Size matters

Like the animals it comes from, leather is irregular in shape and does not lie flat. It is essentially a topographical surface whose irregularity tends to increase as the softness becomes greater. In addition, one of the biggest causes of variability in the area of leather is humidity or moisture level. These factors are the main causes for inaccuracy and reproducibility in leather area measurement. Use of different machine types and calibration add to the variability, particularly between customer and supplier, which can lead to disagreements and disputes. International standard using a pinwheel machine Historically, leather measurement has been performed by mechanical pinwheel, epitomised by the Turner machine. These machines consist of a driven feed roller; resting on this is a row of friction driven wheels, which contain pins in their rims. The pins drop into grooves in the drive roller. The leather passes between the feed roller and the pinwheels, causing the pins to be pressed up and down in response to the shape of the leather. As each pin is lifted, a series of mechanical linkages transmits this movement to the dial. As the leather passes through the machine, the accumulation of the pin movements is transmitted to create the area of the leather. Production machines The main disadvantage of the mechanical pinwheel is that the speed is insufficient for production purposes. Most tanneries prefer to use the quicker and more convenient plane projection system. This uses a row of photoelectric cells, which detect the light emitted from a row of bulbs positioned below. In between the two is a conveyor of nylon strings along which the leather passes, interrupting the beam of light, allowing the computer to calculate and record the area. The main advantages of this system are that operator experience is not required and the production speed is determined by the ability to load and unload the conveyor. The problem with these conveyor machines is that the wrinkles and creases are not opened out and fully accounted for. Soft leathers also exacerbate the problem by sagging slightly between the strings, which means that the photocells do not see and measure the true area. Because the conveyor machines do not hold the leather flat, they tend to under-measure by 3% to 5%. In practice, many tanners apply a 'factor' to allow for this under-measurement. This alone can create variables in the consistency of the measuring system, requiring regular calibration of the machine. Calibration Regular calibration of measuring machines is essential, but not always carried out. Consequently the results from similar machines can vary significantly. The variance of the electronic components can also differ over time. Electronic components do not always give a linear response and calibration of a machine with a 9ft² template cannot be expected to accurately measure 20ft² plus. These factors make it essential to calibrate machines on a daily basis, both before and after use. Moisture content The moisture content of the leather is not adequately controlled in many tanneries. Leather is a hygroscopic material that adjusts its moisture content until equilibrium is achieved between it and the ambient humidity. As the ambient moisture content changes, a corresponding change in the leather area occurs. The standard atmosphere used in ISO 11646 for leather measurement is 65% Relative Humidity. Measuring leathers at a relative humidity level below this, as happens in most tanneries, will result in under-measurement, with a consequent loss of profit. BLC provides a UKAS accredited leather measurement verification service to ISO 11646 IULTCS/ IUP 32 standard. In this process, leather is hung in a humidity controlled room at 20°C and 65% RH for at least 48 hours prior to being passed through a calibrated pinwheel. The verification process usually entails two operators feeding the specimen through the machine to ensure that it is kept as flat as possible. Case study A recent verification for a lambskin tanner showed that once the skins were measured under standard conditions of humidity and temperature, the tanner was found to be under-measuring by 9%. This equated to 0.5 sq ft per skin. With a production of 5,000 skins per day and assuming a modest selling price of $2 per sq ft, this tannery is losing $1 per skin, equating to an annual $1,000,000 of profit being lost! Conclusions and recommendations It is strongly recommended that: * Measuring machines are regularly calibrated at the beginning and end of each working period * Moisture content and humidity are subject to greater control than is currently achieved in most tanneries * Tanners regularly check their measuring procedures to ensure that they are maximising the area yield (and profit) but also ensuring that they are not over-measuring The BLC independent verification service is available to both consumers and producers, offering the opportunity to: * optimise area sold * verify internal leather measurement * resolve disputes * check leather area purchased in addition to standard template * calibration and verification For more details contact Stuart Booth on stuart@blcleathertech.com or tel +44 1604 679956