Pigment particle size is a fundamental factor that influences the performance and quality of paints used by artists. The size of these particles affects not only the aesthetic properties of the paint, such as opacity and gloss but also its long-term durability and resistance to weathering. Understanding the science behind pigment particle size enables artists to make more informed choices when selecting materials, thus allowing for greater control over their work. This article reviews key scientific findings and breaks down how different particle sizes affect critical paint properties, drawing from research in the field.
How Pigment Particle Size Influences Tinting Strength
Tinting strength refers to the ability of a pigment to alter the color of a paint when mixed with other pigments. According to the Kubelka-Munk theory of light scattering in turbid mediums, smaller pigment particles exhibit greater tinting strength due to their larger surface area, which allows more interaction with light. In essence, the more surface area available for light to scatter, the stronger the pigment's influence on the final color of the paint.
This phenomenon is particularly relevant in cases where subtle color mixing is required. Finer particles can significantly alter the hue of a paint mixture, making them ideal for detailed work or when achieving specific tones is critical. However, artists should also be aware that smaller particles may offer more control over tinting, as discussed in the following sections, they can also influence other aspects of paint behavior.
To achieve the highest scattering power, the pigment particle size should be less than one micron (above), if not below 0.5 micron. Notice, however, the scattering power of titanium dioxide as a function of particle size and wavelength shows that the smaller the particle, the greater the blue wavelength, giving it a bluish undertone.
Gloss and Film Appearance: Achieving Smoothness
Gloss in paint refers to the shiny, reflective finish that results from smooth surfaces. Smaller pigment particles play a vital role in achieving a glossy appearance because they enable the formation of smoother paint films. When particles are fine, they pack together more tightly and uniformly on the surface of the painted area, reducing irregularities that scatter light. As a result, light reflects more uniformly, creating what is known as specular reflection—a key factor in producing a high-gloss finish.
For artists seeking to create glossy surfaces, such as those often found in contemporary art or works requiring vibrant, shiny finishes, selecting paints with finer pigments is crucial. Coarser particles, on the other hand, tend to produce a matte effect, as the uneven surface scatters light in multiple directions, reducing overall reflection.
Weather Resistance and Lightfastness: The Downside of Small Particles
While smaller pigment particles offer several advantages in terms of color strength and gloss, they come with a trade-off in weather resistance and lightfastness. These two properties are critical for artists working on outdoor murals or pieces exposed to sunlight and environmental conditions over extended periods.
Small particles have a much larger surface area relative to their volume, which increases their exposure to environmental factors such as UV light, moisture, and pollutants. This increased interaction between each particle and the surrounding environment can lead to quicker degradation of the pigment. In practice, this means that paints with fine particles may fade or degrade more rapidly when exposed to light or harsh weather conditions.
Therefore, while smaller particles may be desirable for certain aesthetic qualities, they can lead to a reduction in the longevity of the artwork. Artists should consider this trade-off when selecting materials, especially for works that are intended to be displayed outdoors or in sunlight.
The effect of particle size on the tinting strength and undertone is perhaps best shown for titanium dioxide (above). As the particle size decreases, the tinting strength improves. However, the undertone also changes, with a move towards a bluish undertone being observed due to the increased scattering efficiency observed at the blue end of the spectrum for fine particles (as predicted by Weber’s formula). A trade-off exists between high tinting strength and an acceptable hue.
Opacity and Hiding Power: Covering the Surface
Opacity, also known as hiding power, refers to a pigment's ability to cover the underlying surface, preventing the background from showing through. As with other optical properties, smaller pigment particles enhance opacity because they scatter light more effectively. When light enters the paint film, the finely ground particles cause it to reflect more, preventing it from reaching the substrate and thus improving the paint’s coverage.
This is a significant advantage for artists working with transparent or semi-transparent surfaces where complete coverage is necessary. Using paints with finer pigment particles can result in fewer layers required to achieve the desired level of opacity, which is both time-saving and cost-effective.
Pigment Particle Size and Your Artwork
Understanding how pigment particle size influences the properties of paint is essential for any professional artist seeking to optimize their materials. Small particles enhance tinting strength and gloss, making them ideal for vibrant, shiny finishes and color mixing. However, the trade-off is reduced lightfastness and weather resistance due to the increased surface area that interacts with the environment. Balancing these factors is vital for artists to produce high-quality, long-lasting work.
When selecting paints, considering the size of pigment particles can help improve control over artistic effects, from the strength of color to the surface appearance and the durability of the finished piece.
Bibliography
Dunn, E. J., Kushner, M., & Baier, C. H. (1941). Effect of Particle Size and Shape on Paint Consistency. Industrial & Engineering Chemistry, 33(9), 1157-1163. National Lead Company. Retrieved from https://pubs.acs.org/doi/10.1021/ie50382a029.
Pigment Particle Size and Hiding Power. (2012). HORIBA Instruments, AN209. Retrieved from https://www.horiba.com/fileadmin/uploads/Scientific/Documents/Particle_Characterization/AN209_Pigment_Size.pdf.
Gueli, A. M., Bonfiglio, G., Pasquale, S., & Troja, S. O. (2016). Effect of Particle Size on Pigments Colour. Color Research and Application. Wiley Online Library. DOI: 10.1002/col.22062.
Why Particle Sizing? (2005). Paint & Coatings Industry Magazine. Malvern Instruments Ltd., Worcestershire, UK.
Salman, M. (2024). Effect of Particle Size Distribution on Performance of Color Dispersions in Paints Industry. University of Engineering and Technology, Lahore. DOI: 10.13140/RG.2.2.23975.56487. Retrieved from https://www.researchgate.net/publication/379039071_Effect_of_Particle_Size_Distribution_on_Performance_of_Color_Dispersions_in_Paints_Industry.
Summary of the Influence of Pigment Particle Size on Paint Properties
Coarse Pigments (75–100 µm)
Tinting Strength: Coarse pigments are characterized by reduced tinting strength due to their limited surface area relative to finer particles. The inefficient light scattering of coarse pigments necessitates higher pigment loading to achieve comparable color strength, making them less effective for tinting high-gloss or transparent finishes.
Opacity: While traditionally considered opaque, coarse pigments can be less efficient at scattering light compared to smaller particles. The larger particle size provides fewer surfaces to interact with light, leading to a less effective blocking of light. As a result, finer particles often achieve greater opacity at lower concentrations.
Lightfastness: Coarse pigments tend to exhibit better lightfastness, as their lower surface area reduces their exposure to light and chemical interactions. The reduced interactivity of larger particles generally enhances chemical resistance and weathering performance, contributing to more stable color retention in outdoor applications.
Gloss and Film Appearance: Coarse pigments create a rough paint film, reducing gloss and resulting in a matte to semi-matte finish. The larger particle size introduces surface irregularities that scatter light, making them suitable for traditional oil painting, decorative finishes, and industrial coatings where some texture is desired.
Medium Pigments (45–75 µm)
Tinting Strength: Medium-sized pigments have moderate tinting strength, with slightly improved light scattering compared to coarse particles. The increased surface area relative to coarser particles allows for better dispersion in the binder, resulting in more vibrant colors with less pigment loading.
Opacity: Medium-sized pigments provide better opacity than coarse pigments due to more efficient light scattering. The greater surface area of these particles allows for more effective blocking of light, leading to improved coverage and hiding power in paint films.
Lightfastness: Medium pigments benefit from good lightfastness, as their surface area is still relatively limited compared to finer particles. This reduces the extent of light and chemical interaction, promoting longer color retention and resistance to UV-induced degradation.
Gloss and Film Appearance: Medium-sized pigments offer a balance between texture and smoothness. Paints with these pigments exhibit a low to moderate gloss finish, as the particles create a moderately rough surface that scatters light to some extent. These pigments are versatile, being suitable for a wide range of traditional and modern paint applications.
Fine and Extra-Fine Pigments (10–45 µm)
Tinting Strength: Fine pigments demonstrate higher tinting strength, as their increased surface area enhances interaction with the medium and improves light scattering. This results in stronger, more saturated colors with less pigment required, making fine pigments more efficient for tinting applications.
Opacity: Fine pigments achieve greater opacity than larger particles, as their enhanced surface area allows for more effective light scattering. This property enables better hiding power, even in thinner paint films, making fine pigments ideal for applications requiring a balance between opacity and smoothness.
Lightfastness: Fine pigments generally exhibit improved lightfastness compared to larger particles. The increased surface area, while allowing more light interaction, is still balanced by the finer distribution within the paint film, which helps maintain color stability and chemical resistance over time.
Gloss and Film Appearance: Fine pigments contribute to smoother paint films with increased gloss compared to coarser classifications. The smaller particle size reduces surface roughness, allowing for better light reflection and higher gloss levels, making these pigments suitable for fine art techniques and semi-gloss finishes.
Ultra-Fine Pigments (1–10 µm)
Tinting Strength: Ultra-fine pigments exhibit the highest tinting strength, as their extremely high surface area maximizes interaction with the binder and enhances light scattering. This leads to intense, saturated colors even with very small quantities of pigment, making ultra-fine particles highly efficient for producing transparent, vivid layers.
Opacity: Despite their smaller size, ultra-fine pigments can achieve superior opacity due to their enhanced light scattering capabilities. The increased surface area enables better coverage with less pigment, offering both strong opacity and the potential for highly transparent effects depending on the application.
Lightfastness: Ultra-fine pigments generally show the best lightfastness among all classifications. The minimal particle size reduces the potential for localized chemical reactions, enhancing resistance to UV exposure, chemical interactions, and weathering. The smaller overall surface area of well-dispersed particles reduces degradation, promoting longer-lasting color and durability.
Gloss and Film Appearance: Ultra-fine pigments create a highly smooth, glossy paint film. Their small size allows for high packing density within the paint matrix, resulting in minimal surface texture and maximizing light reflection. They are ideal for high-gloss finishes, automotive coatings, and photorealistic art.
Frequently Asked Questions
What is the material that holds pigment particles together?
The material that holds pigment particles together is called the binder. The binder is essential in forming a cohesive film when the paint dries, ensuring the pigment adheres to the surface and stays in place. Common binders for artists' paints include oil, acrylic, and egg yolk, each offering different properties such as flexibility, durability, and drying time.
What is the shape of a pigment particle?
The shape of pigment particles can vary significantly depending on the type of pigment. They can be spherical, flake-like, or even irregularly shaped. The shape of the particles affects how they scatter light, which in turn influences properties such as gloss and opacity in the final paint application.
What is pigment composed of?
Pigments are typically composed of finely ground minerals, synthetic chemicals, or organic compounds. Each type of pigment has specific properties, such as lightfastness, opacity, and tinting strength, which are essential for various artistic and industrial applications.
What is the raw material for pigment?
The raw materials for pigments vary, but they are often derived from natural minerals, such as iron oxides for red and yellow pigments, or from synthetic chemicals like titanium dioxide, which is commonly used for white pigments. Organic sources, such as plants and insects, were also used for certain pigments, but today, organic pigments are usually derived from petroleum compounds.
