Physcial properties of pigments

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2. Physical properties of colorants

Chemical Properties vs. Physical Properties

Pigmentology is a fascinating field where chemistry and physics intersect. To fully comprehend how colorants will appear under the skin, analyzing them solely based on their chemical composition falls short. It's essential to grasp the key differences between three terms: Particles, Molecules, and Atoms. When exploring how pigments reflect light and the final color they take on when healed under the skin, understanding both the physical and chemical properties of the colorants is crucial.

Particle Size as a Physical Property

Particle size refers to the physical dimensions of pigment particles and is a physical property. These particles are often aggregates or clusters of individual molecules. The size of these particles impacts various behaviors of the pigment, such as how well it disperses in a medium and how it interacts with light. However, particle size does not alter the basic chemical structure of the pigment.

Chemical Composition

In contrast, the molecular formula is a chemical property. It specifies the types and quantities of atoms that make up a single pigment molecule. This formula determines the chemical behaviors of the pigment, such as how it reacts with other substances, but it's independent of the physical aspects like particle size.

Atoms, Molecules, and Particles

To break it down further, particles of pigment colorants are assemblies of molecules. Molecules, in turn, are composed of atoms that are bonded together. Each atom consists of a nucleus, which contains protons and neutrons, surrounded by electrons.

So, particle size and molecular formula serve different but complementary roles. One focuses on the physical attributes, like how the pigment will spread and reflect light, while the other concentrates on the chemical characteristics, such as reactivity and stability. A nuanced understanding of these aspects is crucial for knowing how pigments will behave in various applications, including how they will look when healed under the skin, how they reflect light, and their long-term stability.

3. From Atoms to Pigment drops

A sequence of sizes from Atom to Molecule to Particle to Aggregate to Pigment drop on the left and an attractive woman on the right.

Artists with limited knowledge of chemistry and physics often confuse "particles" with "molecules," especially when discussing elements like Carbon. It's crucial to differentiate between four distinct categories when examining the structure of colorants.

The smallest unit is the single atom of Carbon (atomic level).
When an atom is combined in a stable way with other elements, such as hydrogen, we are dealing with the molecular level.
Atoms and molecules can join together in a stable form to create particles. These particles are not just 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.
Aggregates refer to 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.

The human eye is generally sensitive to light wavelengths ranging from about 400 to 700 nanometers (nm). Within this range, we perceive different wavelengths as different colors. However, the ability to 'see' - in terms of discerning the size of an object -is a separate matter and is not directly measured in nanometers.

When it comes to recognizing the size of an object, the smallest object the unaided human eye can detect is usually around 0.1 mm or 100,000 nanometers. This is far greater than the size of most nanoparticles or even aggregates.

When we drop a pigment into water, what we typically see are large groups of aggregates suspended in the pigment's liquid carrier.

4. Particle Size and Micronization

In the world of Permanent Makeup (PMU), the terms "particle size" and "micronization" play crucial roles, although they're often not given the attention they deserve.

Particle Size

Particle size refers to the dimensions of the individual pigment particles in a PMU formula. This size is usually measured in micrometers (or microns) and can vary widely among different types of pigments. The particle size has a direct impact on various attributes of the pigment, such as its color intensity, how well it disperses in the skin, and its retention.

Larger particles generally offer more vibrant colors but may not disperse as well in the skin. They may also be more prone to fading over time. On the other hand, smaller particles often provide a softer color, disperse more evenly, and are generally more stable, leading to longer-lasting results.

Micronization

Micronization is a crucial process in producing semi-permanent pigments, and it involves grinding pigment particles into finer sizes. This process uses specialized machinery like ball mills or jet mills. The primary goal is to achieve a smoother application and enhanced color payoff when the pigment is used on the skin.

Basically, micronization is the technique of reducing the average diameter of a solid material's particles. While the term usually refers to reducing average particle diameters to the micrometer range, it can also extend to further reduction to the nanometer scale.

Traditional vs. Modern Techniques

Traditional methods for micronization rely on mechanical means such as milling, grinding, and bashing. In a typical industrial mill, a cylindrical metallic drum contains steel spheres that crush the particles to smaller diameters as the drum rotates. These mechanical methods are based on friction and impact to reduce particle size.

On the other hand, modern techniques exploit the properties of supercritical fluids to manipulate solubility principles. These methods include the RESS process (Rapid Expansion of Supercritical Solutions), the SAS method (Supercritical Anti-Solvent), and the PGSS method (Particles from Gas Saturated Solutions). These techniques offer finer control over particle diameters, distribution, and consistency.

Relevance to Semi-Permanent Pigments

In the context of semi-permanent pigments, micronization is pivotal in influencing the pigment's behavior under the skin. The smaller and more consistent the particle size, the smoother the application. This results in a more uniform color distribution and reduces the risk of blotchiness or uneven fading over time.

Furthermore, micronized pigments can offer better stability and longevity. This is because smaller particles have higher dissolution rates, which can, in turn, increase the efficacy and staying power of the pigment under the skin.

Micronization and pigment business

Understanding Micronization from a Producer's Perspective in the Semi-Permanent Makeup Pigment Industry To grasp the significance of micronization, it's crucial to look at it from a producer's standpoint. Typically, producers aim to create pigments that meet several key criteria: legality, cost-effectiveness, and ease of application.

Legal Constraints. If the pigments don't meet legal requirements, then there's no scope for a viable business. Therefore, producers put considerable effort into ensuring that their products adhere to safety standards. At least on paper, everything has to be ok.

Cost-Effectiveness. The second factor is cost. To ensure profitability, producers are always looking for methods to produce high-enough quality pigments at a reasonable cost. Micronization plays an important part in achieving that.

Ease of Implantation. The third factor is the ease with which the pigment can be applied or implanted into the skin. Most artists are not professionals, and thus, the producer needs to create a pigment that is easy enough to implant with a reasonable number of passes and, at the same time, not "sinking in" too easily to prevent migration and blurry edges.

Role of Particle Size. This is where the concept of micronization comes into play. The particles have to be small enough so that they are more opaque in the skin. Spaces between larger particles make those less opaque, and thus, the artists have to implant more of them with a larger number of passes.

Consistency and Composition. Another consideration is the pigment's consistency. Smaller particle pigments often require more carrier fluids to maintain a usable consistency. This can dilute the concentration of the colorant, sometimes to as low as 10-15% of the overall composition, which can impact the pigment's effectiveness and longevity once implanted.

Thus, for producers, balancing these various factors can be akin to solving multiple Sudoku puzzles simultaneously. Each decision impacts another, requiring a finely tuned approach to achieve the optimal balance between particle size, stability, ease of implantation, consistency, and cost.

5. Physical Properties of Carbon Black

To understand how the physical properties of particle size affect the color of the pigment in the skin, let's examine a well-known example: Carbon Black (CI 77266). Although it shares the same CI code, this colorant can be one of three substances, each with distinct physical properties. These are: Black 6, Black 2, and Black 7. Let's delve into each of these.

Channel Black - Black 6 - 90-100 nm

Channel Black, also known as Black 6, falls under the color index CI 77266. It's usually made from natural gas and generally has a particle size ranging from 90-100 nm. This method, which uses natural gas as the primary feedstock, is one of the older ways to produce Carbon Black. The gas is directed through a channel and undergoes incomplete combustion, resulting in Carbon Black particles. This process is less commonly used today, mainly because more efficient methods have taken its place.

Furnace Black - Black 2 - 150-300 nm

The second version is Furnace Black, or Black 2. This type is mainly produced from heavy petroleum products and has particle sizes ranging from 150-300 nm. In this method, the petroleum products are burned in a furnace with limited oxygen, resulting in Carbon Black formation. This is the most widely used method for producing Carbon Black today. It offers advantages like large-scale production and precise control over particle size and other attributes.

Thermal Black - Black 7 - around 500 nm

The third variant is Thermal Black, also known as Black 7. It is produced using ethylene gas as the feedstock and has the largest particle size among the three, roughly around 500 nm. In this method, ethylene gas is thermally decomposed at high temperatures. Unlike the furnace method, this process doesn't require a furnace for combustion. Instead, the ethylene gas is exposed to high heat in a controlled setting, leading to its breakdown and forming Carbon Black particles. Though less commonly used, this method is notable for yielding Carbon Black with larger particle sizes.

These different methods and versions underscore the importance of micronization in controlling particle size. This, in turn, influences the color properties and applications of Carbon Black.

6. How we see colors

The human eye is limited in the types of electromagnetic waves it can perceive, meaning many wavelengths around us go undetected by our senses. These include radio waves, microwaves, gamma rays, ultraviolet radiation, and infrared radiation, to name a few. Other examples of waves humans cannot visually detect include X-rays and terahertz waves.

Wavelengths in nanometers

Within the range that is visible to the human eye, different wavelengths correspond to different colors. These wavelengths are measured in nanometers (nm) and generally fall within the following ranges.

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 depend on which wavelengths of light are absorbed by an object and which are reflected back to our eyes. For instance, an object appears black when it absorbs all wavelengths of light and reflects none back to us. Conversely, an object appears white when it reflects all wavelengths of light. If an object absorbs all colors except blue, then it will appear blue to our eyes.

So, when discussing pigments, it's crucial to understand that the size of the pigment particles can impact how light interacts with them. This, in turn, affects how we perceive the color of these particles, which is especially relevant in industries like cosmetics and tattooing, where color and its stability are critical.

7. Reflectance and retroreflectance

Both these principles are deeply related to color perception. In the context of pigments and particle sizes, understanding how these properties affect reflectance can be critical. Smaller or larger particles will interact with light differently, changing how much light is reflected, absorbed, or even scattered. This can ultimately influence the color and its brightness, which is crucial in applications like semi-permanent makeup.

Reflectance

Reflectance measures how much light that falls upon a material is reflected away from it. When light hits a surface, some of it will be absorbed, and some will be reflected. The ratio of the reflected light to the incoming light is termed the "reflectance" of the material. Reflectance can be wavelength-dependent, meaning different wavelengths (colors) can be reflected differently. This is why we see objects as having color; their surfaces reflect certain wavelengths while absorbing others. Reflectance determines what color we see based on which wavelengths are reflected back to our eyes. For instance, a red apple reflects red wavelengths while absorbing most of the others, making it appear red to us.

Almost all solid objects, including people, animals, houses, and pigment molecules, are "reflective." When light hits them, some of it gets absorbed, and some is reflected in various directions. This makes the object "appear" to us separate from its background, which reflects light differently, thus appearing in different colors. This is precisely what happens with pigment molecules in the skin. The fact that they absorb and reflect light differently than the surrounding skin makes the pigment in the form of "pixels," "hairstroke lines," or "shading" in brows visible at all.

Retroreflectance

Retroreflectance is a specific type of reflectance where light or other electromagnetic waves are returned in the direction they came from with minimal scattering. This is unlike regular reflective surfaces, which scatter light in multiple directions. You might be familiar with retroreflective materials from "safety" clothing or road signs, which appear very bright when illuminated by a light source like car headlights. These materials are designed to bounce the light back towards the source, making them highly visible.

Retroreflectance is more about visibility rather than color perception, but it can also impact how vivid or bright color appears under specific lighting conditions. When it comes to different organic and inorganic colorants placed into the skin, they can exhibit varying levels of retroreflectance. The brightness of the color correlates with the retroreflectiveness that is directed back to our eyes. Therefore, the contrast and visibility of the pigment inside the skin are not only functions of selective wavelength absorption but also the retroreflectiveness of the pigment particles.

8. The “Mystery of Blue Brows” Solved

Framing Wavelength Against Particle Size

With our understanding of reflectance, retroreflectance, and particle size, we can now answer the puzzling question of "why brows turn blue" straightforwardly. Specifically, if other colorants in the pigment fade from the skin more quickly than Carbon Black (CI 77266)—which they do—then what remains is how the Carbon Black interacts with light within the skin. Considering the information from previous sections, we get a clear comparison. What we need to remember is that the intensity of the light waves reflected back to our eyes is influenced by the particle sizes reflecting them; larger particle sizes reflect light more clearly.

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

It absorbs almost all wavelengths effectively. Given that the intensity of the blue and violet light is the highest, these are the only light waves that get reflected back to us. As a result, the combined color we perceive is bluish.

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

It effectively absorbs red, orange, and yellow wavelengths and starts to reflect back wavelengths from green, light blue, blue, and violet. Consequently, the combined color we see appears greenish or anthracite.

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

Due to its large particle size, it absorbs much less and reflects back a wide spectrum of colors, including red and green. The combination of these reflected colors appears brownish to our eyes.

The solution to the Bluish Color

The key to preventing the bluish appearance is the choice of Carbon Black used in the pigment. A common solution is to mix Elemental Carbon (Black 7) with the pigment that contains Black 6. This combination reduces the tendency of the pigment to reflect blue light, thereby neutralizing the bluish hue. An alternative option is to use Gamma-Black, a specialized type of black pigment. In Gamma-Black, Carbon Black molecules are fused with mineral (inorganic) Iron Oxide molecules (CI 77499). When you're choosing a pigment, you'll often see both color indexes listed on the label (CI 77499 and CI 77266), indicating the inclusion of both types of black.

9. How to Identify Your "Carbon Black”

Identifying the type of Carbon Black in your pigment is a critical step. While a "water test" won't give precise data about the pigment's long-term stability, it does offer a quick and simple way to differentiate between Black 6, Black 2, and Black 7. To perform the test, drop a small amount of pigment into a water container without stirring it. Here's how to interpret the results:

Black 6

This type of black has very small particle sizes, which means it disperses in water almost like a powder, lightly coloring the water.

Black 2

This is semi-soluble in water and is easy to identify as it acts much like a drop of petroleum or oil in the water. It spreads out in all directions, and you won't see any individual particles. This behavior is a giveaway that you're dealing with Black 2.

Black 7

This variant has larger particle sizes and often sinks to the bottom, similar to how minerals behave. It's primarily made of 99% elemental carbon, which is organic, so it makes sense that it would act this way in water.

10. Conclusions

Assessing pigments requires consideration of both chemical and physical properties. No definitive conclusions can be drawn from solely focusing on either aspect. One of the key physical characteristics to consider is "particle size," which is measured in nanometers. Particle size is determined through a process called micronization. When formulating new pigments, manufacturers must balance various factors: particle size, compliance with regulations, opacity on the skin, and ease of implantation.

Larger particles may implant more easily if their chemical structure closely aligns with the skin's internal environment. However, if the match is too close, this can lead to the risk of easy migration within the skin.

A prime example of variations within the same chemical identifier (CI number) is Carbon Black (CI 77266). It comes in three different particle sizes: Black 6 with 90-100nm, Black 2 with 150-300nm, and Black 7 with 500+nm. Through the example of Carbon Black, we gain insights into how reflectance and retroreflectance impact color perception.

Understanding that the smallest particle size—found in Black 6—absorbs all wavelengths except blue, which it reflects back, helps to solve the mystery of why brows colored with Black 6 may appear bluish. The remedy for blue brows involves increasing the proportion of elemental carbon particles in the pigment mix or using Gamma Black, which combines Iron Oxide Black (77499) and Carbon Black.

To quickly determine the type of Carbon Black in a pigment labeled with CI 77266, a simple water test can be conducted. Black 6 will appear like a thin powder, diffusing color throughout the water. Black 2 will resemble petroleum oil, spreading without visible particles. Black 7, which is mainly elemental carbon, will have large particles that tend to sink, behaving more like an organic mineral.