Varnish is the transparent coating applied over finished paintings to protect and saturate the surface. From mastic to dammar to synthetic resins, learn its history.
When the National Gallery in London surveyed the varnish layers on its 19th-century paintings, the researchers found something that every conservator knows but few museum visitors think about: the varnish on a painting is not permanent. It yellows. It cracks. It becomes brittle. And eventually, it has to be removed and replaced. The survey, published in the National Gallery Technical Bulletin, found that mastic was the preferred varnish at the National Gallery in 1853, sometimes mixed with linseed oil, and that dammar was discussed but not used in Britain before that date. The varnish you see on an Old Master painting today is almost certainly not the varnish the artist applied.
Varnish is a transparent coating applied over a finished painting to protect the surface from dust, moisture, and abrasion, and to saturate the colors so they appear deeper and more luminous. In oil painting, varnish serves two functions: it protects the paint film from environmental damage, and it restores the optical quality of the paint surface, which becomes matte and desaturated as the oil binder ages. A good varnish makes the darks darker and the lights more luminous, giving the painting the appearance it had when the paint was still wet.
This entry covers the history of varnish in painting, the different types of varnish used over the centuries, how varnish ages and is removed, and what conservation science tells us about this fragile but essential layer.
A picture varnish is a solution of a natural or synthetic resin dissolved in a solvent, typically turpentine or mineral spirits. When the varnish is applied to a painting, the solvent evaporates and the resin forms a thin, transparent film over the paint surface. The film is meant to be removable, so that a conservator can strip an aged, yellowed varnish and apply a fresh one without damaging the paint beneath.
The optical effect of varnish comes from its refractive index. When a varnish with a refractive index close to that of the oil binder fills the microscopic roughness of the paint surface, light scattering at the surface is reduced. More light penetrates the paint film, reflects off the pigment particles, and returns to the viewer's eye. The result is that colors appear saturated, darks appear deeper, and the painting regains the wet-looking surface it had when the paint was fresh. Without varnish, an aged oil painting looks flat and chalky because the surface roughness scatters light before it can reach the pigment layer.
The protective function is straightforward. The varnish film acts as a barrier between the paint and the environment. Dust settles on the varnish rather than the paint. Moisture and atmospheric pollutants are blocked by the varnish film. Abrasion from handling and display affects the varnish, not the paint. When the varnish has degraded, it can be removed and replaced, sparing the painting from the damage that would otherwise accumulate directly on its surface.
Varnishing has been part of painting practice since antiquity. Ancient Greek and Roman painters applied protective coatings to their works, and the practice continued through the medieval period. Early varnishes were based on natural resins dissolved in oil or, after the development of distillation in the late Middle Ages, in spirits of turpentine.
Mastic was the dominant varnish resin in Europe from the Renaissance through the mid-19th century. Mastic is harvested from the mastic tree (Pistacia lentiscus), which grows primarily on the Greek island of Chios. The resin is collected as tears from cuts in the bark, dissolved in turpentine, and applied as a thin coating. Mastic varnish produces a warm, slightly amber tone that many painters found pleasing, and it is relatively easy to remove with solvents. Its main drawback is that it yellows significantly over time and becomes increasingly brittle.
Dammar resin was introduced to Europe in the early 19th century, probably around the 1820s. Dammar is harvested from trees of the genus Shorea, native to Indonesia and Malaysia. It is collected as tear-like droplets where the bark has been wounded, then dissolved in turpentine. Dammar produces a lighter, less yellow film than mastic and has better aging properties. A 2025 study published in the International Journal of Conservation Science noted that dammar varnish "became common from the mid-19th century onwards" and is "widely used as a paint contact coating in multilayer varnishes and for the final protection in conservation and restoration treatments." Read the study at IJCS.
The National Gallery's survey of 19th-century varnish compositions, published in the Technical Bulletin, confirmed that mastic was the standard varnish in British collections until the mid-19th century. Dammar was mentioned in the 1853 parliamentary Select Committee report but was not in common use before that date. Gas chromatography-mass spectrometry analysis of varnishes applied in the later 19th century confirmed that mastic was commonly used, sometimes with linseed oil and prepolymerized in some cases. Read the survey at National Gallery Technical Bulletin.
The aging of natural resin varnishes is one of the most studied problems in conservation science. Dammar and mastic are both triterpenoid resins, meaning their chemical structure is based on triterpene molecules. When exposed to light and oxygen, these molecules undergo autoxidation, forming new compounds that are increasingly polar and increasingly yellow. Over decades, a clear varnish becomes amber, then brown. Over centuries, the varnish can become so dark that it fundamentally alters the appearance of the painting beneath.
A study published in the Journal of Cultural Heritage examined the aging of six different triterpenoid resin varnishes, including dammar and various grades of mastic, under different conditions: darkness, museum conditions, and daylight through window glass. The research found that the triterpenoid fraction strongly decreases during aging as higher molecular weight compounds form. The aging process also changes the varnish's solubility, making old natural varnishes increasingly difficult to remove without using polar solvents that risk swelling and leaching the paint layers beneath. Read the study at Journal of Cultural Heritage.
A 2025 study published in Artuklu Humanities compared the aging behavior of three varnish types used in conservation: dammar resin, Regalrez 1094 (a synthetic hydrocarbon resin), and MS2A (a ketone resin). The researchers applied accelerated UV aging representing approximately 10 and 20 years of natural aging and documented changes using UV fluorescence imaging, light microscopy, SEM, and NMR spectroscopy. The results showed that dammar yellows more than the synthetic alternatives but has better compatibility with traditional paint films. Read the study at Artuklu Humanities.
Rubens was known for the luminous surface of his paintings, which he achieved partly through careful varnishing. His The Elevation of the Cross (1610-1611, Cathedral of Saint Bavo, Antwerp) shows the warm, saturated surface that a mastic varnish produces over oil paint. Rubens likely used mastic dissolved in turpentine, the standard varnish of his period.
Turner was obsessive about varnish and surface quality. The National Gallery's survey found that Turner used various varnishes, including mastic with linseed oil and, in some passages, dammar. His experimental approach to varnishes reflects the 19th-century search for the perfect surface coating, one that would protect without yellowing.
Constable varnished his paintings to achieve the deep, saturated greens and skies he wanted. His The Hay Wain (1821, National Gallery, London) would have been varnished shortly after completion, and the varnish has since been removed and replaced multiple times in conservation treatments. The painting's current surface is the product of modern conservation, not Constable's original varnish.
Rembrandt's late paintings, including The Jewish Bride (c. 1665, Rijksmuseum), show the deep, warm surface that varnish produces over impasto and glazing. The varnish on The Jewish Bride has been removed and replaced during conservation, but the optical effect of a fresh varnish over Rembrandt's textured surface remains one of the most striking experiences in any museum.
Rothko explicitly forbade varnishing of his paintings. His color field paintings rely on the matte, unvarnished surface of thinned oil paint applied directly to canvas. When the Tate Gallery varnished some of Rothko's Seagram Murals in the 1960s, the artist was furious. The varnish changed the optical quality of the paintings, giving them a gloss and color saturation that Rothko had deliberately avoided. This illustrates that varnish is not universally desired. Some artists design their paintings for an unvarnished surface, and varnishing them changes the work fundamentally.
Removing an aged varnish is one of the most delicate operations in painting conservation. The conservator must dissolve the varnish without affecting the paint beneath. This requires a solvent strong enough to dissolve the aged, cross-linked resin but not so strong that it swells or leaches the oil paint binder. As varnishes age, they become more polar and require more polar solvents for removal, which increases the risk to the paint layers.
A 2025 study in the International Journal of Conservation Science explored an alternative to solvent-based varnish removal: biocleaning with bacteria. Researchers isolated three bacterial strains from an 18th-century painting with damaged dammar varnish and tested their ability to degrade the varnish. The bacteria Pseudomonas stutzeri, Acinetobacter johnsonii, and Micrococcus luteus all showed promise for bioremoval of dammar varnish, offering an eco-friendly alternative to toxic solvents. Read the study at IJCS.
Conservators today choose from several categories of varnish. Natural resin varnishes (dammar and mastic) remain in use for their traditional optical properties and compatibility with old paint films, but they yellow and require replacement. Synthetic resin varnishes, including Regalrez 1094, MS2A, and various acrylic and ketone resins, offer better aging stability and less yellowing but may lack the optical warmth of natural resins. The choice depends on the painting, the period, and the conservator's judgment about what the artist originally intended.
For contemporary painters, dammar varnish dissolved in turpentine remains the standard final coating for oil paintings. It is applied after the paint has fully cured, which can take six months to a year for a typical painting. Retouching varnish, a thinner version, can be applied earlier to restore surface quality while the paint continues to dry.
Varnish is related to painting mediums, which often contain the same resins (dammar, mastic) dissolved in turpentine but mixed with oil rather than applied as a surface coating. The fixative used on drawings is a similar concept applied to dry media. The aging of varnish is part of the broader field of oil paint conservation. For more on how surface coatings affect a painting's appearance, read our post on oil painting and surface quality, or explore our guide to reading a painting.
The best way to understand varnish is to compare a recently cleaned painting with one that still has its old varnish. At the National Gallery in London, you can sometimes see paintings in adjacent rooms that have been recently conserved alongside paintings awaiting treatment. The cleaned paintings look brighter, cooler, and more detailed. The uncleaned ones look warmer, darker, and softer. The difference is not the paint. It is the varnish.
The Rijksmuseum's Operation Night Watch, ongoing since 2019, includes detailed study of the varnish layers on Rembrandt's The Night Watch. The operation's conservation team has mapped the varnish layers using macroscopic X-ray fluorescence and reflectance imaging spectroscopy, providing the most detailed picture yet of how varnish interacts with the paint surface beneath.
For more on painting materials and conservation, read our entries on linseed oil and turpentine, or explore our post on chiaroscuro and the surface of paintings.
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