Don’t be Vein - defects15 November 2007
Defects are one of the biggest sources of non-conformity in leather production. Some of the defects are inherent in the live animal and the tanner is not able to measure the quality of their incoming raw material.
Veins – the problem
Veininess is attributed to hides and occurs where blood vessels can be seen in the wet-blue or finished leathers. This creates aesthetic problems, resulting in downgrading and often rejection of finished leathers. These very prominent blood vessels in leather are not only unsightly, affecting cutting value, but can also cause physical problems.
They can sometimes cause weakness which, as the leather is continually flexed, gives rise to deep creases spoiling the even break of the leather. This applies to veins and arteries, particularly those near the surface of the hide or skin.
During the life of the animal veins and other blood vessels in the skin play several roles. Arteries carry blood to the skin where it provides oxygen and nutrients, veins carry the deoxygenated blood back to the lungs and heart.
Blood vessels are surrounded by a naturally loose fibre structure containing large quantities of elastin to allow the constant expansion and contraction of the vessels during the animal’s life. Veins, on the contrary, return blood to the heart and have significantly thinner walls. A valley effect is more likely to occur with the veins, whereas a rod-like effect is more associated with arteries.
Blood vessels have a distinctive orientation within the hide and skin, being horizontal near the flesh and just below the grain, but have vertical branches joining these horizontal ones.
When the leather is split or shaved it normally cuts across the vertical vessels. However, sometimes the cut falls in line with the horizontal vessels making the veins clearly visible, particularly on the suede side.
Possible causes of prominent veins;
• Poor bleeding after slaughter will leave the vessels in a distended state accentuating their appearance
• If the animal is distressed or hot when slaughtered this causes an increase in blood flow accentuating the veins
• Putrefaction and certain stages of chemical processing can accentuate the natural loose structure around the blood vessels, causing the grain surface to sink and making the veins more prominent
• The sex of the animal. In milking cows veins are more apparent along the belly and shoulder, remaining visible after slaughter
• The diet of the animal and the season of slaughter. In grazing cattle veins are very marked along the belly, but practically invisible in calves. In between the two ages, the veininess falls between the two extremes
• Breed. In certain animals the vein pattern develops in the upper layer of the skin
• Time of slaughter. Animals slaughtered in winter have coarser grain with more visible veins
(a) Live Animal
Washing the animal in cold water or some other method of cooling prior to slaughter can help control dilation of the blood vessels. (It is recognised that the practice of using water is not permitted in some geographical areas for hygiene reasons)
The visibility of the blood vessels can be minimised by ensuring that the fibre structure is well opened by adequate beamhouse processing. The length of soaking of fresh hide is particularly important.
Controlling the swelling at liming is also crucial. The process has to be adequate to remove the hair and open up the fibre structure, but keep the alkalinity as low as possible to reduce swelling. Swelling can accentuate the looseness around the blood vessels resulting in greater visibility of the veins. It may be beneficial to add salt to inhibit the swelling.
Washing after liming is also important as the already swollen hide may take on more water creating further swelling. A 2-3% offer of salt or spent delime/bate liquors may be added at this point.
The vessel walls are surrounded by substantial amounts of elastin. Therefore, excessive breakdown of the elastin in bating (or by using elastolytic enzymes) can cause voids around the vessels. This can be minimised by the selection of a non-elastolytic enzyme or, if an elastolytic enzyme is used, controlling the amounts offered and ensuring correct processing conditions are employed. BLC can test for the presence of elastase.
Chrome tanned leather is more susceptible to ‘veininess’ since, unlike vegetable tannages, it has no real filling action.
Retanning and fatliquoring can also have an influence on the visibility of veins. Good fatliquor penetration is required to ensure that the fibres don’t stick together during drying. Correct choice of retanning/filling agents to fill the loose structure surrounding the blood vessels can minimise the impact of the visibility of the veininess.
To minimise the effect of veininess it is important to ensure adequate conditions prior to softening. If leathers are too dry at milling, the veins will act as a point of weakness and the constant flexing will accentuate the vein pattern. Improvements in the appearance of veins can also be made using mechanical processes.
As with many leather manufacturing problems there is no simple solution, but attention to detail and processing is required at each stage.
For further information contact BLC Leather Technology Centre Ltd on [email protected].