BLC Leather Technology Centre, discusses the problems of metallic staining during leather making. Corroded piping, poorly mantained machinery or rusty pallet fixings can all cause damage. Metallic agents used to make leather may also leave undesirable discolouration and reduce the tanners return on an otherwise quality product. Here the causes are discussed and some simple prevention measures suggested
Published: 16 March, 2003
The problem
Considering the amount of metal that comes into contact with leather during its manufacture, it is surprising that there are not more problems with metallic stains than there are.Metallic stains manifest themselves in a variety of guises ranging from subtle changes in shade to the most intense black stains.
The causes and prevention
Stains are commonly found to be caused by accidental contamination by iron or copper at inappropriate stages of processing, but sometimes they are caused by metals needed in the process, eg chromium.
Iron stains
Depending on the stage of processing at which the contamination occurs, iron stains can appear in many different colours.
If the contamination occurred during the beamhouse processing, then it is likely that black staining will occur due to the formation of the black iron sulfide.
Similar blue/black stains known as 'inks' can occur when iron reacts with vegetable tannins.
These stains are caused by iron tannates and very low levels of iron can cause problems; as little as 0.4mg/100ml in liquors can cause staining.
Ferric tannate precipitates to form a stain at pH2.7 and ferrous tannate precipitates at pH5.7. Therefore, if the iron is reduced to its ferrous form, eg by the use of bisulfite, then the risk of iron staining during vegetable tanning is minimised.
Alternatively, any stains can be removed by treating with an acid at the end of tannage eg with oxalic acid.
Brown stains can also occur with iron when it oxidises (rust). Often these stains will appear in liquor run marks where rusty water has flowed over the surface of the leather.
Pink is an unusual colour for a stain on leather, but iron can complex with chrome tanning chemicals to produce a pink discolouration.
There are many potential sources of iron contamination:
* rusty water or inlet pipes
* machinery, trucks or horses
* contaminated process chemicals
* blood in raw hides or skins
Copper stains
Like iron, copper also reacts with sulfide to produce black stains of copper sulfide which are very difficult to remove as they are insoluble in water and dilute acid.
Treatment with an oxidising agent can convert the insoluble copper sulfide to the more soluble copper sulfate but there is an increased risk of forming chrome VI on chrome tanned leathers with this method.
Sometimes copper stains appear as a black ring. This is known as the 'Leisegang effect' where dissolved copper diffuses out from a spot of contamination and is then precipitated as copper sulfide (Figure 1).
Often the depth of colour of copper sulfide staining is intensified by the presence of iron.
If hides are not thoroughly delimed, copper can react with the sulfide carried over and then complex with chromium sulfate used in tanning to produce a green stain. Since this most commonly occurs in the thicker areas of the hide, it is known as 'olive green neck'.
Chrome stains
Chrome stains usually appear as large blue/green stains often in a liquor run mark pattern. Common causes of this type of chrome stains are:
1. Uneven application of the chrome liquor. Ensure that liquors are thoroughly diluted or mixed in accordance with the suppliers instructions before adding to the process vessel via the axle, or if adding directly onto the skins through the door, that the vessel is rotated for at least ten minutes after the addition to ensure thorough mixing.
2. Patches of uneven pH at the time of tanning. Ensure that pickling is complete before tanning.
3. Heat damage. Heat degraded collagen will take up more chrome as the breaking of the bonds in the collagen molecule by the heat creates more sites for chrome fixation. Heat damage occurring due to hot liquors or undiluted acids can cause dark run marks to show up after tanning.
4. Pooling of chrome liquor. If pools of chrome liquor are allowed to stand on the surface of the skins after unloading from the tanning vessel, the chrome will continue to fix. Wash skins well at the end of tannage and preferably horse up to drain rather than leaving them in skips or tubs for long periods of time.
Sometimes chrome stains appear as small spots. This could be due to contamination of the surface with neat chrome powder or contamination with an alkaline powder during tannage causing rapid chrome fixation in the contaminated areas. Occasionally, these intense stains can give rise to dye resists later on in processing.
Chrome soaps can cause pink stains on wet-blue which can then go on to cause problems in dyeing (Figure 2). Chrome soaps are caused by the decomposition of natural fats to free fatty acids which react with chrome to give pink stains. Sometimes they can also inhibit the penetration of the tannage and cause raw streaks in the centre of the hide. Chrome soaps can be minimised by:
* Ensuring a fat free flesh surface by effective fleshing
* Storing raw hides and skins under cool conditions
* Effective use of fungicides/biocides
Putrefaction
One of the biggest problem in leather processing which sharply increased in summer and hot climate country like Pakistan.
The problem
As soon as a hide or skin is removed from the carcase, it is susceptible to bacterial attack. The bacteria penetrate into the skin via the exposed flesh surface where they can rapidly multiply. Under ideal conditions, a single bacterium can divide every 20 minutes and, therefore, within 24 hours will have multiplied to give a population of 4,000 million.
* Grain damage
* Weakness
* Looseness
* Staining
* Chrome soaps
* Spue
* Uneven dyeing
In addition, putrefaction can weaken the delicate grain surface and make it more susceptible to the rigours of normal leather production, leading to chemical damage and physical abrasion.
The cause
Soon after flaying the hide is moist, warm and full of protein - an ideal breeding ground for bacteria! The longer these conditions prevail, the more putrefactive damage will occur. Some common causes of putrefaction in hides and skins are:
1) A delay between flaying and curing/processing
2) Insufficient salting or brining
3) Poor penetration of the salt or brine due to heavy fat/flesh deposits
4) Insufficient draining of the salted or brined hide or skin allowing liquors to pool and reduce salt concentrations
5) Poor storage conditions of the raw salted hide or skin, eg exposed to the elements allowing salt to be washed off or excessively warm conditions
6) Prolonged storage, particularly under warm conditions
7) Soaking the raw hides or skins without sufficient biocide protection
Sometimes, even when hides have been well salted or brined, bacteria can still grow. These are a particular type of bacteria which are halophilic (salt loving) and are commonly coloured red or purple; affected hides are said to have 'red heat'.
Under normal storage conditions for raw hides or skins, red and purple heat bacteria take a relatively long time to grow; around two to three months. Therefore, their presence is an indication that the hides or skins have been in storage for some time. However, at higher temperatures (30-40ºC), growth will be more rapid. The warm, humid conditions favoured by red heat bacteria are also favoured by other non coloured spoilage bacteria, so if salt levels are not high enough, putrefactive bacteria may also be present.
It was once thought that red heat bacteria caused no harm to the hide, but it is now known that some types of bacteria do produce proteolytic enzymes which are capable of damaging collagen.
Although red heat bacteria are aerobic and, therefore, only grow on the surface of the hide or skin, the enzymes that they produce are mobile and can penetrate further into the skin structure.
Any proteolytic enzymes which may be produced by purple heat are even more likely to cause damage because they can proliferate within the hide structure.
Prevention
Rapid and appropriate curing is a key issue in the prevention of putrefaction: the hide should be cooled immediately after flaying in order to dissipate any remaining body heat, and kept cool until the hide is salted.
As soon as practicable, the hide should be treated with clean salt, in a quantity equivalent to no less than 35% of the green hide weight.
Mechanised agitation will produce a more rapid absorption of salt, while piling will call for a minimum of seven days in salt, and preferably 14 days, to effect the level of saturation required.
When salt penetration is adequate, the remaining salt should be 'biffed' off and a sprinkling of fresh salt applied before the hides are prepared for shipment in accordance with market practice.
Once at the tannery, the hides should be stored appropriately. Cool dry storage conditions for hides help maintain a good standard of preservation. Storage in the open, where hides are not covered, can cause the hides to heat up if in direct sunlight, encouraging bacterial activity and drying out.
Thermal damage may also occur to hides, particularly if stored in direct sunlight under clear polythene. Under humid atmospheric conditions, salt can absorb moisture from the atmosphere which results in a reduced salt content at the surface of the hide. Salt can also be lost from the hides by drainage of this moisture. Leaching out of salt can also occur where hides are left open to rain.
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