Impasto Medium: Additives That Thicken Paint for Texture

Rembrandt's impasto is so thick that scientists spent years trying to figure out what he put in his paint. The peaks and ridges in The Jewish Bride and The Night Watch hold their shape with a crispness that ordinary oil paint, even straight from the tube, cannot match. Something was added. A 2015 study using synchrotron X-ray diffraction found a rare lead compound called plumbonacrite in Rembrandt's thickest paint passages, suggesting he mixed his lead white with an alkaline binding medium to achieve the outstanding rheological properties that let his impasto hold its shape. Rembrandt was, in effect, formulating his own impasto medium three centuries before anyone called it that.

Impasto medium is any substance added to paint to increase its thickness and body so that it holds visible texture, peaks, and marks when applied to a surface. The term covers a range of products: oil-based pastes for oil painting, acrylic gels and modeling pastes for acrylic painting, and historical additives like wax, resin, and lead compounds. The purpose is always the same: to make the paint stiffer and more retainable so that brushstrokes, knife marks, and built-up texture survive the drying process without slumping or flattening.

This entry covers what impasto mediums are made of, how they work, their history, and how to choose and use them.

What Is Impasto Medium and How Does It Work?

Impasto medium works by modifying the rheological properties of paint. Rheology is the study of how materials flow and deform. Oil paint straight from the tube has a certain viscosity (thickness) and yield stress (the force required to make it flow). To hold thick peaks and visible marks, paint needs a high yield stress: it should behave like a solid when at rest, holding its shape, and like a liquid only when force is applied by a brush or knife.

A 2014 study published in the Journal of Colloid and Interface Science reconstructed 19th-century white paint formulations and characterized their rheological properties, finding that both lead white and zinc white paints exhibit a fluid-to-solid transition. The research showed that zinc white exhibits approximately 30 times higher yield stress than lead white, which explains why Van Gogh used zinc white for his brightest, thickest impasto accents. The study is available at ScienceDirect.

Impasto mediums increase yield stress by adding solid particles (extender pigments) or by creating a gel structure within the paint. The result is paint that is stiffer, more buttery, and more capable of holding texture.

Oil-Based Impasto Mediums

Oil-based impasto mediums consist of oil, sometimes resin, and extender pigments. Extender pigments are white inorganic minerals that appear transparent when mixed with oil paint. Common extenders include calcite (calcium carbonate), barite (barium sulfate), talc, kaolin (clay), silica, and bentonite. These minerals add bulk and stiffness without significantly changing the color of the paint.

Natural Pigments, a manufacturer of traditional painting materials, explains that paste mediums are ideal for thickening oil paints because "they dries evenly without cracking or wrinkling." Their Impasto Medium is composed of finely ground calcite, barite, and talc in pale linseed oil. It makes colors slightly transparent while allowing thick, impasted applications. The medium does not contain stearates, solvents, driers, resins, or other additives, and it does not significantly affect drying time. Read the product information at Natural Pigments.

The density of extender pigments affects handling properties. Chalk, with a relative density of 2.7, provides impastos that hold a short peak and stand upon drying. Barite, with a density of 4.4, also holds its shape but tends to produce paint that is "long" or "ropy" rather than "short" and buttery. The oil type also matters: paint made with bodied oils (heat-polymerized oils) flows out and levels, so brushstrokes melt after application. Unbodied oil forms buttery, short paint that holds brushstrokes upon drying.

Acrylic Impasto Mediums

Acrylic impasto mediums are typically gel mediums or modeling pastes. Acrylic gel medium is a thick, transparent polymer emulsion that increases the body of acrylic paint without changing its color. Modeling paste is an opaque white gel that adds bulk and opacity. When mixed with acrylic paint, these mediums create a stiff, sculptural paint that can be applied with a knife or brush and will hold its shape as the water evaporates and the polymer coalesces into a solid film.

Acrylic mediums dry by evaporation and coalescence, which means they dry faster than oil-based mediums. This is an advantage for painters who want to build up texture quickly, but it also means there is less working time. Once acrylic medium dries, it cannot be reactivated.

Origins and History

Painters have been modifying the consistency of their paint for as long as they have been painting. The Early Netherlandish painters added oils and resins to achieve specific handling properties. Rembrandt's use of plumbonacrite, discovered in the 2015 study published in Angewandte Chemie, is evidence that 17th-century painters were formulating their own impasto mediums. The research, conducted on microscopic samples from the Louvre, the Rijksmuseum, and the Mauritshuis, identified plumbonacrite as a fingerprint of Rembrandt's impasto recipe. Read the study at Wiley.

In the 19th century, commercial paint manufacturers began producing ready-made mediums. The most famous were the "gumtions" and "megilps" used by British painters like J.M.W. Turner. These gel mediums were made from mastic resin, linseed oil, turpentine, and lead compounds. A 2017 study in Angewandte Chemie analyzed these mediums in depth, describing them as "functional hybrid materials" based on lead metallogels co-existing with lead oxide nanoparticles. The gel structure gave Turner's paint extraordinary handling properties, allowing him to easily tune the paint's consistency and drying behavior. However, the over-use of these mediums was blamed for severe cracking and disruptions in the paint films of many 19th-century British paintings. Read the study at Wiley.

A related study published in Advanced Functional Materials further investigated the nanocomposite structure of these 19th-century mediums, describing them as "hybrid nanocomposites" based on lead metallogels. The research showed that gumtions are formed through complex chemistry involving the coordination of lead cations with carboxylic acid moieties of triterpenoids from mastic resin, creating a supramolecular lamellar organization. Read the study at HAL Archives.

In the 20th century, the development of acrylic paint introduced a new category of impasto mediums. Acrylic gel mediums and modeling pastes allowed painters to build up thick, sculptural surfaces with a fast-drying, flexible polymer. These mediums became standard in contemporary painting and are widely available from manufacturers like Golden, Liquitex, and Winsor and Newton.

Key Artists and Their Impasto Mediums

Rembrandt van Rijn (1606-1669)

Rembrandt formulated his own impasto medium using lead white and an alkaline binding medium that produced plumbonacrite. His The Jewish Bride (1665, Rijksmuseum) shows the results: impasto so thick that individual brush hairs are visible in the dried paint surface. A 2025 study in npj Heritage Science as part of Operation Night Watch measured impasto heights up to 850 micrometers on The Night Watch (1642), revealing that Rembrandt built up the textured surface with multiple lead white underpaint layers. Read the study at Nature.

Vincent van Gogh (1853-1890)

Van Gogh used the natural rheological properties of zinc white to achieve his impasto, rather than adding a separate medium. The 2014 rheological study showed that zinc white's high yield stress, approximately 30 times that of lead white, explains its specific use by Van Gogh to create thick impastos. He mixed lead white for covering power in underlayers and zinc white for bright surface accents where he wanted maximum texture.

J.M.W. Turner (1775-1851)

Turner used gumtions and megilps, the 19th-century gel mediums made from mastic resin, linseed oil, and lead compounds. These mediums gave his paint a gel-like consistency that could be manipulated for atmospheric effects and luminous textures. The downside was severe cracking in many of his paintings, which contemporary writers attributed to the excessive use of these mediums.

Frank Auerbach (1931-2024)

Auerbach took impasto to such extremes that his paintings have paint layers several inches deep. He did not use a commercial impasto medium. He achieved his surfaces by scraping off and reapplying paint over weeks and months, building up a surface that is less like a painting and more like a relief sculpture. The Tate holds a significant collection of his work.

Anselm Kiefer (b. 1945)

Kiefer's impasto operates at architectural scale, incorporating not just thick paint but lead, ash, straw, and shellac. His large paintings from the 1980s and 1990s use impasto as a metaphor for the accumulation and destruction of history. Kiefer's approach extends the concept of impasto medium to include non-traditional materials.

How to Choose and Use Impasto Medium

For oil painting, paste mediums are the most stable option. They consist of extender pigments in oil and do not significantly increase the fat content of the paint, which means they do not violate the fat over lean rule. Natural Pigments recommends applying paste mediums the same as oil colors, since they have a high pigment load. When applying thick layers, allow ample time to dry before repainting.

For acrylic painting, gel mediums and modeling pastes are the standard options. Gloss gel mediums are transparent and do not change the color of the paint. Matte mediums and modeling pastes contain white pigments that make the mixture more opaque and slightly lighter in color. Start with a small amount and add more until the desired consistency is reached.

A key consideration is the fat over lean rule. Adding oil-based mediums that do not contain solid particles increases the fat of the paint. Adding paste mediums that contain extender pigments does not significantly increase the fat, because the pigments maintain the pigment volume concentration. This is why paste mediums are recommended for impasto in oil painting: they allow you to build thick texture without creating a layer that is too fat for the layers beneath it.

Related Techniques

Impasto medium is directly related to impasto, the technique of applying paint thickly enough that brush or knife marks remain visible. The medium is the material that makes the technique practical. It connects to fat over lean because the choice of medium affects the paint's fat content and must be considered in the layering structure. It relates to glazing because some impasto mediums can also be used to create thick, translucent glazes. The technique also connects to scumbling, which uses dry, opaque paint for texture, though scumbling is thin while impasto is thick.

For more context, read our blog post on oil painting techniques, or explore our entry on impasto for the full history of the technique.

See Impasto Medium in Practice

The effects of impasto medium are visible in every museum with paintings from the 17th century onward. At the Rijksmuseum in Amsterdam, look at Rembrandt's The Jewish Bride and The Night Watch from an angle to see the raised impasto that his custom medium made possible. At the Musee d'Orsay in Paris, look at Van Gogh's paintings to see how zinc white's natural rheological properties created thick, textured surfaces without the need for added medium. At the Tate in London, look at Turner's paintings to see both the possibilities and the risks of gel mediums: luminous textures alongside severe cracking.

For contemporary practice, look at Anselm Kiefer's large paintings, which extend the concept of impasto medium to include non-traditional materials like lead, ash, and straw. The surfaces are part painting, part relief, part ruin, and they show how far the idea of thickening paint can be pushed. For more on the science of paint rheology, read our entries on impasto and fat over lean, or explore our post on texture in art.