As a ‘rule of thumb’, the hair or wool should be sufficiently tight to enable the whole skin or part of a hide to be lifted from the surface by the hair or wool without any slipping. Hair slip is an indicator that the preservation of the hide or skin is not good and that the resulting leather is likely to show evidence of putrefactive damage, eg grain damage or weakness.
The cause
Most commonly, the enzymes that cause hair slip originate from bacteria, but the same effect can be caused by certain enzymes added in the process, eg soaking enzymes or enzyme assisted unhairing if used to excess. The proteolytic enzymes, like the tannery’s alkaline unhairing system, cause degradation to the hair beginning in the less stable unkeratinised parts of the hair which is usually the lower third of the hair shaft (Figure 1). Once this part of the hair has been degraded, the hair shaft can be pulled from the skin almost intact. In an ideal world, this would be considered an excellent method of unhairing since it would reduce the use of other unhairing chemicals and hair recovery would be easy thus reducing costs and effluent problems.
Indeed, ‘controlled’ hair slip, known as sweating, has been used with effect on sheepskins. Studies into the sweating of sheepskins at BLMRA1 (BLC’s former name) found that a species of Psuedonomas which penetrated the skin from the flesh side was responsible for effective wool loosening during the sweating process. The presence of nematode worms was also found to be beneficial, possibly by distributing and controlling the bacteria population. However, the world is far from ideal and, whilst many sheepskins are unhaired successfully by the sweating process, the effect is not uniform and some skins within a batch do not unhair well and others get damaged by putrefaction. Studies at BLC have been undertaken to help understand the mechanism of hair slip in more detail with a view to eventually developing a chemical-free unhairing process2.
In more advanced hair slip, the proteolytic enzymes begin to degrade the more stable keratinised parts of the hair. In the keratinised part of the hair, the walls of the cortex cells, which comprise almost 90% of the hair mass, have been found to be relatively unaffected by alkali, but are more easily broken down by proteolytic enzymes. Consequently, hair in the follicle broken down by enzymes often has a ‘splintered’ appearance as the long narrow cortex cells are separated (Figure 2). Conventional tannery unhairing systems utilising alkali treatments show a quite different type of hair breakdown as these target the sulfur bonds in the keratin protein3.
If the enzymes from putrefactive bacteria have penetrated far enough into the hide or skin to cause this type of hair degradation, it is an indication that putrefaction is relatively advanced and some damage to the leather is likely. Eventually, the enzymes will have penetrated all of the way through from the flesh side of the hide or skin and then the epidermis is likely to slip as well as the hair. At this stage, extensive damage to the leather is likely.
As in life, the epidermis on the outer surface of the skin acts as an impenetrable barrier against bacteria and many chemicals. Consequently, unless there are any breaks in the epidermis due to disease or injury, putrefactive bacteria penetrate the hide after flaying from the flesh side.
Some types of bacteria do not penetrate because they need air to survive, ie aerobic bacteria. However, the putrefactive enzymes that some aerobic bacteria produce can easily penetrate the hide and cause hair slip to occur. Therefore, the absence of bacteria within the hide does not necessarily discount putrefaction as the cause.
Since putrefaction is the most common cause of hair slip, the prevention of bacterial growth is the key to preventing it. Putrefaction in hides can occur due to:
i) A delay between flaying and curing/processing the raw hide. Research has shown that degradation of the hair shaft in the unkeratinised area begins within 2 hours of flaying and hair slip is evident after 12 hours if the hide is not cooled2
ii) Insufficient curing of the raw hide. Ideally, 35% of the green hide weight of salt should be used for salting
iii) Poor penetration of the salt due to heavy fat/flesh deposits. Heavy fat deposits should be removed, not only because they impede the penetration of salt, but their breakdown products (fatty acids) can lead to problems later in processing, eg chrome soaps
iv) Insufficient draining of the salted/brined hide allowing liquors to pool and reduce salt concentrations. Hides and skins should be piled to drain for at least a week to allow saturation to occur. When the salt has fully penetrated, they should be re-salted and folded for storage
v) Poor storage conditions of the raw salted/brined hide, eg exposed to the elements allowing salt to be washed off or excessively warm conditions. Keep cured hides and skins under cover and as cool as possible. If outside storage is necessary, do not cover with clear polythene as this can cause damaging high temperatures in direct sunlight
vi) Prolonged storage, particularly under warm conditions
vii) Soaking the raw hides without sufficient biocide protection. An effective short dirt soak to remove excess dung and blood will reduce bacterial loading followed by a main soak containing sufficient biocide to minimise bacterial growth. This can be easily monitored by using dipslides.
If proteolytic enzymes are used in the process, it is important to ensure that they are used at the correct level, temperature and pH. Your chemical supplier should be able to advise.