Abstract

One of the main criteria for assessing leather production is the yield of finished leather, and as most leather is sold by area, yield is generally expressed as the area of leather produced from a given quantity of raw material. The influence of processing conditions on yield is under constant observation and differences in yield as small as 1% are important in terms of financial return. Therefore it is essential that either yields can be calculated sufficiently precisely in order to detect such small differences, or that the limitations to the calculation are fully appreciated. Many fluctuating factors enter this calculation, and the most important is the basis on which the raw material is quantified. In arriving at the tail weight as defined in BS 3935:1974 a visual assessment of dung and fat is made and it is inevitable that such an assessment will lack precision and that differences between hide markets will exist. Data collected from 477 hides received from two hide markets revealed a difference of 6% in the area of raw hide per lb tail weight depending on the hide market origin. The fleshed weight is therefore a more accurate basis for an estimate of potential leather-making raw material. Even so, there is a poor degree of correlation between the fleshed weight and the area of a hide. From the data collected, considerable differences in thickness were found between individual hides within the same weight range: in the butt, hides varied between 3.5 and 6.5 mm. It is obvious, then, that hides of the same weight are likely to vary widely in area, and it was found that a hide, for example, weighing 22.5 kg when fleshed could vary between 37 and 47 ft², i.e., ± 12% of the total area. This poor degree of correlation between weight and area points to the fleshed weight as being an insufficiently precise basis for calculation of yield if small changes in yield need to be detected with any degree of confidence. A high degree of correlation was found between the area of a raw hide and the area of the final leather, and for this reason the raw area would appear to be a far more precise basis for calculation of yield. A change in the basis of sale of raw hide from one of weight to one of area should greatly simplify internal costing. Firstly, the quantity of raw material purchased would be unaffected by the presence of dung and fat. Secondly, the prediction of yield of grain split from any given purchase would be more precise. A change of this kind would require hides to be classified into ranges according to thickness, so producing packs of hides of relatively uniform thickness. This would have the advantage of uniform action of chemicals and simplifying the prediction of flesh split yield from any purchase lot. Basing yield on raw area would allow small changes in yield between packs to be detected with greater confidence than when yield is based on weight. Consequently in the regular monitoring of yield of packs processed in the same way, real changes in yield due to unscheduled changes in processing are likely to be detected at an earlier stage. One of the difficulties of applying a process control of this kind to chrome side production is that packs are sorted at the wet-blue stage and hides from the same pack may be retanned in several different ways. Under these circumstances it would be particularly valuable to measure the area of the wet-blue after sorting. This would serve as a reliable basis on which to calculate the yield of the finished leather.