Is leather made in the beamhouse?26 November 2009
Common problems in beamhouse processing are discussed by Amanda Michel of Leather Wise
It is often said that leather is made in the beamhouse. Personally, I regard that to be a generally true statement in as much as the final physical qualities of the leather are dictated to a significant degree by the processing that the hide or skin receives prior to tanning. However, I am also acutely aware that leather can just as easily be ruined in the beamhouse. This article discusses some, but by no means all, of common problems that manifest themselves in the final leather that have their origins in the beamhouse.
The beamhouse incorporates several different processes. According to the International Union of Leather Technologists and Chemists Societies’ glossary of leather terms, the beamhouse is the ‘section of the tannery where hides or skins are prepared for tanning, which includes the operations of soaking, unhairing, fleshing and deliming etc’, so there are plenty of opportunities for things to go wrong. Let us start at the beginning:
It should be remembered that no matter how fresh or well preserved the raw hides or skins are, putrefaction can still occur if they are not soaked properly at the start of beamhouse processing. Ideally, following a short dirt soak to remove free salt from the surface of the hide and blood or dung contamination, a more extensive soak should take place in the presence of a good bactericide. The effectiveness of soaking bactericides can be monitored by using dipslides. These are plastic strips that have a coating of agar. After dipping into the soak liquor they can be incubated overnight and then the number of colonies of bacteria that grow on the agar estimated. If bacterial growth is not effectively controlled during soaking, grain damage may ensue.
Most unhairing systems are based on the use of lime, sodium sulphide and sodium hydrosulphide – all highly alkaline chemicals. When we add alkalis to collagen it becomes more negatively charged causing the collagen molecules to repel each other thus forcing themselves apart. When the collagen molecules are pushed apart in this way, more water is taken up to fill the now larger gap between them, ie swelling occurs. This is why hides become thick and turgid during liming. Because the molecules have been pushed apart by the swelling effect, they are now much less stable and, as such, much more easily damaged by abrasion and heat. Therefore, when hides and skins are at liming pH they must not be exposed to high temperatures otherwise the normally robust collagen structure will be irreversibly damaged; the shrinkage temperature of limed collagen can be as low 35°C. So, if temperatures rise much above 30°C there is a serious risk of the delicate grain surface becoming damaged.
Whilst the swelling that occurs in the liming process makes the collagen vulnerable, it is an important part of the leather making process as it helps to remove much of the inter-fibrilliary matter such as albumins and globulins. If not properly removed, these substances act like glue sticking the leather fibres together when dried resulting in firm, inflexible leather.
However, an excessive amount of swelling can result in too much separation of the fibres which causes an over-opening up of the structure and looseness. The osmotic uptake of water in the hide increases significantly above pH 12.0. If excessive amounts of sodium sulphide are used in the liming process it can react with water to produce sodium hydroxide, which can raise the pH above 12.0; over swelling is the result. Replacing some of the sodium sulphide with sodium hydrosulphide can resolve this problem.
Other problems caused by over swelling during liming include prominent growth marks on bovine hides and ‘leopard grain’ in sheepskins. The grain layer tends to swell more than the corium structure which, because of the naturally loose structure between the grain and corium layers in woolled sheep, tends to force the grain up into small raised areas. Because of the grain’s vulnerability towards abrasion in the limed state, the top of these raised areas can become abraded. This results in dull spots on the final leather that often appear a different colour, hence the term ‘leopard grain’.
Apart from opening up the fibre structure and softening the leather, the other principle purpose of liming is to remove the hair and epidermis. If this is not successfully achieved, the penetration of later process chemicals is impeded, resulting in stains on the grain surface.
If limed hides are exposed to air for a long time, the lime (calcium hydroxide) can react with carbon dioxide in the atmosphere to produce insoluble calcium carbonate. To prevent this happening, the liming vessel should not be left static for too long and when limed hides are unloaded in the limed state, eg for fleshing or splitting, they should be covered in polythene or a hide turned grain side down to prevent exposing the grain surface to the air.
Deliming reduces the pH to that suitable for bating and is normally achieved by the use of ammonium salts or carbon dioxide. Apart from a suitable pH, bating enzymes also need warmth to work effectively. The ideal temperature for bating is 35°C, but it is very important that deliming is not started at such a high temperature – remember, collagen at liming pH can be damaged at this temperature. So, the very start of deliming must be carried out cold until the pH of the delicate grain surface has been reduced. The temperature can then be raised without risk of grain damage.
If deliming is not properly conducted, lime (calcium hydroxide) may remain which can subsequently react with sulphuric acid used in pickling to produce insoluble calcium sulphate. This causes white stains seen on the wet-blue. The progress of the deliming process should be monitored by using a suitable indicator to a cut in the thicker neck area. Phenolphthalein is clear between pH 8.2 and 10.0 and is therefore suitable for checking hides delimed with ammonium salts, which will have a pH around 8.5. However, CO2 deliming is normally conducted around pH 7.0 in order to form soluble calcium bicarbonate rather than the less soluble calcium carbonate. It should be noted that phenolphthalein will not distinguish pHs below pH 8.2. Therefore, CO2 delimed hides should be checked with bromo thymol blue which is clear between pH 6.0 and 7.6.
From this brief synopsis of the potential pitfalls that the beamhouse holds for the tanner, it can be seen that whether or not leather is made in the beamhouse, there is no doubt that this stage of processing is critical and needs the utmost control in order to prevent damage that can persist into the final leather rendering it useless.