Hair dye allows individuals to change their hair color for various reasons, from covering gray hairs to experimenting with new aesthetics. Altering hair color involves intricate chemical reactions and interactions with the hair’s natural structure. Understanding these mechanisms provides insight into how different dye types achieve their results and impact the hair.
Hair’s Natural Blueprint
Each hair strand is primarily composed of keratin, a fibrous protein. The visible hair shaft has three main layers. The outermost cuticle consists of overlapping cells that protect the inner hair. Beneath it, the cortex is the thickest layer, providing strength, elasticity, and containing natural pigment. The innermost medulla is not always present, typically found in thicker hair strands.
Hair’s natural color is determined by melanin, a pigment produced in the cortex. Two primary types exist: eumelanin for brown and black hues, and pheomelanin for yellow, red, and auburn tones. The specific combination and concentration of these two melanin types result in the wide range of natural hair colors.
How Temporary Dyes Work
Temporary hair dyes offer a non-committal way to change hair color because they do not chemically alter the hair’s structure or natural pigment. These dyes contain large molecules that simply coat the outside of the hair cuticle, meaning they do not penetrate the hair shaft.
Since the dye molecules only adhere to the hair’s exterior, they wash out easily with a single shampoo. Temporary dyes are often used for vibrant, short-term changes, providing a superficial color that fades rapidly. They are akin to painting the hair, with the color sitting on the surface.
How Semi-Permanent and Demi-Permanent Dyes Work
Semi-permanent dyes provide a longer-lasting color change than temporary dyes, yet they avoid significant chemical alteration. These dyes use smaller molecules that penetrate slightly beyond the cuticle into the outermost layer of the cortex. They do not contain ammonia or peroxide, so they cannot lighten the hair’s natural color. The color gradually fades over several weeks with shampooing as the dye molecules slowly exit the hair shaft.
Demi-permanent dyes offer an intermediate level of longevity and penetration. These dyes contain a low-volume hydrogen peroxide developer, but no ammonia. The peroxide gently opens the cuticle, allowing dye molecules to penetrate deeper into the cortex than semi-permanent dyes. While depositing color effectively, demi-permanent dyes cannot significantly lighten natural pigment because they lack ammonia. The color typically lasts for about 4 to 6 weeks, or up to 24 shampoos, before gradually fading.
How Permanent Dyes Work
Permanent hair dyes create a lasting color change through complex chemical reactions within the hair’s cortex. This process involves an alkaline agent and a developer, typically hydrogen peroxide. An alkaline agent, often ammonia, swells the hair shaft and raises the cuticle scales. This allows the dye mixture to penetrate deeply into the cortex, where natural pigment is located.
Inside the cortex, hydrogen peroxide, an oxidizing agent, performs a dual role. It breaks down the hair’s natural melanin pigments, lightening the original hair color. Simultaneously, small, colorless dye precursors from the mixture enter the cortex, where hydrogen peroxide causes them to react and link, forming larger, colored molecules. These new, larger dye molecules become trapped within the cortex, too big to escape through the resealed cuticle. This combination of lightening natural pigment and depositing new color results in a permanent change that resists washing out.