HUMAN HAIR IS TRANSPARENT
You can see right through the cuticle into the center of the hair strand (cortex). Human hair's physical-chemical properties make it possible to be colored to suit personal tastes and fashion trends.
To become skilled in using hair color, a basic understanding of hair structure is needed.
Cuticle: this is the shingle-like protective covering on the hair shaft that is water-repellant. To understand this, place a strand of hair in a glass of water. If the cuticle is intact, the hair floats; if not, it will sink.
Cortex: this is the primary structural part of the hair shaft that has the protein chains. These long, high weight chains give hair its strength and elasticity. The cortex absorbs water easily. That is why highly bleached hair is porous. It has lost the cuticle layer, exposing the water-loving cortex. The pigment in the cortex determines the hair's natural color.
Medulla: this is the hollow core of the hair shaft. The medulla is like a small hollow tunnel in the center of the hair shaft. It is not necessarily continuous but may occur in a random manner.
Now that you have a sense of hair's structure we can talk about its texture, density, elasticity and strength.
Texture: refers to the diameter or thickness of an individual hair strand (coarse hair is thicker than fine hair). Texture has no bearing on wave patterns. Density: refers to the number of hairs on a person's head. The average head has 100,000 - l50,000 hairs. Normal hair loss is 50-100 hairs per day.
Elasticity: One of the key indicators of hair's condition is elasticity. Elasticity is affected by such things as chemicals, excessive heat, sun, and refers to hair's ability to return to its normal length after being stretched. Elasticity is only seen on wet hair. The healthier the hair the better its ability to return to normal length after stretching.
Strength: determined by the condition of hair's cortex. Hair's strength is affected by chemicals and heat. Strength is determined by the amount of stress a hair can withstand without breaking. The healthier the hair the more stress it can withstand.
Melanin is the natural hair color pigment in the cortex layer. As a result of the melanin's molecular weight, it cannot be changed --except by intense oxidation or concentrated alkaline solutions.
Natural hair color differs from person to person and within hair on the same scalp. It may also differ from one area of a hair strand to another. The distribution of natural pigment (melanin) inside the hair and the amount of natural pigment both determine each strand's color.
Previous treatment of the hair shaft determines hair's condition and porosity. The more cuticle damage there is, the more porous the hair.
Before beginning any chemical service, determine the porosity to better understand how the hair will respond.
When coloring, the protective cuticle layer must be lifted to allow the color gel to penetrate the hair cortex. This promotes longer lasting and more natural looking color. If the cuticle has been damaged by excessive lightening, or chemical applications that are too harsh and frequent certain steps must be taken to insure natural looking color. Overly porous hair may not hold color and is prone to end grabbing.
PERMANENT OXIDATIVE HAIRCOLORING
Permanent hair dyes result in permanent change to natural hair color. Permanent hair colors are used to cover gray or to impart a lighter or darker shade. The two advantages of permanent hair coloring are natural looking colors and permanent waving compatibility.
Modern permanent hair coloring is done by mixing a hair dye base (containing oxidative dyes) with a developer (oxidizing agent) before use. These oxidative dyes are generally small colorless molecules that penetrate the hair shaft where they are oxidized (developed) into large, insoluble colored pigments, hence the name oxidation dyes. These large pigments are trapped within the cortex and cannot be washed out easily.
As the dyes oxidize some of the natural pigment is lifted (lightened) by the product. This lifting action depends on 3 things: the strength of the developer; the level of ammonia in the product; and processing time. Less peroxide and ammonia and shorter processing time result in less natural pigment loss.