Why do brows turn “blue”?

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1. Background

Many artists have inquired why pigments in the skin turn blue and how to prevent this outcome. These questions are often easier to understand after the fact but can be counterintuitive when trying to predict. For this article, we compiled information from interviews with 46 PMU artists and from research conducted by the Powderbrows.com Research Center between 2019 and 2023. Several of these research efforts are ongoing. Of the interviewed artists, 32 are from EU countries, nine from the UK, and five from the US.

We also consulted two chemists, a dermatologist, and a physics expert specializing in optometry to organize the results and make them more accessible. While there were no significant differences in the artists' agreement with the reviewed findings, it's important to note that even seasoned professionals sometimes hold incomplete or sub-optimal views about the causes of certain skin pigment phenomena. In this article, we aim to provide a clear and thorough explanation of why pigments appear bluish on the skin and how to avoid it.

2. Understanding “Seeing” Colors

There are no inherent colors

As we perceive them, colors are not intrinsic qualities of objects but are created by the interaction of light—composed of electromagnetic waves—with objects and how our brains interpret them.

Interactions of Light with Objects

Light can be absorbed, reflected, scattered, transmitted, or transformed into thermal energy when it strikes an object. The colors we see are mainly due to the reflected and scattered light. An object absorbs certain wavelengths and reflects others, reaching our eyes and influencing our perception of color. This is why objects that absorb more light, appearing darker, can also feel warmer; they convert more light into heat.

The Role of the Retina in Color Perception

Our retinas have specialized cells called photoreceptors - cones for color and rods for low-light conditions. These cells convert light into electrical signals that the brain interprets to produce the sensation of color. This process is influenced by biology and cognitive factors, as our past experiences and the context of our environment can influence how we perceive color.

Color Constancy - Brain’s Interpretation

Consider color constancy, where we perceive the color of an object as constant despite changes in illumination. Our visual system allows us to recognize colors reliably, regardless of lighting. This understanding is relevant when discussing semi-permanent makeup, such as Powder Brows or Hairstrokes. The pigments implanted into the skin don’t inherently turn blue. Instead, the perception of a blue tint in brow color arises from the interplay of light's reflection, the skin's unique absorption properties, and our brain's interpretation.

Limited Detection Range of the Human Eye

The human eye can detect only a limited range of electromagnetic waves, with many other wavelengths, such as radio waves, microwaves, gamma rays, ultraviolet radiation, infrared radiation, X-rays, and terahertz waves, beyond our sensory perception.

Visible Wavelengths in Nanometers

The visible spectrum to the human eye corresponds to specific wavelengths measured in nanometers (nm), generally spanning these 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 perceived color of an object is determined by the wavelengths it absorbs and reflects. For instance, an object appears black if it absorbs all wavelengths and white if it reflects them all. If an object absorbs all colors except blue, it will appear blue.

Importance of Particle Size in Pigment Interaction

Understanding particle size is crucial as it influences how light interacts with these particles, particularly in cosmetics and tattooing, where color stability is vital.

Difference Between “Recognizing Colors” and “Seeing” Objects

While we can detect colors in the 380 to 700 nm range, the smallest object we can "see" in discerning its shape and space occupation is much larger, around 100,000 nanometers. For example, the following.

A human hair is about 70,000 to 100,000 nanometers in diameter.
A grain of table salt is around 100,000 nanometers across.
Refined sand grains are also about 100,000 nanometers in size.

It's impossible to detect individual pigment particles in water with the naked eye; we see aggregates suspended in the pigment's liquid carrier.

Different Sensitivity to Wavelengths of Light

Not all wavelengths within the visible range are equally detectable. We're generally more sensitive to shorter wavelengths like violet and blue than to longer wavelengths like red due to the higher sensitivity of our photoreceptor cells, particularly cones, to shorter-wavelength light.

3. How "Blue" Light Reflects

Understanding Reflectance

Reflectance is the measure of the proportion of light that is reflected away from a material. When light strikes a surface, it's either absorbed or reflected. The ratio of reflected to incoming light is the material's reflectance. It can be wavelength-dependent, varying across different light wavelengths (or colors). This property is why objects have colors; they reflect specific wavelengths while absorbing others. For instance, a red apple appears red because it reflects red wavelengths and absorbs most others.

Reflectivity of Solid Objects

Solid objects like people, animals, houses, and pigment molecules are typically reflective. They absorb or reflect light, distinguishing themselves from their background. This distinct reflection gives them their unique colors, similar to how pigment molecules in the skin become visible as "pixels," "hairstroke lines," or "shading" in brows due to their absorption and reflection patterns.

Reflection and Particle Size

How light reflects off an object is affected by its size, influencing light scattering. For particles around 100 nm and larger, "Mie scattering" is relevant and more complex than "Rayleigh scattering," which accounts for the sky's blue appearance.

Mie scattering considers particle size, light wavelengths, and the refractive index of the surrounding medium. It explains how spheres of specific sizes and materials scatter different light wavelengths, considering absorption and scattering.

Particles 90-100 nm

At this size, particles are close in size to blue and violet light wavelengths (approximately 380-495 nm). Mie scattering at these wavelengths is efficient, causing a bluish appearance.

Particles 200-300 nm

These larger particles interact with a wider range of wavelengths, including green and light blue (approximately 450-570 nm) and blue and violet, potentially appearing anthracite greenish due to scattering these colors.

Particles 500 nm and Larger

At this size, particles behave more like bulk materials, absorbing various wavelengths and scattering shorter violet and longer dark red wavelengths, resulting in a brownish appearance.

The perceived color is influenced by efficiently scattered wavelengths and our eyes' sensitivity to different wavelengths. While the relationship between particle size and color is complex due to Mie equations, generally, they interact with more wavelengths as particles grow.

Mie Scattering and Retroreflectance

Retroreflectance is a type of reflectance where light returns in the direction it came from with minimal scattering. Common in "safety" clothing and road signs, these materials appear bright when illuminated, reflecting light toward the source for high visibility.

While more related to visibility than color perception, retro reflectance can affect how vivid or bright color appears under certain lighting. Organic and inorganic colorants in the skin can show varying levels of retroreflectance. A color's brightness correlates with the retroreflectiveness directed back to our eyes. Thus, the contrast and visibility of skin pigment are influenced by both selective wavelength absorption and the pigment particles' retroreflectiveness.

Mie scattering and retroreflectance contribute to pigment perception in the skin in different ways. Mie scattering focuses on color creation through particle-light interaction, while retro reflectance enhances visibility by reflecting light to its source. Together, they influence the color and brightness of pigment particles in the skin based on their size and retroreflective properties.

For instance, a pigment particle good at Mie scattering might show a specific color, but its visibility can be boosted if it also has strong retroreflective properties. This dual interaction offers a nuanced understanding of pigment appearance under various lighting conditions, aiding in applications like cosmetics and tattooing.

4. Conclusions from Physical Reasons

Understanding why pigment appears blue within the skin is rooted in recognizing how light interacts with pigment particles. When we perceive something as "blue," it's due to the reflection of blue (450-476 nm) and violet (380-450 nm) light wavelengths back to our eyes, while other wavelengths are absorbed.

Physically, this effect can be achieved using “Gas Carbon Black” (CI 77 266), also known as "Pigment Black 6" or "Channel Black." This pigment employs the smallest possible particles to maximize opacity and richness.

However, this is merely one physical aspect. As we'll explore further, the outcome of the procedure and the amount of pigment entering the skin significantly influence the result, making “Oil Carbon Black,” or Carbon Black 2, potentially the most hazardous.

The size of the pigment particles is crucial in determining their perceived color. Smaller particles tend to appear blue because they scatter light less, producing a more concentrated color. Conversely, larger particles (500 nm and above) behave more like bulk materials, absorbing various wavelengths. Their size also facilitates the scattering of shorter violet and longer dark red wavelengths, leading to a brownish appearance.

Preliminary Conclusion from Physics

To achieve a visual effect in the skin where the pigment appears "blue," a deeply opaque color is necessary. This color must interact with light to absorb all colors except blue and violet. The most efficient way to accomplish this is by using Carbon Black with small particle sizes, like Channel Black or "Gas Black," approximately 100 nm in size. This specific composition creates the conditions for a "blue" appearance. Achieving this effect with larger particle sizes or pigments of any color other than black is improbable from a purely physical standpoint.

5. The Tyndall Effect

Understanding the practical possibility of brows turning blue involves analyzing the behavior of pigment particles, specifically CI 77 266, within human skin. This requires understanding the Tyndall effect, the skin's pH level, and the pigment volume inserted.

The Tyndall effect occurs when light encounters particles smaller than its wavelength, scattering the light in different directions. This phenomenon is observable when a beam of sunlight passes through a dusty room, or a flashlight shines into the fog.

From Blue Sky to Blue Brows

The Tyndall effect more significantly scatters shorter wavelengths like blue and violet light, often resulting in a bluish tint. This scattering is why the sky appears blue during the day—sunlight is dispersed by the Earth's atmosphere.

Tyndall Effect Observations in Dermatology

Understanding the Tyndall effect is crucial in dermatology, particularly skin pigmentation procedures. As pigments are injected into the skin and exposed to light, the Tyndall effect can significantly impact their perceived color. The size, type, and skin placement of pigment particles can all influence the Tyndall effect, thus affecting the final color seen.

In conclusion, the bluish appearance of pigment in the skin is influenced by the size and distribution of carbon particles and their interaction with blue wavelengths. When these factors are optimized and other color-distorting elements are minimal, the result can be a more pronounced bluish tint.

6. Skin’s pH Level

In semi-permanent makeup, such as Powder Brows and Hairstrokes, a critical but often overlooked factor is the skin's pH level and its impact on pigment implantation. Particularly, the type of carbon black used can significantly influence the ease of implantation and the risk of pigment migration, affecting the color outcome and the potential for a "blue" effect.

Different Types of Carbon Black and Their Characteristics

Let's examine three common types of carbon black pigments. Channel Black (Black 6) - This gas-derived pigment features particle sizes ranging from 90-100 nm. It contains about 19% hydrocarbons and a high percentage of elemental carbon, varying between 30-90%. Its smaller particle size and composition make it highly suitable for creating deep, rich black tones.

Furnace Black (Black 2). Originating from oil, this variant has larger particles between 200 and 300 nm. It comprises up to 55% hydrocarbons and significantly less elemental carbon, approximately 8%. The larger particle size and different composition can influence how it settles and appears on the skin.

Thermal Black (Black 7). Also derived from gas, this pigment has the largest particles, around 500 nm. It is almost entirely made up of elemental carbon, at 99%. Its large particle size and purity might affect its behavior in the skin differently from the other types.

Understanding these types of carbon black and their unique properties is crucial for predicting how they'll interact with the skin's pH and ultimately influence the final color seen in semi-permanent makeup applications.

7. Differences in Water

Gas-derived Carbon Black

When Thermal Black (Black 7), a gas-derived carbon black, is introduced to water, it behaves similarly to mineral-based particles like Black Iron Oxide. It's relatively dense, causing some particles to sink to the bottom. The water remains mostly clear, with visible clumps of insoluble particles. Thermal Black is an inorganic component composed of elemental Carbon, which doesn't dissolve in water.

Conversely, Channel Black (Black 6) has up to 20% organic components (C-H bonds). This characteristic allows it to disperse more evenly in water, causing the water to take on a lighter color due to smaller particle aggregates.

In a comparative image, the differences are evident. Gas-derived carbon blacks - Thermal and Channel Black - would be shown on the right side. Thermal Black would have larger, heavier aggregates, mostly at the bottom. At the same time, Channel Black would display a more uniform coloration in the water owing to its smaller and more evenly distributed particles.

Oil-derived Carbon Black

In contrast, Furnace Black (Black 2), which is oil-derived, behaves distinctly when introduced to water. It acts more like petroleum oil, being semi-soluble and oily. It spreads in various directions, creating a unique visual effect in the water on the left. This behavior is due to its oil-based origin and different physical properties, resulting in a visibly different interaction with water compared to gas-derived carbon blacks.

8. Implantation Factor

Ease of Implantation and Skin pH: The Hydrocarbon Factor

Initially, one might think smaller particles, like those in Channel Black (Black 6), would implant more easily into the skin. However, the reality is different. With its larger particles, oil-based Furnace Black (Black 2) implants more effectively in a single pass than gas-based Channel Black.

The reason lies in the chemical composition of the pigment and its interaction with the skin's pH. Smaller particles often need more oxygen bonds for stability, making them less acidic. Since the skin's pH involves hydrogen bonds, pigments with larger particles, rich in organic hydrocarbons with hydrogen bonds, integrate more readily into the skin. Thus, Black 2, with its higher hydrocarbon content, is more compatible with the skin than Black 6.

Larger Particles Can Be Easier to Implant

Theories suggest that smaller particle sizes could make implantation more challenging due to less compatibility with the skin's pH. While it's inaccurate to say that a higher oxygen concentration in smaller particles makes them more alkaline, many smaller pigments have a less compatible pH. On the other hand, many larger particle pigments, though not inherently more acidic, have a pH that aligns better with the skin due to more functional groups that can donate or accept hydrogen ions, facilitating easier implantation.

Human Aspect of Implantation

The human element, the skill and experience of the artist, is crucial when considering why pigments might turn "blue" once implanted. Oil-based Carbon Black (Black 2) is particularly risky due to its chemical and physical properties, making it easy to implant. In inexperienced hands, this ease can lead to excessive organic carbon introduction into the skin, potentially resulting in a "blue" appearance.

"Blue Brows," Skin Melanin, and Fitzpatrick Scale

Individuals with higher Fitzpatrick skin types may more frequently experience a bluish hue due to increased melanin influencing the pigment's underlying tones. A common countermeasure is to add magenta pigment to the color formula, warming up the brow color and neutralizing cooler blue tones. This approach doesn't change the carbon's intrinsic reflective properties but adds complexity to how light is absorbed and scattered, affecting our perceived color.

Conclusion on Why Pigment Turns Blue

No pigment is inherently "blue;" it's an optical phenomenon. The smallest particles of Channel Carbon Black (90-100nm) can reflect light in ways that appear bluish or violet. The Tyndall effect and the skin's pH level play significant roles in this optical behavior. Understanding why pigment appears blue requires considering all factors that create an environment conducive to this optical phenomenon. Using Black 2, especially by less experienced artists, and treating it like mineral pigments can lead to a "blue" appearance in treatments like eyebrow or eyeliner pigmentation.

9. How to Prevent "Blue" Pigmentation

Preventing pigments from appearing blue involves three main strategies: Incorporating Larger Elemental Carbon Particles.

Creating a "pseudo-hybrid pigment" with a substantial amount of larger elemental Carbon can mitigate the blue appearance. This involves using larger Carbon particles (500 nm or larger) in the formulation.

For applications like eyeliner, utilizing Oxide Fused with Carbon is beneficial.

Opting for mineral or hybrid pigments incorporating Furnace Black or large Carbon particles can significantly reduce the chances of a blue hue.Adding Elemental Carbon

The choice of Carbon Black is crucial in preventing a bluish appearance. One effective method is to blend Elemental Carbon (Black 7) with pigments containing Black 6. This mix can diminish the pigment's tendency to reflect blue light, thereby neutralizing the blue tone.

Adding larger particles of Elemental Carbon into the pigment helps scatter light differently, reducing the prominence of shorter, bluish wavelengths.Using Iron Oxide Fused with Carbon

Iron Oxide Fused with Carbon, sometimes called Gamma-Black, is a specialized black pigment where Carbon Black molecules are fused with mineral (inorganic) Iron Oxide molecules (CI 77499).

When selecting a pigment, look for both color indexes (CI 77499 and CI 77266) on the label, indicating the presence of both types of black. This combination offers a broader range of light absorption and reflection properties, which can help significantly reduce the blue tint.

The fusion of Iron Oxide with Carbon creates a more complex pigment that interacts with light to diminish the bluish reflection, offering a more neutral and true black appearance.

By carefully choosing the type and size of Carbon particles and considering the combination of different pigments, practitioners can significantly reduce the risk of blue pigmentation in semi-permanent makeup applications.

10. Black Iron Oxide Fused with Carbon

Black Iron Oxide (Fe3O4) fused with carbon results in a stable, non-magnetic black pigment, commonly listed as "CI 77266 + 77499" in semi-permanent makeup pigments.

Carbothermal Reduction Process

Process Overview. Carbothermal reduction fuses Black Iron Oxide with carbon. This chemical process involves heating iron oxide and carbon at high temperatures in an inert atmosphere, typically using gases like nitrogen or argon to prevent oxidation.
Reducing Agent. Elemental carbon, often in graphite or charcoal, acts as the reducing agent, stabilizing the iron oxide and eliminating its magnetic properties.
Procedure. The process starts with mixing elemental carbon and Black Iron Oxide (Fe3O4). The mixture is then heated to temperatures between 800°C and 1300°C. At these temperatures, the carbon reduces Fe3O4 to produce elemental iron (Fe) and carbon dioxide (CO2) or carbon monoxide (CO).
Chemical Equation. The reaction can be represented by the following balanced equations:

3Fe3O4+4C→4CO2+9Fe or

3Fe3O4+4C→4CO+9Fe

During the reaction, carbon and Fe3O4 interact at the boundary layers of their particles. The produced gases (CO2 or CO) escape, leaving behind elemental iron and a stabilized iron-carbon compound, typically indicated as C Fe3O4.

Advantages of CI 77266 + 77499 in Semi-Permanent Makeup Non-Magnetic. The fusion process eliminates the magnetic properties of Black Iron Oxide, making it safer and more stable for use in cosmetic applications.

Opacity and Color. This pigment provides an opaque, rich black color desired in various makeup products, particularly in creating sharp, defined lines.

Stability. The fused pigment is stable, ensuring consistent color and performance over time.

Ease of Implantation. Its properties make it fairly easy to implant into the skin, which is crucial for the precision and quality of semi-permanent makeup applications.

Thus, Black Iron Oxide fused with carbon (CI 77266 + 77499) is created through carbothermal reduction. It is prized in the semi-permanent makeup industry for its stability, color, and ease of use.

11. Conclusion

The Cause

The "blue" pigmentation phenomenon is an optical rather than an intrinsic characteristic of the pigment. Understanding this requires recognizing that objects, including brow pigmentation, have no inherent color; what we perceive is determined by reflection, retro-reflection, and light scattering (Mie scattering in pigmentation's context). Specifically, Channel Carbon Black's small particles (90-100nm) reflect bluish or violet wavelengths due to their optical properties.

Adding factors like the Tyndall effect, which emphasizes unabsorbed blue wavelengths, the skin's pH, and the technique of implantation, we deduce that the most significant risk of "blue" appearance is linked to the number of small particles implanted that absorb all wavelengths except blue. Therefore, pigments containing CI 77266 Carbon particles produced via the Furnace method from petroleum oil (Black 2), rich in hydrocarbons and more compatible with skin pH, are primarily responsible for blue pigmentation in brows and eyeliner. This is particularly true when implanted like larger Carbon Black particles (Thermal Black) or mineral pigments.

The Solution

To mitigate this optical effect, three main strategies are recommended:

Incorporating Larger Elemental Carbon ParticlesEmploying larger elemental Carbon particles in the pigment or creating a "pseudo-hybrid" by adding Thermal Black can modify the optical behavior and reduce the scattering of blue wavelengths.
Using Iron Oxide Fused with Carbon. Especially for eyeliner applications, using Iron Oxide Fused with Carbon (often labeled as CI 77266 + 77499) can offer a pigment that interacts differently with light, lessening the blue appearance.
Opting for Mineral or Hybrid Pigments with Larger ParticlesChoosing mineral or hybrid pigments containing larger Carbon particles (500 nm or larger), like Thermal Black, can reduce the likelihood of a blue hue due to their different light scattering properties.

In conclusion, a deep understanding of the optical principles involved in pigment reflection and scattering, combined with strategic pigment selection and formulation, can significantly reduce the occurrence of undesired "blue" pigmentation in semi-permanent makeup.