Chapter 6 - Characteristic of the Hair - CHI Education (2024)

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It is critical to understand the seven key characteristics of hair: texture, density, porosity, elasticity, tenacity, formation and color. These characteristics all must be considered before deciding how to cut, style, color or texturize a client’s hair.

Every study of hair structure begins with the composition of each hair. The hair is comprised of three basic sections or layers. The outermost layer is the cuticle. These overlapping dead cells are similar to shingles on a roof. They protect the softer, inner layers and they give hair its outward appearance.

The center layer is called the cortex. This the thickest layer and it contains the pigment that determines hair color. The innermost layer is called the medulla. It is round in shape. It is always present in medium or coarse hair but may be absent in fine blonde hair.

The two most common types of hair found on the body are terminal hair and vellus hair.

Terminal hair is the coarse, long, pigmented hair found on the scalp, legs, arms and bodies of females and males. It is coarser than vellus hair and, with the exception of gray hair, it is pigmented. It usually contains a medulla.

Vellus hair, also known as lanugo hair, is fine, short, non-pigmented and downy. It rarely has a medulla. It is usually found on infants and may remain present on children until puberty. In adults, vellus hair is present in areas considered “hairless” such as the eyelids, forehead and bald scalp. It’s also on nearly all other areas of the body, with the exception of the palms of the hands and soles of the feet. Vellus hair aids with perspiration evaporation, and women normally retain 55 percent more vellus hair than men.

Hormonal changes during puberty cause some areas of vellus hair to shift to thicker, terminal hair. All hair follicles are capable of producing vellus and terminal hair, and when they do so is predicated upon genetics, age and hormonal activity.

Structures of the Hair Shaft

There are three main layers of the hair shaft; the hair cuticle,cortex, and medulla.

The hair cuticle (hayr kyoo-ti-kul) is the outermost layer of the hair. It consists of a single layer of clear, scale-like cells, similar in appearance to shingles on a roof. The cuticle layer provides a barrier that safeguards the inner structure of the hair as it lies firmly against the cortex… The smooth, shiny, and silky feel of healthy hair is created by the hair cuticle.

To feel the hair cuticle, pinch a single strand of hair between your thumb and forefinger. Starting near the scalp, gently slide your fingers up on the strand. A healthy stand of hair will feel smooth and silky. Next, while holding the end of the hair strand with one hand, pinch the strand with the thumband forefingers of your other hand. Slide your fingers down the hair shaft. In this direction, the hair feels rougher because you are going opposite of the natural growth of the cuticle layer. A healthy cuticle is needed into order to fight against damage of the hair strand.

Each overlapping scales which are attached to the cortex is the main defense against damage. It is a hard shingle like layer of overlapping cells; some have five to twelve scales deep. These overlapping scales are called the cuticle layer. These layers can be opened by using chemicals or substances like permanent wave solution or hair color.

These chemicals open the cuticle or swell the cuticle to allow the chemicals to go into the cortex.

The cortex (kor-teks) is the middle layer of the hair. The cortex is the main part of the hair that is placed under the cuticle. The cortex is comprised of fibers or strands that are toughly intertwisted together. They are rather flexible and can be significantly stretched and return to the original length with no adverse consequences. The color ofhair is defined by melanin and pheomelanin, the main color pigments that are placed in the cortex. Changes take place in the cortex when hair coloring, wet setting the hair, chemical texturizing, and blow drying.

The medulla, or central core that may be present in the hair, is comprised of round cells. Thick, coarse hair and male beard hair contain medullas; other types of hair may not. The function of the medulla has not yet been determined.

Chemical Composition of Hair

Hair is composed of protein that originates in cells within the hair follicle. As soon as these protein cells form, they begin a journey upward through the hair follicle. They mature via a process called keratinization. During keratinization, the cells fill with a fibrous protein called keratin. By the time the hair shaft emerges from the scalp, these cells have lost their nuclei and died. At this stage, the cells of the hair are completely keratinized and are no longer living. Hair, therefore, is a non-living fiber composed of keratinized protein. It is composed of 90 percent protein, and this protein is comprised of a long chain of amino acids. Five main elements form these amino acids.

Hair is around 90 percent protein. The protein is made up of long chains of amino acids, which, in turn, are made up of elements.

The Cohn’s Element Table

The main elements that make up human hair are Oxygen, carbon, hydrogen, nitrogen and sulfur are the main elements that make up human hair. They are commonly referred to as the COHNS elements. These elements are also found in the skin and nails.

Proteins, Peptides and Polypeptide Bonds

Proteins are comprised of long chains of amino acids.These strands are connected end to end–like a child’s pop beads toy. A strong chemical bond called a peptide or end bond links these amino acid chains.

A long chain of amino acids linked by peptide bonds is called a polypeptide chain. When these long polypeptide chains become coiled and intertwined, the spiral is called a helix.

Side Bonds of the Cortex

The cortex is composed of millions of polypeptide chains. Polypeptide chains are cross-linked by three different types of side bonds that connect the polypeptide chains and are responsible for the powerful concentrated strength and elasticity of human hair. They are not significant to services such as wet setting, permanent waving, chemical hair relaxing, and thermal styling. The three types of side bonds are hydrogen, salt, and disulfide bonds.

• A hydrogen bond is a cross-linked side bond that can be easily broken by water or heat. While a single hydrogen bond is quite frail, hydrogen bonds are so numerous, they account for about a third of hair’s overall strength. Hydrogen bonds are broken when the hair is wet with water, which allows it to be reshaped when wrapped around rollers or brushes. Hydrogen bonds then reform into the new shape when the hair
• The second type of bond between protein chains is the salt bond. A salt bond is also a weak, physical, cross-link side bond between adjoining polypeptide chains. This bond is the result of the attraction of contrasting charges. Salt bonds depend on pH, and they are broken without difficulty by strong alkaline or acidic Salt bonds assist in the organization of the protein chains. They account for 35% of the hair’s resistance to change. Like the hydrogen bond, salt bonds are not particularly strong.
• A disulfide bond is a strong, chemical side bond that is unlike the physical hydrogen and salt bonds. Disulfide bonds link the sulfur atoms of two, adjoining cysteine amino acids to form a single cysteine. This cysteine links two polypeptide strands. While there are fewer disulfide bonds than hydrogen or salt bonds, disulfide bonds are much stronger, and also account for a third of hair’s total strength.

Disulfide bonds are not broken by water. The bonds are broken by chemical hair relaxers and permanent waves that alter the shape of hair Also, normal quantity of heat, such as the heat used in standard thermal styling, do not break disulfide bonds. The bonds are usually broken by extreme heat created by boiling water and some extreme temperature thermal styling tools such as flat irons and straightening irons.

Thio permanent waves break disulfide bonds and reform the bonds with thio neutralizers. Hydroxide chemical hair relaxers break disulfide bonds and convert them to lanthionine bonds once the relaxer is rinsed from the hair. With hydroxide relaxers, disulfide bonds are broken permanently and cannot be reformed.

Hair Pigment

All natural hair color is the product of the pigment positioned within the cortex. Is comprised of small grains of pigment in the cortex that furnish the hair with its natural color. The two types of melanin are pheomelanin and eumelanin.

• Pheomelanin supplies the hair’s natural color in tones ranging from red to yellow to blonde.
• Eumelanin supplies the hair with its natural dark brown to black tones.

All natural hair color is the result of the ratio of eumelanin to pheomelanin, accompanied with the overall number and size of pigment granules.

Wave Pattern

The wave pattern of hair is the shape of the actual hair strand. It can be curly, extremely curly, wavy or straight.

Natural wave patterns are the result of genetics. Though there are numerous exceptions, as a general rule, Asians and Native Americans are inclined to have extremely straight hair, Caucasians are inclined to have straight, curly, or wavy hair, and African Americans are likely to have extremely curly hair. However, members of every race, or mixed race individuals, may also exhibit any degree of straight, wavy or curly hair. Wave patterns may also vary on a single person. For example, curly hair is often straighter in the crown and tighter in other areas of the head.

Hair Shapes

While there are many theories, there is no conclusive evidence as to why some hair grows straight and some grows curly. The most popular theory claims that the cross-section of the hair determines the amount of curl. A round cross-section results in straight hair; an oval-to-flattened cross-section results in wavy hair and a flattened to oval flattened cross-section results in extremely curly hair.

Modern microscopes have shown that a cross-section of hair can be nearly any shape, so the shape of the cross- section does not always relate to the degree of curl.

Another theory states that in curly hair, one side of the hair strand grows faster than the other side. As a result, the side that grows more quickly is a bit longer than the slower-growing side, and tautness/tension within the strand causes the long side to curl around the short side. Conversely, hair that grows at the same rate on each side emerges straight. This theory has not been proven.

Extremely Curly Hair

Extremely curly hair grows in long, twisted spirals. Cross-sections show extremely curly hair to be flattened, with variations in shape and thickness along the length. Compared to straight or wavy hair, which possess a fairly uniform diameter along each strand, extremely curly hair may be irregular, with variations in diameter along a single strand. Some extremely curly hair has a tendency to coil. Spiraled or coily hair may be fine in texture, with many individual strands twisted together to form the coiled locks. Extremely curly hair may also exhibit low elasticity, a tendency to break easily and a tendency to form knots…mainly on the ends. Soft, gentle scalp manipulation; conditioning shampoos and detangling rinses will help minimize tangles in this hair type.