Growing Pains28 May 2008
Growth lines, growth marks or growthiness are terms used to describe lines in leather that extend across the hide perpendicular to the backbone. Cattle hides display this feature most but they also occur in small skins such as goat. They are sometimes also described as neck wrinkles or fat wrinkles.
Sometimes these lines are flat but very often they form a depression or channel in the leather that can be up to 2mm deep. They tend to be most prevalent in the neck and shoulder area of the hide. Growth lines are an inherent feature of the hide and, therefore, it would be wrong to describe them as a defect as such. Describing them as a problem is a matter of perception - some like them because they are a natural feature and, therefore, give each hide a unique character. However, others loathe them because they make matching panels or components difficult and, therefore, reduce cutting efficiency. Being an inherent feature, there is nothing that the tanner can do to avoid them. However, there are process factors that can influence their appearance, both for the better and for the worse. What are growth lines? To describe these lines in leather as ‘Growth Lines' is a little erroneous since their origin is not entirely down to the growth of the animal, although age does have some influence on their appearance. Actually, they are a result of constant bending and flexing of the hide or skin. The hides and skins that we use for leather making are invariably from grazing animals and, as such, they spend a significant amount of their time grazing and browsing on grass and other vegetation. This inevitably involves a great deal of bending and stretching of their neck. The skin in this area has to be able to accommodate all this movement which it does by forming deep creases in the skin that stretch out flat as the neck is extended. Sometimes much fainter and less deep lines can be seen further down from the neck and shoulder area. These are most commonly found on the hides and skins from younger animals and are a result of the skin on younger animals always being a little over-sized. By the time they reach adulthood, they have grown into their skin and these folds taken up. Minimising the impact For the tanner, the problem of growth lines in the neck is compounded by the differences in structure of the skin in this area. The fibre structure is denser and the skin much thicker in this region, partly as a defence mechanism against predators. This makes it more difficult for the tanner to get his process chemicals to penetrate into the neck area. Consequently, it is more difficult to open up the fibre structure and allow the growth lines to relax. There is also more elastin to be found in the growth lines. Elastin is a fibrous protein found in abundance in the grain layer of skin and, as its name suggests, helps to maintain the skin's elasticity. The neck has a good deal more stretching to do than some other areas of the hide, hence more elastin, especially in the growth lines. The problem for the tanner is that once tanned, elastin can become quite hard thus making the growth lines more pronounced. For the tanner to minimise the appearance of growth lines it is important that in the early stages of processing he does nothing to increase the depth of the lines and gives them every opportunity to relax and flatten out. Then, in later processing, he maximises the potential for the dyeing and finishing processes to conceal them. An holistic approach is best; there is no single magic wand that will make them go away. Because of the thicker structure in the neck area, a thorough soaking process is important. If the neck is not well rehydrated and the soluble proteins not properly removed, the structure will always remain firm and unrelaxed. Green fleshing and the use of soaking agents may be beneficial. During unhairing, it is important that the hides or skins are not permitted to swell or plump excessively. This will only cause the growth lines to become deeper and hair and scud to become trapped in the grain leading to un-level dyeing which will accentuate the appearance of the growth lines in later processing. Excessive swelling during unhairing can be minimised by:
- Keeping drum speeds down to a level that ensures adequate unhairing and mixing of chemicals, but no more than that. Slow speeds and intermittent agitation, eg 5 minutes every half hour, are ideal. Beware of extended draining times at the end of liming; continuous running in low floats, or no float at all, is sure to result in excessive swelling.
- Keep the amount of alkali in the liming process sufficient to enable adequate unhairing, but not excessive. It may be possible to reduce the amount of sodium sulfide or hydrosulfide by the use of unhairing assists or enzymes.
- Liming in warm liquors can help the skin relax, but it is very important that the temperature does not exceed 28°C during the liming process or early part of deliming, otherwise grain damage may result.
- After unhairing, the addition of a small amount of salt during the wash before deliming can help minimise swelling.