementioned areas on the cape and stockings, suggesting an older application of overpainting that might exist underneath the varnish layer.

As you can see from the image on the right, taken during the varnish removal process, the older areas of overpainting that surrounded the figure’s feet remain visible after the varnish is removed from that area. This confirms that these overpaintings were indeed underneath the varnish and were less prominent under UV light because the veil of varnish was obstructing them.

Such analyses are incredibly useful to conservators during the initial documentation process. Knowing where these areas of overpainting exist allows conservators to be ready for overpaint removal, flags them to areas of potential damage, and in extreme cases can assist a conservator in deciding against further intervention.

3. Revealing Organic Pigments and Materials

UV light can illuminate organic materials that might otherwise remain invisible. For example, organic pigments often fluoresce under UV light, which helps conservators identify and document them without invasive sampling. Additionally, UV light can be used to detect biological growth like mould or bacteria on a painting’s surface.

Example: The Ghent Altarpiece, an early 15th-century polyptych by Hubert and Jan van Eyck, displayed fluorescence under UV light, which helped conservators to detect organic varnishes and assess areas affected by mould. This information guided the cleaning process, allowing conservators to remove these contaminants carefully without damaging the paint layers. Read more about this project’s restoration here.

Example: Joshua Reynolds’ 1760 portrait of Anne, Countess of Albemarle is an example of fading red lake due to light exposure. Through further examination, it was discovered that the skin on the pale portrait’s face consisted of lead white mixed with red lake, which would have originally created a pink hue. You can read more about this example in a 1994 National Bulletin “Light-induced Colour Changes in Red and Yellow Lake Pigments.” here

4. Assessing Structural Stability and Condition

UV examination can reveal cracks, abrasions, and flaking paint that might not be visible in regular light. This is crucial for assessing the structural condition of a painting, guiding conservators in determining areas requiring urgent stabilisation or consolidation.This approach essentially highlights paint losses and areas of lifting paint through differing fluorescence instead of differing texture. These types of issues (cracks, abrasions and lifting paint) are very common problems that painting conservators face and they are important to identify so that the surface of the artwork can be properly consolidated and stabilised before continuing interventions. Digital UV microscopes are extremely useful in cases like these and are ideal for non-destructive analysis, characterisation tests and condition recording.

Go to Digital UV Microscopes for art inspection

Precautions for Using UV Light in Conservation

1. Avoiding Prolonged Exposure

Although UV light is useful in analysis, prolonged exposure can damage certain pigments and organic materials by accelerating degradation. UV rays can cause colour fading, especially in sensitive materials, and weaken the molecular structure of some organic compounds.

Precautionary Measure: Conservators limit the duration and intensity of UV exposure to mitigate potential damage. For example, portable UV lamps are preferred over continuous high-intensity UV sources, and conservators adhere to strict time limits during examination sessions. Read more here about monitoring colour fading and precautions you can take to preserve colour.

2. Protecting Sensitive Paintings and Materials

Not all artworks respond well to UV light exposure. Pigments like red lakes and some organic compounds can degrade when exposed to UV light. Furthermore, certain binders and varnishes might become brittle under repeated UV illumination.

Precautionary Measure: Conducting tests on smaller, less visible areas helps conservators gauge a painting's reaction to UV exposure. For example, they may start by analysing the painting's corners to ensure there is no adverse reaction before examining the central portions of the artwork. Small UV torches are extremely useful for such controlled inspections.

3. Using Proper Protective Gear

UV light can be harmful to human skin and eyes. Conservation professionals must use appropriate protective gear, including UV-blocking goggles, gloves, and protective clothing, to minimise exposure risks.

Precautionary Measure: In many museums, such as the Getty Conservation Institute, UV examinations are conducted in rooms with UV-filtered lighting and equipped with protective barriers to shield conservators from exposure.

Conclusion

UV light has transformed the field of painting conservation, allowing for non-invasive examination and an in-depth understanding of artworks’ materials, conditions, and histories. When used carefully and in combination with other methods, UV light enables conservators to respect and preserve the original integrity of paintings while enhancing their longevity. By observing precautions, conservation experts can use UV light safely and effectively, revealing hidden histories without compromising the artwork’s future.

Digital Resources for Further Study

• Smithsonian Institution - UV Light in Art ConservationThe Smithsonian provides an accessible guide to how UV light is used in art conservation, including technical details on UV fluorescence in different materials.
• The Getty Conservation Institute - Resources on UV Examination The Getty Library offers extensive online resources, videos, and publications covering UV light's role in examining paintings, including conservation studies of Renaissance art.
• National Gallery, London - Technical Bulletins The National Gallery’s Technical Bulletins include studies on how UV analysis aids in understanding historical paintings and provides insights on best practices for using UV in conservation. Similarly, you can find studies on radiation and color change, analyses on paint mediums, full conservation reports, and additional documentation methods.
• ARTEnet - Ultraviolet Fluorescence ARTEnet’s comprehensive introduction to UV Examination provides basic information with accompanying photos for emerging conservators or students as well as a scientific overview of how this imaging technique is useful in the field.
• YouTube - Painting Inspection with Dino-Lite Digital Microscopes Dino-Lite’s Youtube video offers a quick and comprehensive look at how handheld digital microscopes can be useful for conservators.