Most of the hides and skins that we use to make leather are covered with hair and these hairs grow from the hair follicles. Very basically, the hair follicle is a tube inside the skin that produces the cells that the hair is made from, packs them together and moulds them into a hair shape. As more cells are produced, the hair shaft gets pushed out at the top of the hair follicle into the outside world. Simple as that explanation might sound, the process is in fact quite complex.
In an earlier article in this series the structure and function of the epidermis was discussed1. Essentially, the epidermis comprises of complex layers of cells that cover the underlying dermis that we use to make leather from. During the embryonic development of a mammal, the hair follicles are formed by parts of the epidermis growing inwards. If you imagine the epidermis to be a thin flexible film, an indentation of hair follicle shape can be formed by pushing your finger into the film. At the same time that this indentation in the epidermis, known as a dermal plug, begins to form, special cells in the underlying dermis begin to accumulate just below the epidermis in the same area. These cells form what will eventually become the dermal papilla that produces the cells from which the hair shaft is made. As the epidermal plug grows downwards, it comes into contact with the dermal papilla and pushes it downwards further into the dermis. The epidermal cells of the dermal plug communicate with the dermal cells of the dermal papilla and they begin to organise themselves into what will eventually become the hair follicle and associated structures.
The epidermal plug differentiates into three concentric cylinders. The innermost cylinder forms the hair shaft. The outermost cylinder forms the outer root sheath that separates the whole structure from the dermis and the middle cylinder, the inner root sheath, moulds the growing hair into shape and guides it upwards.
The follicle
As the embryonic hair follicle begins to develop, small ‘buds’ begin to develop. One becomes the sebaceous gland, which produces oils that coat the hair as it grows and another forms what is called the hair follicle ‘bulge’. The bulge is the area of the hair follicle to which a small muscle, known as the ‘arrector pili’ will attach. The other end of the arrector pili muscle attaches to the underside of the epidermis above. When this muscle contracts, it causes the hair follicle to become more perpendicular and the hair stand upright. One of the reasons for this taking place is to trap air between the hair and skin which acts as an insulating layer in cold climates. This is why we get ‘goose
bumps’ when we are cold.
Ordinarily, the follicle lies at a slight angle to the outer surface of the skin. This means that the lower side of the follicle is slightly longer than the upper. When the follicle is pulled more upright by the arrector pili
muscle, the longer side inevitably is pushed slightly above the otherwise flat skin surface – ‘goose bumps’.
Sometimes this effect can be produced on leather, especially if a particularly astringent tannage is used which causes the hair muscle to contract.
The dermal papilla at the base of the hair follicle continuously produces cells that form the hair. Cells known as melanocytes produce a pigment known as melanin that colours the hair. As the hair cells move upwards in the follicle the melanin is also carried upwards in the inner part of the hair. The rate of hair growth is dependant on the amount of natural light, which varies according to the time of year: it grows more quickly in winter when the days are short. The hair cells that have just been produced are soft and malleable and are moulded into the hair shape by the more rigid hair follicle wall. At this stage the hair is made from what is termed ‘soft keratin’.
Hair shaft
When the hair cells reach approximately one third of the way up the follicle, they die and harden; a process known as keratinisation, which gives this region of the hair shaft its name of the ‘keratogenous zone’. The hair is now extremely tough and resistant to wear and referred to as ‘hard keratin’. Hard keratin also makes up feathers, claws, nails and hoofs. For the animal, such resistance is important but, for the tanner, it makes hair removal one of the more difficult processes in the manufacture of leather.
The outer layer of the hair, known as the cuticle, comprises of flat cells that overlap, rather like the tiles on a roof. The central part of the hair shaft is called the cortex which contains keratin and melanin pigment. Sometimes the cortex has a hollow centre known as the medulla. The medulla contains air that acts as an insulating layer and plays an important part in the regulation of body temperature. Animals that come from colder climates, eg reindeer, tend to have a larger hollow medulla in the hair. This can cause problems for the tanner in the early stages of processing since it is difficult to get the hides to submerge; the air inside the hairs makes them naturally buoyant.
Hair removal
In the majority of instances, it is vitally important that the tanner removes all traces of hair from the outer surface of the hide or skin and to a certain extent from within the hair follicle also. If hair removal is poor, process chemicals are not able to penetrate down the hair follicles properly, which is one their main entry points into the grain layer of the leather. Uneven colour and firmness often results if hair removal is poor.
The principle protein in hair, keratin, comprises of chains eighteen different amino acids. Its resistance towards chemical degradation is largely due to sulphur-sulphur bonds that link adjacent protein chains together. These links occur between one particular amino acid called cystine.
In order for the tanner to dissolve the hair away, the sulphur-sulphur bonds need to be broken. This can be achieved eventually with alkali alone, but the reaction is much faster if a ‘sharpening agent’ such as sodium sulphide is used. Once the sulphur-sulphur bonds have broken, the keratin protein is destabilised and the hair begins to dissolve as it is further decomposed by the alkali unhairing chemicals. This process is the conventional hair-burn or pulping system.
If hair is left for a long time in an alkaline solution without such sharpening agents, eg in just lime solution, the cystine amino acid undergoes chemical changes and is converted into another amino acid called lanthionine. Once this chemical change has taken place, the keratin protein is rendered much more resistant towards unhairing chemicals; it becomes ‘immunised’ and will not dissolve away.
Accidental immunisation of the hair can be problematic in hair-burn processes resulting in incomplete removal of the hair and consequently inadequate penetration of process chemicals. However, if properly controlled, it forms the basis of the hair-save method of unhairing. In a typical hair-save process the hair is immunised by exposure to high pH (usually lime) followed by the degradation of the unkeratinised portion of the hair found in the lower part of the follicle with either sodium sulphide/hydro-sulphide or enzymes. This releases the almost intact hair shaft from within the follicle for subsequent removal from the drum by filtration. Whilst hair-save methods do need to be more carefully controlled than hair-burn processes, there are benefits due to the reduced
levels of solid matter in the effluent.
References:
1. Hide and Skin – What’s in it for you?, Amanda Michel, p16, Leather International, April 2009
2. Diagram modified from The effects of hair shaving on unhairing reactions. Part 2: a new mechanism of unhairing, H A M El Baba, A D Covington, D Davighi. JSLTC Vol 84, Jan 2000.
