Physical Properties of Pigments

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1. Background

In pigment and colorant analysis, many artists primarily focus on chemical properties, using color indexes as a reference. However, this method often falls short in predicting how a pigment will look on the skin. This article gathered insights from 32 top-performing pigmentation artists based on interviews conducted from 2020 to 2023, with ongoing research into these topics. Additionally, we sought input from a chemist, a dermatologist, and a physics specialist with expertise in optics to enrich our understanding.

Among the participating artists, 21 are from the European Union, eight from the UK, and three from the US. Our data analysis revealed that many artists lack crucial information to choose, adjust, or optimally tone pigments for their clients in different situations.

2. Physical Properties of Colorants

Pigmentology, where chemistry and physics meet, requires more than chemical analysis to understand how colorants appear under the skin. It's essential to distinguish between particles, molecules, and atoms. The physical and chemical properties of colorants are crucial in determining how pigments reflect light and their final appearance when healed under the skin.

Particle Size as a Physical Property

Particle size, the physical dimensions of pigment particles, is a significant physical property. These particles, typically aggregates of molecules, affect the pigment's behavior, like dispersion in a medium and interaction with light. However, particle size doesn't change the pigment's chemical structure.

Chemical Composition

In contrast, the molecular formula, a chemical property, defines the types and quantities of atoms in a pigment molecule. This formula influences the pigment's chemical behaviors, such as reactions with other substances, but is separate from physical properties like particle size.

Atoms, Molecules, and Particles

Pigment particles are collections of molecules, which are themselves composed of bonded atoms. Each atom has a nucleus with protons and neutrons surrounded by electrons.

Particle size and molecular formula play distinct yet complementary roles. Particle size relates to physical attributes, affecting how the pigment spreads and reflects light. The molecular formula focuses on chemical characteristics like reactivity and stability.

Understanding these aspects is key to predicting pigment behavior in various applications, including their healed appearance, light reflection, and long-term stability.

3. From Atoms to Pigment Drops

Understanding the structure of colorants requires differentiating between atoms, molecules, particles, and aggregates, particularly for artists with limited chemistry and physics knowledge.

Atomic Level: The smallest unit is a single atom, like a carbon atom.
Molecular Level: When an atom combines stably with other elements (e.g., hydrogen), it forms a molecule.
Particle Formation: Atoms and molecules can join to form particles, more than just a collection of atoms. They're often aggregates of molecules containing thousands to tens of thousands of atoms, varying in size.

Aggregates are assemblies of particles, either loosely bound or strongly bonded together. Aggregates can be substantially larger than individual particles, multiple times up to tens of times their size.

The human eye is sensitive to light wavelengths between approximately 400 to 700 nanometers (nm), perceiving different wavelengths as different colors. However, the ability to discern the size of an object is separate and not measured in nanometers. Typically, the smallest object the unaided human eye can detect is around 0.1 mm or 100,000 nanometers, significantly larger than most nanoparticles or aggregates.

In pigment applications, such as dropping pigment into water, we often see large groups of aggregates suspended in the liquid carrier of the pigment.

4. Particle Size and Micronization

In Permanent Makeup (PMU), "particle size" and "micronization" are crucial terms, but they often don't receive the attention they deserve.

Particle Size

Particle size in PMU pigments refers to the dimensions of individual pigment particles measured in micrometers (microns). This size varies among pigments and affects color intensity, dispersion in the skin, and retention. Larger particles offer more vibrant colors but may not disperse as well and fade over time. Conversely, smaller particles provide a softer color, disperse more evenly, and have better stability, leading to longer-lasting results.

Micronization

Micronization, crucial in semi-permanent pigment production, involves grinding particles into finer sizes using machinery like ball mills or jet mills. This process aims to achieve smoother application and enhanced color payoff on the skin. Micronization refers to reducing particle diameters to the micrometer or even nanometer scale.

Traditional vs. Modern Techniques

Traditional micronization methods include mechanical milling and grinding, where particles are crushed to smaller sizes via friction and impact. Modern techniques, like the RESS, SAS, and PGSS methods, use supercritical fluids for finer control over particle sizes and consistency.

Relevance to Semi-Permanent Pigments

Micronization is key in semi-permanent pigments, influencing the pigment's behavior under the skin. Smaller, consistent particles allow for smoother application, uniform color distribution, and reduced risk of uneven fading. Additionally, micronized pigments offer better stability and longevity due to higher dissolution rates.

Micronization from a Producer's Perspective

Producers of semi-permanent makeup pigments focus on legality, cost-effectiveness, and ease of application. Legal constraints ensure safety compliance, while cost considerations aim to produce quality pigments affordably. Ease of implantation is vital, as most artists aren't professionals, requiring pigments that implant easily without causing migration or blurry edges.

Role of Particle Size and Composition

Micronization is critical here. Smaller particles increase opacity in the skin, reducing the need for multiple passes. However, smaller particle pigments often need more carrier fluids for consistency, diluting colorant concentration and possibly affecting effectiveness and longevity once implanted.

Balancing these factors is complex, akin to solving multiple Sudoku puzzles simultaneously. Producers must finely tune particle size, stability, implantation ease, consistency, and cost to achieve the optimal pigment formula.

5. Physical Properties of Carbon Black

Understanding how particle size affects pigment color in the skin can be illustrated by examining Carbon Black (CI 77266). Despite sharing the same CI code, Carbon Black can be one of three substances with unique physical properties: Black 6, Black 2, and Black 7.

Channel Black - Black 6 - 90-100 nm

Channel Black, or Black 6, belongs to the color index CI 77266. It is typically produced from natural gas and has a particle size of 90-100 nm. This older production method involves directing natural gas through a channel, undergoing incomplete combustion to create Carbon Black particles. Today, this process is less common, overtaken by more efficient methods.

Furnace Black - Black 2 - 150-300 nm

The second type, Furnace Black or Black 2, is primarily made from heavy petroleum products, with particle sizes ranging from 150-300 nm. In this method, the petroleum products burn in a furnace with limited oxygen to form Carbon Black. It's the most common method for Carbon Black production today, offering advantages like large-scale production and precise control over particle size and other properties.

Thermal Black - Black 7 - around 500 nm

The third variant, Thermal Black or Black 7, uses ethylene gas as feedstock and has the largest particle size, about 500 nm. Ethylene gas is thermally decomposed at high temperatures without a furnace. This method, though less common, is known for producing Carbon Black with larger particle sizes.

These various production methods highlight the role of micronization in controlling particle size, which significantly influences Carbon Black's color properties and applications.

6. How We See Colors

The human eye can only detect a limited range of the electromagnetic spectrum, leaving many wavelengths invisible to us. These include radio waves, microwaves, gamma rays, ultraviolet radiation, infrared radiation, X-rays, and terahertz waves.

Wavelengths in Nanometers

The visible spectrum for humans includes wavelengths measured in nanometers (nm), each corresponding to different colors.

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 see depend on the wavelengths of light that an object absorbs and reflects. An object looks black if it absorbs all wavelengths and reflects none, while it appears white if it reflects all wavelengths. For example, an object appears blue if it absorbs all colors except blue.

When it comes to pigments, the size of the pigment particles influences how they interact with light, thereby affecting our perception of their color. This is particularly important in cosmetics and tattooing, where the exact color and consistency are vital.

7. Reflectance and Retroreflectance

Reflectance and retroreflectance are key concepts in understanding color perception, particularly relevant in the context of pigments and particle sizes in semi-permanent makeup.

Reflectance

Reflectance measures how much light hitting a material is reflected away. When light contacts a surface, some is absorbed, and some is reflected. The reflectance ratio is the proportion of reflected light to incoming light. Reflectance can vary with different wavelengths, so objects have color; they reflect certain wavelengths while absorbing others. For example, a red apple looks red because it reflects red light and absorbs other colors.

Solid objects, including pigment molecules, reflect light. This reflection allows us to distinguish objects from their backgrounds, as they reflect light differently. In semi-permanent makeup, the pigment molecules in the skin absorb and reflect light differently than the surrounding skin, making the pigment visible as pixels, hairstroke lines, or shading in brows.

Retroreflectance

Retroreflectance is a special type of reflectance where light is reflected in the direction it came from with minimal scattering, unlike regular reflective surfaces that scatter light in various directions. Common examples include safety clothing and road signs, which appear bright when illuminated by a light source like car headlights. These materials are designed to reflect light toward the source for high visibility.

While retro reflectance primarily affects visibility, it can also influence how vivid or bright color appears under certain lighting conditions. Different organic and inorganic colorants in the skin can exhibit varying levels of retroreflectance. The brightness of a color is partly due to the retroreflectiveness directed back to our eyes. Thus, the contrast and visibility of pigment in the skin depend not only on selective wavelength absorption but also on the retroreflectiveness of the pigment particles.

8. The “Mystery of Pigment turning Blue Solved

Understanding the relationship between wavelength, reflectance, retro reflectance, and particle size helps us solve the puzzle of "why pigment turns blue." If other colorants in a pigment fade faster than Carbon Black (CI 77266) - which they often do - what remains is how Carbon Black interacts with light in the skin. Remember, the intensity of reflected light waves is influenced by the particle sizes reflecting them, with larger particles reflecting light more distinctly.

Black 6 (Channel Black or "Gas Black"), particle size 90-100 nm

It absorbs almost all wavelengths effectively. However, the intensity of blue and violet light is the highest, leading these wavelengths to be reflected to us, resulting in a perceived bluish color.

Black 2 (Furnace Black or "Oil Black"), particle size 150-300 nm

This type absorbs red, orange, and yellow wavelengths, reflecting back wavelengths from green to violet. The combined color appears greenish or anthracite.

Black 7 (Thermal Black or "Elemental Carbon"), particle size around 500 nm

With its large particle size, it absorbs less and reflects a broader spectrum, including red and green. The mix of these reflected colors appears brownish.

The solution to the Bluish Color

Choosing Carbon Black in the pigment is crucial to prevent a bluish appearance. A common approach is mixing Elemental Carbon (Black 7) with Black 6, reducing the tendency to reflect blue light and neutralizing the bluish hue. Another option is using Gamma-Black, combining Carbon Black molecules with mineral (inorganic) Iron Oxide molecules (CI 77499). Pigments often list color indexes (CI 77499 and CI 77266) on their labels, indicating the inclusion of black types.

9. How to Identify Your "Carbon Black”

Determining the type of Carbon Black in your pigment is crucial. Although a "water test" won't provide detailed data about the pigment's long-term stability, it can help distinguish between Black 6, Black 2, and Black 7. Drop a small amount of pigment into a water container without stirring for the test. Here's how to interpret what you observe.

Black 6

With very small particle sizes, Black 6 disperses in water like a powder, subtly coloring the water. This light dispersion is indicative of Black 6's fine particles.

Black 2

Black 2 is semi-soluble in water and behaves like a drop of petroleum or oil, spreading out without showing individual particles. This distinct spreading pattern clearly signifies that you're dealing with Black 2.

Black 7

Black 7, characterized by larger particle sizes, tends to sink to the bottom, resembling the behavior of minerals. Composed mainly of 99% elemental carbon, which is organic, its tendency to sink is a hallmark of Black 7 in water.

This simple water test provides a straightforward method to identify the type of Carbon Black in your pigment based on how it interacts with water.

10. Conclusions

Evaluating pigments necessitates examining chemical and physical properties, as focusing solely on one aspect doesn't provide a complete picture. A key physical property is "particle size," measured in nanometers and achieved through micronization. Manufacturers must balance particle size, regulatory compliance, skin opacity, and ease of implantation in creating new pigments.

Larger particles might implant more easily if they chemically align with the skin's internal environment. However, too close a match can lead to migration within the skin.

Carbon Black (CI 77266) illustrates how the same CI number can have varying particle sizes: Black 6 at 90-100nm, Black 2 at 150-300nm, and Black 7 at 500+nm. This example helps us understand how reflectance and retroreflectance affect color perception.

Realizing that Black 6's small particle size reflects only blue light explains why brows colored with it may appear bluish. A solution is to increase elemental carbon particles in the mix or use Gamma Black, combining Iron Oxide Black (77499) with Carbon Black.

A water test can identify the type of Carbon Black in a pigment with CI 77266. Black 6 disperses like a powder, coloring the water lightly. Black 2 spreads like petroleum oil without visible particles. Black 7, mostly elemental carbon, has larger particles that sink, behaving like an organic mineral.

Physical Properties of Pigments

"Understanding the physical properties of colorants is crucial in understanding how pigments act in the skin. You can't fully grasp colorants by looking only at their chemical makeup. Both physical and chemical properties play a vital role in the process."

1. Background

2. Physical Properties of Colorants