Hair dyeing is fundamentally a chemical modification process designed to change the color of the hair fiber. This alteration is achieved through the precise delivery and chemical reaction of various compounds that either mask, lift, or permanently replace the hair’s natural pigment. The chemistry requires specific agents to prepare the hair shaft, facilitate the color change, and finally lock the new molecules into place. The final result depends entirely on whether the process involves simply depositing a pre-formed color molecule or initiating a deeper chemical reaction that changes the fiber’s internal structure.
The Target: Hair Structure and Melanin
Successfully altering hair color requires understanding the hair shaft’s architecture. The visible portion of hair is composed mainly of the protein keratin and consists of three main layers: the cuticle, the cortex, and sometimes the medulla.
The cuticle is the transparent, outermost layer, formed by overlapping, scale-like cells that function as a protective barrier. For permanent color change, substances must first penetrate this tightly sealed layer. Beneath the cuticle lies the cortex, the thickest layer, which accounts for up to 90% of the hair’s total mass.
The cortex is the primary location for the hair’s natural color, determined by the pigment melanin. Melanin exists mainly as eumelanin (brown and black shades) and pheomelanin (red and yellow tones). Chemical treatments must target the cortex to deposit new color or alter these existing melanin granules.
Surface-Level Alteration: Direct Dye Chemistry
Non-oxidative methods, which include temporary and semi-permanent dyes, achieve color change through direct dye chemistry. These formulations contain color molecules that are already fully formed and ready to deposit onto the hair. Since these dyes do not require a chemical reaction to create color, they do not use alkalizing or oxidizing agents like hydrogen peroxide.
These pre-formed dye molecules, such as large acid dyes, are generally too large to penetrate deeply past the cuticle layer. Instead, they adhere primarily to the hair shaft’s surface or slip minimally into the outermost cuticle scales. The resulting color is vibrant, but it is temporary because it lacks a chemical bond to the hair structure.
Because the dye molecules coat the exterior, they are easily washed away by shampooing and water. This means the color typically fades significantly after just a few washes. Direct dyes offer a low-commitment option that alters the hair’s color without changing its internal chemical structure.
The Core Process: Oxidative Dyeing Chemistry
Oxidative hair dyeing forms the basis of both demi-permanent and permanent color, relying on a multi-step chemical process for a lasting color change. This method is defined by the use of an oxidizing agent, which initiates the reaction that forms the final dye molecule inside the hair shaft. The entire process hinges on three simultaneous chemical actions: alkalization, oxidation and lightening, and color formation.
Alkalization
The first step in oxidative dyeing is the application of an alkalizing agent, most commonly ammonia or an ammonia substitute like ethanolamine. This chemical raises the hair’s pH level from its natural acidic state to an alkaline state, typically between pH 8 and 11. This elevation in pH causes the tight, overlapping cells of the cuticle to soften and swell, creating pathways into the inner cortex.
The opened cuticle allows the small dye precursors and hydrogen peroxide to diffuse into the hair’s core. Without this alkalization step, the chemical process that creates permanent color cannot begin. While ammonia is highly effective at this swelling action, its volatility causes the characteristic strong odor associated with permanent hair color.
Oxidation and Lightening
The oxidizing agent, hydrogen peroxide, performs a dual function in the process. First, it acts as a bleaching agent by oxidizing the natural melanin pigments found in the cortex. This oxidation breaks the chemical bonds that give melanin its color, converting the dark pigment into a colorless compound.
This lightening action allows the final color to be lighter than the natural hair shade. The peroxide also serves as the catalyst for initiating the polymerization reaction that forms the new color molecules. The concentration of the peroxide, often referred to as the developer volume, determines the degree of both lightening and color formation.
Color Formation (Coupling)
The ultimate goal of the oxidative process is the creation of large, insoluble color molecules that become trapped within the hair structure. The dye formulation contains two types of small, colorless molecules: primary intermediates (like p-phenylenediamine) and color couplers (like resorcinol). These precursors penetrate the cortex through the pathways created by the alkalizer.
Once inside the cortex, hydrogen peroxide oxidizes the primary intermediates, creating highly reactive compounds. These reactive molecules immediately combine, or couple, with the color couplers in a polymerization reaction. This chemical linking creates much larger, colored dye molecules that are too bulky to exit the hair shaft, effectively locking the new color inside the cortex. This trapping mechanism is the chemical reason permanent hair dye resists washing out.
Chemical Factors Affecting Color Longevity and Integrity
Even permanently dyed hair is susceptible to fading and degradation due to chemical and environmental factors. The high pH required during the alkalization phase of dyeing can chemically stress the hair’s keratin structure, causing the fiber to become more porous. This increased porosity makes it easier for the newly formed dye molecules to leach out of the cortex over time with repeated washing.
External factors, particularly ultraviolet (UV) radiation from the sun, generate free radicals that actively break down the chemical bonds of the dye molecules, leading to color loss and shifts in tone. Additionally, minerals found in hard water, such as iron and magnesium, can build up on the hair surface and react with the color, causing it to appear dull or discolored.
To counteract the chemical impact of the dyeing process, many color treatments conclude with an acidic conditioner or “finisher.” This acidic product lowers the hair’s pH, which helps to flatten and reseal the cuticle scales that were opened by the alkalizer. Closing the cuticle is a necessary chemical step to physically contain the dye molecules and minimize future color washout.