Understanding Extender Pigments: Enhancing Paint Properties for Artists
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Extender pigments like chalk, calcite, marble, silica, talc, and mica can play an important role in paint. These minerals, often considered secondary to primary pigments, are vital in modifying paint’s properties. Extender pigments affect aspects such as opacity, coverage, texture, and mechanical performance, which are essential for artists. This article reviews recent studies on extender pigments and their impact on paint properties, providing insights into best painting practices.

How Particle Size Influences Paint Properties

The particle size of extender pigments significantly influences a paint’s optical and mechanical properties. According to DeArmitt and Hancock’s study, finer particles generally enhance a paint’s opacity and hiding power, while coarser particles contribute to a textured finish. For example, silica improves tensile strength and flexibility with its harder and finer particles, making the paint more durable.

Similarly, when used in their coarser forms, chalk and marble increase the paint’s texture and add bulk, making them ideal for creating underpainting or impasto techniques. On the other hand, finer particles are better suited for smoother layers and provide improved adhesion, as noted by Mohiuddin in his study on limestone-based paints.

Enhancing Paint Rheology and Application

Rheology, the flow and consistency of paint, is another property affected by extender pigments. Talc and mica, known for their platy particle shapes, improve the paint flow, making it easier to spread. This property is particularly beneficial in oil paints, where smooth application is often desirable.

According to Harben’s analysis of ground nepheline syenite (a nepheline and alkali feldspar), mica, and silica in coatings, these minerals enhance rheology by reducing the viscosity of paint without compromising its color intensity. Artists using acrylics or oil mediums can benefit from adding these extenders to achieve more controlled brushstrokes, especially during detailed work.

Mechanical and Protective Qualities

Extender pigments also enhance the paint’s mechanical properties. As detailed in the work by DeArmitt and Hancock, minerals like talc and silica not only increase tensile strength but also improve flexibility. This flexibility is crucial for paints applied on surfaces that may expand or contract, such as wood or canvas.

Harben’s study emphasizes that feldspar minerals and similar extenders add resistance to abrasion, which is vital for works intended for high-traffic areas or outdoor murals. Paints formulated with these extenders tend to have better scrub and burnish resistance, making them suitable for interior and exterior applications.

Coverage and Opacity

Chalk, calcite, and marble are well-known for improving a paint’s coverage and hiding power. As described by Phillips, these minerals scatter light effectively, increasing a paint’s opacity without significantly altering its color. This property allows artists to achieve even coverage with fewer layers, making these extenders valuable in both traditional and contemporary painting techniques.

Chemical Stability and Longevity

The chemical stability of paint is critical for artists, especially when creating works meant to endure. Feldspar and silica provide excellent resistance to chemical reactions, making them ideal for use in paints exposed to harsh environments. Harben’s research notes that feldspar, for instance, resists yellowing over time, maintaining color vibrancy.

Talc and mica contribute to the paint’s resistance to moisture, reducing the risk of mold growth and flaking. These characteristics make them suitable for creating works in humid environments or murals in urban areas.

Practical Tips for Artists

Artists can leverage the benefits of extender pigments by considering their particle size and the desired properties of the paint:

  • For better coverage and opacity, use finer grades of chalk, calcite, or marble.
  • To achieve textured effects, opt for coarser grades of extenders like calcite and marble.
  • For enhanced flexibility and durability, incorporate silica and talc into oil or acrylic mediums.
  • For moisture resistance in outdoor applications, prioritize feldspar and mica as extenders.

Versatile Tool for Artists

Extender pigments offer artists a versatile tool to modify paint properties and achieve desired effects. Understanding how these minerals influence paint can enhance artistic techniques and help achieve specific results, from better coverage to increased durability. As research evolves, artists can refine their materials further, making informed choices about the extenders they incorporate into their work.

Extender Pigments: How to Enhance Your Paintings without Altering Color

Watch this episode of Artist Materials Advisor on extender pigments and additives that can extend paint without affecting its color. This session focuses on calcite (chalk), barite (baryte), and the additive organoclay. Learn the differences between extender pigments and additives, how to incorporate them into your paint, and the best type for your painting practice.

For more information on this topic, here are additional articles:

Oil Paint Extenders: Enhance Your Oil Painting with Texture and Transparency

What Are Extender Pigments and How to Use Them in Oil Paint

Extender Pigments Mentioned in this Article

Comprehensive Classification of Extender Pigment Particle Sizes

Very Coarse Pigments

  • Micrometer (µm) Range: >100 µm
  • U.S. Standard Sieve Size: Particles retained on No. 150 Mesh (106 µm)
  • Descriptive Term: Very Coarse
  • Visual Reference: Comparable to coarse sand or table salt crystals

Pigments of this size were common in historical periods before the advent of fine grinding tools. Artists like those from the medieval and early Renaissance periods would often use pigments with very coarse particles, producing heavily textured surfaces with noticeable granularity, which enhanced certain tactile effects and luminosity in their artworks.

Coarse Pigments

  • Micrometer (µm) Range: 75–100 µm
  • U.S. Standard Sieve Size: Particles retained on No. 200 Mesh (75 µm)
  • Descriptive Term: Coarse
  • Visual Reference: Comparable to granulated sugar

As grinding techniques improved, pigments began to be processed more finely, though coarse pigments remained in use for specific applications. Coarse pigments are associated with visible brushwork and thick impasto techniques, often employed by painters aiming to emphasize texture and bold color.

Medium Pigments

  • Micrometer (µm) Range: 45–75 µm
  • U.S. Standard Sieve Size: Particles retained on No. 325 Mesh (45 µm)
  • Descriptive Term: Moderately Fine
  • Visual Reference: Comparable to beach sand

Pigments in this range are characteristic of traditional paint formulations used from the Renaissance to the 19th century. They offer a balance between texture and smoothness, making them suitable for both fine detail and broader applications. In modern artistic practice, these are still favored by those who prefer some degree of texture in their paint.

Fine Pigments

  • Micrometer (µm) Range: 20–45 µm
  • U.S. Standard Sieve Size: Passing through No. 325 (45 µm) and retained on No. 635 (20 µm) 
  • Descriptive Term: Fine
  • Visual Reference: Comparable to flour or fine sand

Fine pigments are well-suited for techniques that require moderate texture, such as classical oil painting, fresco, and specific industrial coatings where some degree of opacity and visible brushstrokes are desired. The particle distribution in this size range contributes to a slight granularity, offering greater color opacity and enhanced surface coverage.

Extra-Fine Pigments

  • Micrometer (µm) Range: 10–20 µm
  • U.S. Standard Sieve Size: Passing through No. 635 (20 µm)
  • Descriptive Term: Extra-Fine
  • Visual Reference: Comparable to powdered sugar or baking flour

Extra-fine pigments are optimal for techniques that demand a high degree of smoothness and reduced texture, such as fine detail painting, precision coatings, and conservation work. Their size range allows for better pigment dispersion, facilitating thin layering and achieving more even, transparent color applications without visible grain.

Ultra-Fine Pigments

  • Micrometer (µm) Range: 1–10 µm
  • U.S. Standard Sieve Size: Not applicable; below practical sieve classification
  • Descriptive Term: Ultra-Fine
  • Visual Reference: Comparable to copier toner or airborne particles

Ultra-fine pigments are critical in high-performance applications, including photorealistic painting, digital printing, and advanced thin-film coatings. Their size enables exceptional light scattering and superior color uniformity, allowing artists to create highly transparent glazes, achieve fine details, and produce even color films with minimal surface texture. These properties make them indispensable for cutting-edge artistic techniques and specialized industrial processes.

Implications for Artists and Conservators

The above classifications provide artists, conservators, and manufacturers with a detailed understanding of pigment particle sizes in relation to both historical and contemporary usage. Coarser pigments, often associated with early painting practices, are ideal for those seeking to replicate historical textures or explore expressive brushwork. Medium and fine pigments, which offer a balance between texture and smoothness, are appropriate for a wide range of applications from classical to modern techniques. Very fine and ultra-fine pigments, on the other hand, offer unique opportunities for achieving smooth, even layers with enhanced color saturation and minimal texture, making them indispensable in contemporary fine art and industrial applications.

This classification system also highlights the importance of particle size in determining the aesthetic and functional properties of pigments. Larger particles, for instance, provide increased texture, which can affect the refractive qualities of the surface and give the paint a distinct tactile quality. In contrast, smaller particles allow for more even distribution of color, smoother surfaces, and greater transparency, which are critical in fine detailing, glazing techniques, and high-performance coatings.

We have included descriptions similar to those found in the Wentworth scale, which help artists visualize and better understand the relationship between particle size and its impact on the physical properties of pigments. This detailed classification system not only enhances artistic practice but also provides valuable insights for conservators tasked with preserving historical works created with pigments across these various size ranges.

Important Notice

While the classifications presented on this website reflect commonly accepted particle size ranges for pigments and extender pigments (fillers), it is essential to recognize that not all pigments are produced with such tightly controlled particle size distributions. Manufacturing processes, such as milling methods, raw material properties, and production techniques, may result in broader particle size distributions than those specified here. Therefore, the classifications provided here should be regarded as a guide intended for informational purposes only and may not accurately represent the characteristics of our pigments and extender pigments. Users are encouraged to consult specific product data on our product detail pages and conduct appropriate testing to ensure suitability for their intended applications.

Glossary of Terms

Binder

The liquid component in paint binds pigment particles together and helps them adhere to a surface once the paint dries. Binders contribute to the film formation, gloss, and overall durability of the paint. Common binders in artists' paints include oil, oil-alkyd, acrylic polymer, gum arabic, and egg yolk.

Coverage

The ability of a paint to coat a surface effectively. High coverage indicates that the paint can hide the surface beneath it, often requiring fewer coats to achieve an even and opaque finish.

Dispersion

The process by which pigment particles are evenly distributed within the binder or medium. Good dispersion is critical for achieving uniform color, gloss, and durability in the paint film.

Film Appearance

The visual characteristics of a dried paint layer include gloss, texture, and evenness. Film appearance is influenced by pigment particle size, binder type, and application technique.

Glazing

A painting technique that involves applying thin, transparent layers of color over a dried layer of paint. Glazing creates depth and luminosity in the paint film and is commonly used to enhance color vibrancy and achieve realistic effects.

Gloss

The degree of shininess or reflectivity of the paint film's surface. Gloss is typically measured on a scale from matte (low gloss) to high gloss, with particle size, binder composition, and surface texture influencing the gloss level.

Hiding Power

Also referred to as opacity, hiding power is the ability of a pigment to obscure the surface underneath it. High hiding power means that the paint effectively blocks light from passing through, resulting in a more opaque finish.

Lightfastness

The resistance of a pigment to fading or color change when exposed to light, particularly sunlight. Pigments with high lightfastness retain their original color and brightness over time, making them ideal for both interior and exterior applications.

Masstone

The appearance of a paint color when applied in its pure, undiluted form. Masstone is the color of the paint as seen in a thick, opaque layer and is distinct from the color’s appearance in a thin glaze or tint.

Matte

A finish characterized by a lack of shine or gloss. Matte paints have a rougher surface that scatters light, resulting in a more diffuse reflection. They are commonly used to reduce glare and create softer visual effects.

Opacity

The degree to which a paint film can block the underlying surface from view. Opacity is influenced by the particle size, refractive index, and dispersion of the pigment within the paint. High-opacity paints provide complete coverage, while low-opacity paints allow some visibility of the substrate or previous layers.

Packing Density

The extent to which pigment particles are closely packed within the paint film. Higher packing density reduces gaps between particles, leading to a smoother surface, improved gloss, and better overall film integrity.

Particle Size

The size of individual pigment particles, usually measured in micrometers (µm). Particle size influences a wide range of paint properties, including gloss, opacity, tinting strength, and texture. It is a fundamental factor in the classification of pigments, ranging from very coarse to ultra-fine.

Primer

A preparatory coat of paint applied to a surface before the final paint layers. Primers often contain larger, more opaque pigments to enhance adhesion, increase opacity, and provide a suitable base for subsequent layers.

Refractive Index

A measure of how much light is bent or refracted when passing through a material. The refractive index of a pigment influences its opacity and color intensity, affecting the overall appearance of the paint film.

Saturation

The intensity or purity of a color. Highly saturated colors appear vivid and intense, while less saturated colors can look more muted or pastel. Pigment dispersion and tinting strength directly impact color saturation.

Semi-Gloss

A paint finish that falls between matte and high gloss. Semi-gloss finishes have moderate reflectivity and are often used for surfaces that require both durability and a degree of sheen, such as trim, cabinets, and furniture.

Sheen

A qualitative term that refers to the level of luster or shininess in a paint film. Sheen is closely related to gloss but can encompass a wider range of visual characteristics, including the soft glow of eggshell or the subtle shine of satin finishes.

Tinting Strength

The ability of a pigment to influence or modify the color of another pigment when mixed together. High tinting strength indicates that a small amount of pigment can produce a strong color change in the mixture, while low tinting strength requires a larger quantity to achieve the same effect.

Transparency

The opposite of opacity, transparency refers to the ability of a paint film to allow light to pass through. Transparent paints are commonly used for glazing, layering, and creating luminous effects. The level of transparency is influenced by the particle size, shape, and refractive index of the pigments.

UV Stability

The resistance of a paint film to degradation caused by ultraviolet (UV) radiation. UV stability is critical for exterior applications, where prolonged exposure to sunlight can lead to fading, chalking, or breakdown of the paint film.

For more information about sieve sizes, please visit GIlson Company.

Bibliography

Ciullo, Peter. Industrial Minerals and Their Uses: A Handbook & Formulary. Noyes Publications, 1996.
This examines extender pigments like talc, marble, and chalk in coatings, detailing their impact on paint performance, such as durability and finish. Available here.

DeArmitt, Chris, and Michael Hancock. Particulate-Filled Polymer Composites. ResearchGate, 2003.
This book explores the mechanical effects of fillers like talc and silica on paint, emphasizing tensile strength, flexibility, and other mechanical properties​.

Harben, Peter W., and David Lobdell. Ground Feldspar: The Potential for Sales in North America. Geoscience Data Queensland, 2005. Available here.
Analyzes the application of feldspar, silica, mica, and talc in protective coatings, detailing their role in improving paint’s durability, resistance, and finish.

Mohiuddin, A.K. An extensive review of face powders: Functional uses and formulations. International Journal of Pharmacy and Pharmaceutical Science. Academia.edu. Available here.
Focuses on how extender pigments such as limestone, chalk, and marble affect paint adhesion, opacity, and rheology.

Phillips, K.A. Industrial Minerals in Arizona’s Paint Industry. Arizona Department of Mines and Mineral Resources Open-file Report 89-1. May, 1989. Available here.
This paper examines marble and talc, focusing on their use in paints to enhance coverage, hide power, and create texture.

Frequently Asked Questions

How do you make oil paint spread more?

To make oil paint spread more easily, you can use extenders such as calcium carbonate, barium sulfate, or kaolin. These substances, when added with additional oil, reduce paint viscosity, making it more workable and fluid. Additionally, using mediums like linseed oil, stand oil, or turpentine can further enhance paint spreadability, improving its flow and leveling.

What are the different types of extenders in paint?

Extenders in paint include substances like calcium carbonate, barium sulfate, kaolin, silica, talc, and mica. Each extender has unique properties that can modify paint characteristics, such as opacity, gloss, and drying time. For example, calcium carbonate increases paint body and opacity, while talc adds a soft, matte finish.