Physical properties of pigments

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1. Background

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When analyzing pigments and colorants, many artists predominantly focus on the chemical aspects, often relying on color indexes as their primary guide. However, this approach can be insufficient for accurately predicting how a pigment will manifest on the skin. To address this gap, we compiled insights from interviews with 32 top-earning pigmentation artists between 2020 and 2023. This research is part of an ongoing effort to deepen our understanding of pigment behavior. Additionally, to enrich our analysis, we sought the expertise of a chemist, a dermatologist, and a physics specialist focusing on optics.

Among the artists in our study, 21 are based in the European Union, eight in the UK, and three in the US. Our data analysis shows that many artists are not fully equipped with the necessary knowledge to make well-informed decisions regarding the selection, modification, or toning of pigments to meet their clients' specific needs. This article aims to shed light on the physical components of pigments, enhancing artists' understanding and ability to choose and apply pigments more effectively.

2. Physical Properties of Colorants

Chemical Properties vs. Physical Properties

Pigmentology is an intriguing field at the crossroads of chemistry and physics. To fully understand how colorants will manifest beneath the skin, it's insufficient to analyze them solely based on their chemical composition. A comprehensive understanding requires differentiating between particles, molecules, and atoms. Grasping the physical and chemical properties of colorants is crucial when exploring how pigments reflect light and the final color they present when healed under the skin.

Particle Size as a Physical Property

Particle size, a critical physical property, refers to the dimensions of pigment particles. These particles are typically aggregates or clusters of molecules. The size of these particles significantly influences how the pigment disperses in a medium and interacts with light. However, it's important to note that particle size does not modify the pigment's fundamental chemical structure.

Chemical Composition

Conversely, the molecular formula is a chemical property. It details the types and amounts of atoms comprising each pigment molecule. This formula is pivotal in determining the pigment's chemical behaviors and reactions with other substances. Yet, this formula remains distinct from physical properties like particle size.

Understanding Atoms, Molecules, and Particles

Delving deeper, pigment particles are assemblies of molecules. These molecules are themselves composed of atoms bonded together. Each atom has a nucleus — housing protons and neutrons — orbited by electrons.

Therefore, particle size and molecular formula fulfill different yet complementary functions. Particle size pertains to the pigment's physical attributes, influencing how it spreads and reflects light. In contrast, the molecular formula relates to chemical characteristics such as reactivity and stability. An intricate understanding of these aspects is vital for predicting how pigments will perform in various scenarios, including their appearance when healed under the skin, their interaction with light, and their long-term stability.

3. From Atoms to Pigment Drops

In the realm of pigmentology, a clear understanding of the structural hierarchy from atoms to pigment drops is essential. Artists with a foundational knowledge of chemistry and physics might sometimes conflate "particles" with "molecules," particularly when discussing elements like Carbon. To clarify, let's differentiate between four distinct categories within the structure of colorants.

Atomic Level. The most fundamental unit is the single atom, such as a Carbon atom. At this level, we're considering the individual building blocks of matter.
Molecular Level: When atoms combine stably with other elements, like hydrogen, we transition to the molecular level. Molecules are atoms bonded together, representing the smallest units of a chemical compound that maintain the compound's properties.
Particle Level. Atoms and molecules can join to form particles. These particles aren't merely a collection of atoms; they are often aggregates of molecules and can consist of thousands to tens of thousands of atoms, depending on their size.
Aggregate Level. Aggregates are assemblies of particles that can be loosely bound together or have very strong bonds. They can be substantially larger—multiple times up to tens of times—than individual particles. Perception of Light and Size

The human eye is generally sensitive to light wavelengths ranging from about 400 to 700 nanometers (nm), with different wavelengths perceived as different colors. However, discerning the size of an object—a separate matter—is not directly measured in nanometers. The smallest object the unaided human eye can typically detect is around 0.1 mm or 100,000 nanometers. This size is far greater than most nanoparticles or even aggregates.

Observing Pigment in a Medium

When we introduce a pigment into a medium like water, we generally observe large groups of aggregates suspended in the liquid carrier of the pigment. These aggregates are the visible manifestations of the pigment's color and are a testament to the intricate structure that determines the pigment's final appearance and behavior, from atoms to aggregates.

4. Particle Size and Micronization

In the specialized world of Permanent Makeup (PMU), understanding "particle size" and "micronization" is critical, though these concepts often don't receive the attention they deserve.

Understanding Particle Size

Particle size refers to the dimensions of individual pigment particles within a PMU formula, typically measured in micrometers (microns). This size significantly influences the pigment's color intensity, dispersion in the skin, and retention. Larger particles offer more vibrant colors but may not disperse as evenly and could be more prone to fading. Conversely, smaller particles provide a softer color, disperse more uniformly, and yield longer-lasting results.

The Micronization Process

Micronization is a pivotal process in pigment production, involving the grinding pigment particles into finer sizes using specialized machinery like ball mills or jet mills. The aim is to enhance the smoothness of application and color payoff on the skin. Micronization reduces the average diameter of solid material particles, often extending to the nanometer scale for more refined results.

Traditional vs. Modern Micronization Techniques

Traditional micronization methods rely on mechanical actions such as milling and grinding. For instance, steel spheres in a rotating drum crush particles to smaller sizes in industrial mills. Modern techniques, however, utilize supercritical fluids to control particle size more precisely through processes like RESS, SAS, and PGSS. These advanced methods offer greater control over particle size and consistency.

Relevance in Semi-Permanent Pigments

In semi-permanent pigments, micronization's role is paramount. Finer, more consistent particle sizes result in smoother application, uniform color distribution, and reduced blotchiness or uneven fading. Additionally, smaller particles can dissolve more readily, enhancing the pigment's efficacy and longevity under the skin.

Micronization from a Producer's Perspective

For producers, micronization is a balancing act between legality, cost-effectiveness, and ease of application. Legal Constraints. Producers must ensure their pigments comply with safety standards and legal requirements. Cost-Effectiveness. Finding efficient micronization methods that don't inflate costs is crucial for maintaining profitability. Ease of Implantation. Pigments should be easily implantable by artists, requiring minimal passes while preventing migration and blurry edges.

Particle Size and Opacity: Smaller particles increase opacity in the skin as larger gaps between bigger particles can make the pigment appear less opaque, necessitating more passes.

Consistency and Composition. Smaller particles often need more carrier fluids to maintain a usable consistency, which can dilute the colorant concentration and potentially affect the pigment's effectiveness and longevity once implanted.

Producers must navigate these complexities, akin to solving multiple puzzles simultaneously, to achieve the optimal balance between particle size, stability, ease of implantation, consistency, and cost.

5. Physical Properties of Carbon Black

Understanding particle size's impact on skin pigment color requires examining a well-known colorant, Carbon Black (CI 77266). Despite sharing the same CI code, Carbon Black can be one of three distinct substances: Black 6, Black 2, and Black 7, each with unique physical properties.

Channel Black (Black 6) - 90-100 nm

Channel Black, or Black 6, is traditionally produced from natural gas and typically has a particle size ranging from 90-100 nm. This older production method involves directing natural gas through a channel and subjecting it to incomplete combustion to create Carbon Black particles. Although less commonly used today due to the emergence of more efficient methods, Channel Black represents a significant historical method of Carbon Black production.

Furnace Black (Black 2) - 150-300 nm

Furnace Black, or Black 2, is primarily produced from heavy petroleum products with particle sizes between 150-300 nm. The production process involves burning petroleum products in a controlled-oxygen furnace, forming Carbon Black. This method is the most prevalent today, offering advantages such as large-scale production and precisely controlling particle size and other characteristics.

Thermal Black (Black 7) - around 500 nm

Thermal Black, also known as Black 7, is produced using ethylene gas and is notable for having the largest particle size among the three, approximately 500 nm. Rather than combustion in a furnace, ethylene gas is thermally decomposed at high temperatures in a controlled environment, forming Carbon Black particles. While it's a less common method, Thermal Black is particularly known for yielding Carbon Black with larger particle sizes.

Implications of Micronization and Particle Size

These various production methods and resultant types of Carbon Black highlight the critical role of micronization in controlling particle size, which significantly influences the color properties and applications of Carbon Black in semi-permanent makeup. A deeper understanding of these properties allows artists and producers to make more informed choices, ensuring the pigment's performance aligns with the desired aesthetic and longevity outcomes.

6. Understanding Color Perception

Human vision is limited to a narrow spectrum of the electromagnetic waves surrounding us. We're blind to many wavelengths in the universe, such as radio waves, microwaves, gamma rays, ultraviolet radiation, infrared radiation, X-rays, and terahertz waves.

Wavelengths and Visible Colors

Within the visible spectrum, different wavelengths correspond to different colors, measured in nanometers (nm). The general ranges for these colors are as follows:

Red: approximately 620-750 nm
Orange: approximately 590-620 nm
Yellow: approximately 570-590 nm
Green: approximately 495-570 nm
Light Blue: approximately 476-495 nm
Blue: approximately 450-476 nm
Violet: approximately 380-450 nm

The colors we perceive are dictated by the wavelengths of light that objects absorb and reflect. An object appears black if it absorbs all wavelengths and reflects none. Conversely, an object appears white if it reflects all wavelengths. For example, an object will appear blue to us if it absorbs all colors except blue.

The Role of Particle Size in Pigment Color

When discussing pigments, understanding that the size of the pigment particles significantly impacts how light interacts with them is crucial. This interaction affects how we perceive the color of these particles. In industries like cosmetics and tattooing, where precise color and its stability are paramount, the physical properties of pigment particles, particularly their size, play a crucial role. Larger particles might scatter light differently than smaller ones, influencing the vividness and tone of the color as perceived by the human eye.

7. Reflectance and Retroreflectance

Understanding the principles of reflectance and retroreflectance is crucial when analyzing pigments and their interaction with light, particularly in applications like semi-permanent makeup. Pigment particles' size and physical properties significantly influence how they reflect, absorb, or scatter light, ultimately affecting the color's appearance and brightness.

Reflectance

Reflectance is the measure of the proportion of light that is reflected away upon striking a material. When light encounters a surface, it can either be absorbed or reflected. The ratio of reflected light to the total incoming light is defined as the material's "reflectance." This property can vary with wavelength, causing different colors to be reflected more than others. It's the foundation of why we perceive objects as colored; surfaces selectively reflect specific wavelengths while absorbing the rest. For instance, a red apple appears red because it reflects red wavelengths and absorbs most others.

Solid objects, including pigment molecules, reflect light in various directions. This reflection differentiates them from their background, each reflecting light differently, resulting in distinct colors. In semi-permanent makeup, the pigment molecules' absorption and reflection of light, contrasting with the surrounding skin, make the desired color effects visible.

Retroreflectance

Retroreflectance is a specialized form of reflectance where light or other electromagnetic waves are directed back towards their source with minimal scattering. Unlike typical reflective surfaces, which scatter light indiscriminately, retroreflective materials return light towards its origin. This property is evident in safety clothing or road signs that shine brightly when illuminated by headlights, as they're designed to bounce light back towards its source, enhancing visibility.

While retro reflectance primarily relates to visibility rather than color perception, it can influence the apparent vividness or brightness of color under certain lighting conditions. Various organic and inorganic colorants used in the skin may exhibit differing levels of retroreflectance. The color's brightness partly depends on how much-retroreflected light reaches our eyes. Consequently, the contrast and visibility of pigments embedded in the skin depend on selective wavelength absorption and the particles' retroreflective properties.

8. Solving the “Mystery of Blue Brows”

Wavelength and Particle Size Interaction

The phenomenon of brows turning blue in semi-permanent makeup can be elucidated by understanding the interplay between reflectance, retroreflectance, and particle size. This phenomenon typically occurs when colorants other than Carbon Black (CI 77266) fade more rapidly, leaving behind the distinct interaction of Carbon Black with light in the skin. The particle size of Carbon Black variants significantly influences the intensity and color of the light reflected to our eyes.

Black 6 (Channel Black) - 90-100 nm

This variant, also known as Gas Black, effectively absorbs almost all wavelengths. However, due to its smaller particle size, it predominantly reflects high-intensity blue and violet light, leading to a perceived bluish tint in the brows.

Black 2 (Furnace Black) - 150-300 nm

Furnace Black or Oil Black tends to absorb red, orange, and yellow wavelengths more effectively and starts to reflect light in the green to violet spectrum. The resulting combined color perceived is often greenish or anthracite.

Black 7 (Thermal Black) - 500 nm

With its larger particle size, Thermal Black reflects a broader spectrum of colors, including red and green. This leads to a brownish appearance as the combined reflected colors merge.

Preventing the Bluish Hue

Selecting the appropriate type of Carbon Black is crucial to counteract the blue appearance. One common solution involves mixing Elemental Carbon (Black 7) with Black 6, reducing the pigment's tendency to reflect predominantly blue light. An alternative is Gamma-Black, a fusion of Carbon Black and mineral (inorganic) Iron Oxide (CI 77499). Pigments containing CI 77499 and CI 77266 are often designed to mitigate the bluish hue, providing a more neutral or natural color.

9. The Water Test

Identifying Carbon Black Variants
Determining the type of Carbon Black in your pigment is crucial for ensuring the desired color and behavior in semi-permanent makeup applications. A simple water test, while not a substitute for detailed chemical analysis, can quickly indicate the type of Carbon Black you're working with. Here's how to conduct the test and interpret the results:

Materials Needed

A small container of water
A small amount of the pigment in question

Procedure

Gently drop a small amount of the pigment into the water without stirring.
Observe the behavior of the pigment in the water.

Interpreting the Results

Black 6 (Channel Black)

Particle Size- 90-100 nm
Behavior. Disperses almost like a fine powder, subtly coloring the water.
Interpretation. The very small particle size of Black 6 causes it to disperse widely in water, lightly tinting it without forming clumps or sinking rapidly. This is indicative of the finer particle size typical of Channel Black.

Black 2 (Furnace Black)

Particle Size. 150-300 nm
Behavior. Acts like petroleum or oil in water, spreading out in all directions without forming distinct particles.
Interpretation. The semi-soluble nature and medium particle size of Black 2 cause it to spread smoothly in water, much like oil. This behavior is characteristic of Furnace Black, making it identifiable through this dispersion pattern.

Black 7 (Thermal Black)

Particle Size. Approximately 500 nm
Behavior. Tends to sink to the bottom of the container, behaving similarly to mineral particles.
Interpretation. The larger particle size and predominantly organic composition (99% elemental carbon) of Black 7 lead it to act more like a dense material in water, often settling to the bottom rather than dispersing. This behavior is typical of Thermal Black.

While this water test provides a quick and accessible method for distinguishing between the three types of Carbon Black, it's important to remember that it doesn't replace a comprehensive chemical analysis for precise identification and understanding of the pigment's long-term stability and behavior. However, it serves as a practical initial assessment for artists seeking to understand the physical properties of their pigments.

10. Conclusions

A holistic approach that considers chemical and physical properties is essential when evaluating pigments for semi-permanent makeup. One cannot draw reliable conclusions by examining only one aspect in isolation. Among the crucial physical attributes, particle size is a fundamental characteristic impacting the pigment's behavior and appearance. Measured in nanometers, particle size is meticulously refined through a process known as micronization.

Manufacturing Considerations

Manufacturers engaged in formulating new pigments must judiciously balance a multitude of factors. These include ensuring the optimal particle size for desired opacity and ease of skin implantation while adhering to stringent regulatory standards. The goal is to create pigments that meet legal requirements and deliver exceptional performance and aesthetic appeal.

Particle Size and Skin Interaction

Larger particles might implant more readily, particularly if their chemical composition is compatible with the skin's internal environment. However, a too-close match may predispose the pigment to undesired migration within the skin, highlighting the need for a delicate equilibrium.

Carbon Black - A Case Study

Carbon Black (CI 77266) exemplifies the impact of particle size variation within the same chemical identifier. It manifests in three distinct forms.

Black 6 (90-100 nm)
Black 2 (150-300 nm)
Black 7 (500+ nm)

This variation underlines the influence of particle size on reflectance and retroreflectance, consequently affecting how we perceive color. For instance, the smallest particles in Black 6 predominantly reflect blue light, elucidating the common issue of brows appearing bluish. To counteract this, artists may increase the proportion of larger elemental carbon particles in the mix or opt for Gamma Black, a fusion of Iron Oxide Black (CI 77499) and Carbon Black.

Quick Identification with Water Test

A practical water test can swiftly hint at the type of Carbon Black in a pigment. In this test, Black 6 diffuses like a fine powder, coloring the water uniformly. Black 2 spreads out like oil, without distinct particles, while Black 7, primarily elemental carbon, has larger particles settling to the bottom.

Therefore, a nuanced understanding of pigment's chemical and physical properties is indispensable for artists aiming for excellence in semi-permanent makeup.