Alleviating vein defects5 June 2008
Veinous defects on the surface of the leather have been familiar to tanners for many years. CTC has evaluated which stages of processing a box-calf leather offer opportunities to alleviate these defects whilst conserving the functionalities required from this type of leather.
The veinous network can appear in a very superficial and thin layer. It becomes more pronounced in skins which are naturally thin. Theories as to the cause of this defect are numerous: the age of the animal, the climate, ‘the species', its diet, the period of slaughter, the conservation method, the manufacturing process, are factors thought to have a degree of influence on the development of this defect. Characterisation of the vein The vein can be compared to a tube which is either empty or full of blood and which is located in the deeper or very superficial layers of the dermis. When the skin is subjected to dehydration and the production processes, this tube is flattened and stands out in a fairly obvious way. The closer the network is situated to the surface, the more difficult it is for the tanner to hide the effect whilst conserving a transparent surface imposed by certain finishing processes, notably those for a box-calf article as shown. Analysis of the structure of the defect The following illustrations show the main structural and optical characteristics of the vein. General presentation of the vein defect The photos taken with the vision machine and automatic classification machine Selectra 3000, illustrate this defect from a global perspective on a skin. CTC observed a skin in the wet-blue stage and semi-finished condition (below). Drying renders the vein less prominent in the semi-finished skin but finishing operations generally make it reappear straight away. Scanning electron microscopy Scanning electron microscopy enables one to enlarge the surface of the skin up to 30,000 times and, therefore, observe the fibres from a diameter of 0.1 µm. Analysis by scanning electron microscopy of cross-sections shows the difference in size between a thick vein (around 300µm in width) and a finer vein (around 175µm wide). Optical microscopy The optical microscopy used allowed CTC to enlarge the surface up to 60 times. It is less precise than the previous equipment but, nevertheless, sufficient to analyse the main characteristics of the veins (location in the cross-section and average width). Observation of veins at different levels on the grain of the leather were made. CTC split the leather at a variable distance from the grain (under 0.15mm, under 0.3mm, under 0.5mm) and the photographs can be seen on the right. Advancement of work The first phase of the study consisted in identification of the stages in which the defect could be mitigated during processing of box type leather. Starting with one production type, an initial series of tests were carried out on calf skins which were at different stages of production. The skins were never veined in the same way. Throughout the study, CTC compared the strips onto which they had added their own variants, with the constant/control strips processed according to the classic tanning process. The main factors that were studied up until now are as follows:
- the impact of unhairing
- the impact of the shaving thickness (13/15, 16/18, 22/24)
- the impact of samming and setting (with and without)
- the impact of the drying method (drying under glass, hot air, straining)
- the impact of mechanical finishing treatments (glazing/ rotopress)