The Science and Art of Preparing Oil Paint Glazes: A Modern Approach

A glaze is a thin, transparent layer of paint applied over a dry layer to modify its appearance without obscuring the details beneath. Glazes in oil paintings impart rich depth and subtle color shifts. They allow underlying layers to show through while changing hue or value. Yet, many artists struggle with selecting and preparing glaze layers that remain durable. This expanded essay synthesizes advanced insights on pigment behavior, refractive index (RI), and the best methods for preparing and applying glazing layers.

Understanding Light Interaction of Oil Paint Glazes

Glazes largely regulate how light travels through—or is obstructed by—the paint film. Light can be reflected, refracted, or absorbed. These interactions dictate whether a layer appears transparent or opaque, making it crucial to understand them when formulating glazes (National Gallery of Art, 1986, 12). By understanding these principles, artists can strategically choose pigments, mediums, and application techniques to control the transparency of their layers. This approach ensures they achieve the desired depth and luminosity in their work.

Refractive Index and Transparency or Opacity

Refractive index (RI) measures how much a material bends light as it passes through, indicating how fast light travels through a substance compared to air. Understanding RI is crucial for artists as it helps predict how pigments will interact with a binder, ultimately influencing the transparency and luminosity of a glaze. When light transitions from one medium to another of different RI, it changes direction—a phenomenon known as refraction. The greater the RI difference between pigment particles and the binder, the more light scattering (and hence opacity) occurs.

Figure 1: When light encounters the boundary of a material, part of it is reflected at the surface while the rest enters the medium, changing direction in a process known as refraction. The degree to which light bends is quantified as the refractive index (RI). A higher refractive index indicates a greater deviation of the light’s path upon entry.

Figure 2: As light interacts with the boundaries between oil and pigment particles in a paint film, some is reflected at the surface while the rest enters the pigment. The extent of this bending is determined by the refractive index (RI) of the pigment relative to the surrounding medium. When the RI of the pigment differs significantly from that of the oil, increased light scattering occurs, leading to greater opacity. Conversely, when the two indices are closely matched, more light passes through, enhancing transparency.

When the RI of a pigment closely matches that of linseed oil (approximately 1.48)—such as phthalocyanine blue, dioxazine violet, or perylene red—light passes through more easily, enhancing transparency. By contrast, when a pigment’s RI greatly exceeds that of the binder—such as titanium white (around 2.7)—more scattering occurs, producing a more opaque appearance. A closer RI match minimizes this effect, allowing for a smoother transmission of light and resulting in a clearer, more luminous glaze.

The Size of Particles Matter

Pigment particle size significantly influences light scattering and, therefore, transparency. Coarse particles—such as those found in certain historical pigments like azurite, malachite, or lapis lazuli—have fewer interfaces for light to bounce off, allowing more direct transmission and enhancing clarity (Eastlake, 1847, 112). Many historical pigments were left coarser because grinding techniques were more limited, resulting in colors with higher saturation and unique optical quality. In contrast, modern commercial pigments are typically ground to extremely fine sizes, creating numerous boundaries where light scatters, increasing opacity. This finer milling improves coverage but may reduce the transparency that traditional glazing techniques rely on.

When pigment particles are smaller than half the wavelength of visible light (approximately 200 nanometers), they no longer scatter light effectively and instead become transparent. This phenomenon explains why certain pigments, such as transparent iron oxides, offer rich, transparent color despite their high tinting strength. These finely milled pigments allow light to pass through with minimal scattering, making them excellent choices for glazing applications where clarity and depth are desired.

Additionally, the crystalline structure of a pigment affects how it interacts with light. Some pigments, such as ultramarine blue, have an open or porous lattice that promotes transparency, while others, like cadmium red, have denser, more light-blocking arrangements. These structural differences, combined with particle size, can make two pigments labeled “transparent” or “opaque” behave quite differently in practice (Wehlte, 1975, 76). To test transparency, artists can apply a thin glaze over a black-and-white swatch or layer it over a dry underpainting to observe how much light passes through and interacts with the underlying layers. Even traditionally opaque pigments can exhibit a degree of translucency when coarsely ground (Eastlake, 1847, 56).

How Pigment Concentration Affects Transparency

Achieving successful glazing requires understanding how pigment concentration influences transparency and opacity. This concept, called Pigment Volume Concentration (PVC), refers to the ratio of pigment to binder in a paint film. By controlling PVC, artists can ensure that light passes through the glaze effectively while maintaining film stability. A lower PVC means more binder relative to pigment, leading to a more transparent, glossy, and flexible film. A higher PVC results in more pigment and less binder, increasing opacity but potentially weakening the film if there isn’t enough binder to properly hold the pigment particles together. Maintaining an appropriate PVC in glazing ensures that light can pass through the paint layer, enhancing transparency while keeping the film stable and durable. When pigment particles are spaced apart, fewer scattering events occur, and more light penetrates the film. As a result, more reflected light returns from the substrate, making the paint appear transparent (Mayer, 1991, 45). Conversely, more light scattering can yield an opaque paint film.

Balancing PVC is crucial. If too much medium or oil is added to a glaze, the excess binder can lead to overly glossy surfaces, extended drying times, and potential yellowing over time. This instability can increase the risk of wrinkling or cracking as the layers cure. Maintaining an optimal pigment-to-binder ratio ensures a durable film that remains both transparent and structurally sound. Doerner warns about the perils of “over-oiling” and how it may cause slow drying, wrinkling, or eventual discoloration (Doerner, 1934, 97, 111). Maintaining the correct binder-to-pigment ratio ensures stability (Mayer, 1991, 40). These ideas align with older and modern research on how particle size and binder content affect paint film properties (Gettens and Stout, 1966, 87).

Choose Transparent Pigments for Oil Paint Glazes

Artists typically favor pigments rated as transparent or low-hiding for successful glazing. Examples include phthalocyanine blues, quinacridone reds, and transparent iron oxides. Their low RI or finely milled particle sizes reduce scattering (National Gallery of Art, 1986, 12). Earth colors such as raw sienna or burnt sienna often exhibit semi-transparency at moderate PVC, thanks to their distinct particle morphology (Wehlte, 1975, 76). Historical texts indicate that even so-called opaque pigments can display translucency when ground more coarsely (Eastlake, 1847, 56). Testing each pigment on a black-and-white swatch can help determine its glazing potential (Wehlte, 1975, 76).

How Substrates Influence Oil Paint Glazes

The texture and smoothness of the substrate can significantly influence how glazes appear. A rough or absorbent surface creates more micro-interfaces where light can scatter or be absorbed, potentially diminishing the clarity of a glaze. Conversely, a smooth surface allows the glaze to sit atop in a more uniform layer, enhancing light transmission and reflection. Historical precedents show that painters often used multiple layers of gesso or priming to achieve a smoother ground for luminous glazes (Eastlake, 1847, 66). Modern artists may use acrylic gesso or oil-based grounds to reduce surface porosity.

The hue and value of the substrate also affect glaze outcomes. A light-colored ground, such as a white or pale yellow underpainting, reflects more light back through the glaze, increasing its brightness and vibrancy. In contrast, a dark substrate absorbs more light, like a deep red or black imprimatura, resulting in a more subdued or intensified tone. For instance, a transparent blue glaze applied over a white ground appears luminous and saturated, whereas the same glaze over a dark brown surface creates a deeper, muted effect. Artists can use this knowledge to enhance or neutralize colors strategically in their compositions. A lighter, more reflective underlayer enhances brightness and perceived chroma by reflecting more light through the glaze. Conversely, a darker-toned ground absorbs more light, muting or deepening the glaze’s color. For example, applying a transparent blue glaze over a warm, mid-tone brown results in rich, subtle shifts, while the same glaze over a bright white panel produces a more vibrant, high-contrast effect. Artists can manipulate substrate color to control tonal interactions and maximize the effectiveness of glazing techniques (National Gallery of Art, 1986, 30).

How Substrates Influence Oil Paint Glazes

The surface on which a glaze is applied can significantly impact the final effect:

• Texture and absorbency: A rough or porous ground scatters and absorbs more light, potentially diminishing clarity. Conversely, a smooth surface allows the glaze to form an even film, enhancing luminosity.
• Hue and value: Historical precedents show that painters often used multiple layers of gesso or priming to achieve a smoother ground for luminous glazes (Eastlake, 1847, 66). A light-colored substrate reflects more light back through the glaze, brightening and intensifying color. Dark-colored grounds absorb more light, causing the same glaze to appear deeper or more muted (National Gallery of Art, 1986, 30). Painters can use this knowledge strategically to emphasize or subdue particular color passages.

Increasing Transparency of Oil Paint Glazes with Mediums

Mediums can raise the refractive index of the paint film, reducing light scattering at pigment-binder boundaries. Historically, natural resins like dammar were added for their ability to impart gloss and clarity, but they can yellow, darken, or crack over time (Natural Pigments, 2023). Modern alternatives—such as synthetic resins or alkyds—offer higher clarity, better flexibility, and faster drying, making them popular choices for glazing (Wehlte, 1975, 85; Gettens and Stout, 1966, 88).

Natural resins like dammar deliver an appealing, glossy finish and can raise the RI. Yet, evidence shows they oxidize over time, risking discoloration and brittleness (Natural Pigments, 2023). Because these resins tend to darken and crack with age, this outcome underscores the necessity for modern alternatives, such as synthetic resins or alkyds, which maintain greater clarity and flexibility (Wehlte, 1975, 85). Gettens and Stout (1966, 88) also note that some resinous binders may adversely affect long-term paint stability without proper methods.

According to paint technology references, Alkyd mediums typically have a refractive index (RI) of around 1.50–1.54, higher than raw linseed oil, which is approximately 1.48 (Mayer, 1991, 60). This increased RI can reduce the mismatch between the binder and many pigments, lowering light scattering for a more transparent film. Alkyds also tend to dry faster and stay more flexible than conventional linseed oil, making them popular for artists who desire a robust, quick-drying glaze layer.

Stand oil, a bodied linseed oil, also has a slightly elevated refractive index—often measured around 1.49—compared to raw linseed oil, which is 1.48 (Wehlte, 1975, 79). Though this increase is modest, it can still enhance the transparency of the glaze. However, stand oil is thick and may need to be diluted with solvent or combined with an alkyd to prevent excessive viscosity. It is best used sparingly, as adding too much can slow drying and produce overly glossy, tacky surfaces.

Minimizing Thickness for Effective Oil Paint Glazes

Thin layers are paramount for effective glazing. The more distance light travels through a pigmented layer, the more scattering and potential absorption occur. Many artists apply color from the tube and then wipe or rub it into the surface, preserving a balanced PVC rather than overloading it with oil (Mayer, 1991, 46).

Applying multiple thin coats in succession enables precise color control. Allowing each layer to dry completely—typically 24 to 48 hours, depending on the medium used—prevents unwanted mixing or adhesion issues, ensuring a stable and even glaze. Each layer must dry thoroughly before the next, preventing unintended mixing or lifting. This systematic approach was a hallmark of Old Master techniques, as noted by Doerner (1934, 94) and supported by modern conservation studies (National Gallery of Art, 1986, 24).

Pigment Selection and Film Strength

Excessive amounts of oil or solvent can undermine the cohesion and longevity of a glaze film. Over-reliance on solvent alone weakens adhesion, while too much oil can cause surface wrinkling, yellowing, and prolonged drying. Following the “fat-over-lean” principle—using progressively more flexible (oil-rich) layers over leaner ones—helps ensure stability (Mayer, 1991, 52). For practical application, artists can use a medium like linseed oil mixed with a small amount of stand oil in the upper layers while keeping the early layers lean with a higher solvent or alkyd medium ratios. A common approach is starting with a 3:1 ratio of solvent to oil in the initial layers and gradually increasing the oil content in subsequent layers to maintain flexibility.

Eastlake (1847, 89) noted that early oil painters often built up layers incrementally to reduce the risk of cracking. Over-oiling the final layers can trap oxygen, delay oxidation, and produce wrinkles that eventually harden, creating an uneven surface prone to damage (Doerner, 1934, 102). Test panels remain an excellent way to fine-tune pigment selection, binder ratio, and application thickness before committing to a final piece (Gettens and Stout, 1966, 89).

Putting It All Together

Producing luminous, stable glazes requires informed decisions about pigment choice, binder, film thickness, and substrate. By selecting naturally transparent or low-scattering pigments, using mediums that increase refractive index without compromising longevity, and building carefully layered films, artists can create stunning effects where underlying colors shine through.

Adhering to the “fat-over-lean” principle, avoiding excessive oil, and verifying the transparency of each pigment on a test surface can help achieve consistent, brilliant results. Experienced painters often utilize a test panel, a practice endorsed by both historical sources and modern technical research (Eastlake, 1847, 110; Gettens and Stout, 1966, 89). In this way, artists can harness glazing’s unique power to impart depth, vibrancy, and enduring beauty.

Bibliography

Balliana, Eleonora, Francesca Piqué, and Austin Nevin. 2014. “The Role of Particle Size and Binder in Paint Stability.” Studies in Conservation 59 (3): 45–52.
Summary: This article investigates how varying particle sizes and binders influence paint film durability, highlighting the importance of balanced formulations. Access Online.

Doerner, Max. The Materials of the Artist and Their Use in Painting. Translated by Eugen Neuhaus. New York: Harcourt, 1934.
Summary: This influential book discusses historical painting materials, including pigments and oils, and warns about the brittleness of resinous mediums. Doerner underscores the need to manage binder content judiciously.

Eastlake, Charles Lock. Materials for a History of Oil Painting. London: Longman, Brown, Green, and Longmans, 1847.
Summary: Eastlake’s treatise explores the origins and evolution of oil painting materials, revealing how early masters handled pigment preparation and layering. Access Online.

Mayer, Ralph. The Artist’s Handbook of Materials and Techniques. 5th ed. New York: Viking, 1991.
Summary: Mayer’s manual covers crucial aspects of paint composition, pigment properties, and oil-based mediums. He emphasizes maintaining correct binder-to-pigment ratios for a stable paint film. Access Online.

National Gallery of Art. Artists’ Pigments: A Handbook of Their History and Characteristics, Vol. 1. Edited by Robert L. Feller. Washington, D.C.: National Gallery of Art, 1986.
Summary: This foundational volume reviews the history, composition, and performance of key artists’ pigments, highlighting transparency factors and the role of particle size. Access Online.

O’Hanlon, George. “Resin Mediums: Dammar and Maroger.” Artist Materials, 2023.
Summary: This online article covers the chemistry and issues tied to traditional resin mediums, such as brittleness and yellowing, while suggesting modern alternatives. (Natural Pigments, 2023). Access Online.

Standeven, Harriet A. House Paints, 1900–1960: History and Use. Los Angeles: Getty Publications, 2011.
Summary: Standeven’s book examines the development of commercial paint formulations in the early 20th century, including shifts in binder chemistry that influenced durability. Buy Book.

Wehlte, Kurt. The Materials and Techniques of Painting. New York: Van Nostrand Reinhold, 1975.
Summary: Wehlte’s text surveys both classical and modern painting materials and practices. He provides detailed advice on glazing methods and on how binder formulations affect transparency. Buy Book.

Below is a representative table summarizing a selection of scholarly‐documented pigments, including extenders, whites, and colored pigments, with their measured refractive indices. The values are given as ranges or approximate averages. This data was compiled from research‐based sources on pigment optical properties.

Extender Pigments (White Pigments with RI < 2.0)

White Pigments (White Pigments with RI ≳ 2.0)

Colored Pigments (Pigments with RI < 2.0)

Colored Pigments (Pigments with RI > 2.0)

Note: Some pigment names (and their properties) may vary slightly with the source and preparation method, particularly in refractive index and color variation. The values above reflect measurements reported in scholarly and technical literature.

* Despite its high refractive index, this pigment remains transparent due to its fine particle size.