The cause
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 and veins carry the deoxygenated blood back to the lungs and heart. The small capillaries which are close to the skin surface not only distribute the blood to where it is required, but also act as a heat regulatory system; they become dilated when the body is hot, allowing more blood close to the surface of the skin where the heat can easily dissipate. Blood vessels are surrounded by a naturally loose fibre structure containing a lot of elastin (a stretchy, fibrous protein) to permit the constant expansion and contraction of the vessels during life (Figure 2).
Blood vessels have a distinctive orientation within the skin, being horizontal near the flesh side and just below the grain layer, with vertical vessels joining the horizontal ones (Figure 3). When the leather is split and shaved to the required substance, it normally cuts across the vertical vessels. However, if thin skin is being used to make a thick leather, or a thick skin being used to make a thin leather, there is a risk that the splitting may occur at the same location as the horizontally aligned vessels. In such cases, the veins become clearly visible on the suede side of the leather.
Blood vessels which are very prominent on the grain side can occur for a variety of reasons:
i) If the animal is slaughtered when very hot or distressed
ii) Poor bleeding after slaughter
iii) Putrefaction and certain processing conditions can accentuate the naturally loose structure surrounding blood vessels and cause it to become more visible or allow the grain surface above to sink
Prevention of veininess
Accentuating the visibility of blood vessels during processing can be minimised by ensuring that the fibre structure is well opened up evenly throughout by the provision of adequate beamhouse processing. Excessive swelling during liming can accentuate the loose structure around the blood vessels, so swelling should be controlled.
The vessel walls are surrounded by a substantial amount of elastin, therefore, excessive elastin breakdown during bating can cause voids around the vessels. This could be minimised by selecting a non-elastolytic enzyme or, if an elastolytic enzyme is used, ensuring that it is used at an appropriate concentration for the processing conditions employed. Similarly, enzymes produced by putrefactive bacteria can also cause excessive breakdown of the elastin and over-opening up of the fibre structure.
Retanning and fatliquoring can also influence the visibility of veins. Good fatliquor penetration ensures that re-sticking of the fibres upon drying is minimised. Some retanning agents and filling agents fill the loose structure surrounding the blood vessels, thus minimising the impact it can have upon the visibility of veininess.
