Problems with staining

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