Mould - cause and prevention

5 February 2001

The problem
Like bacteria, mould spores are everywhere and most of the time they cause no harm. However, every now and again mould raises its ugly head and can cause untold damage. Not only is mould growth unsightly, it can be a health hazard.

The pigments produced by some moulds are virtually impossible to remove, they can cause problems with dye uptake and cause fatty spues. Most responsible tanners use a suitable level of an effective fungicide to protect their leather whilst it is in process or if it is to be transported in the wet state. But even then, sometimes the mould wins. Mould is not only a problem on wet part processed leather; with the increasing manufacture of leathergoods in hot and humid countries, we are seeing more and more finished leather items which become mouldy during transit. The cause Mould thrives in moist, warm conditions with little movement of air. Unlike most synthetic fibres, leather readily equilibrates to the moisture content of the surrounding atmosphere, which in some countries can exceed 90% relative humidity. The minimum level of relative humidity for the inhibition of mould growth is about 70%. The moisture in the leather when at equilibrium with the atmosphere will depend on many factors such as the type of tannage and protein content. However, the moisture uptake by a typical chrome tanned leather from the atmosphere increases significantly above 65% relative humidity (Figure 2). So, when leather with a high moisture content resulting from exposure to high relative humidity in tropical climates is wrapped in a non-porous material such as polythene, the moisture within the leather is trapped. When the ambient temperature changes, either as a result of climatic change or shipment of the goods to another part of the world, the moisture can condense, most probably on the inside of the packaging material causing localised wetting. This could raise the moisture content of the leather in that area creating a micro-climate that is favourable for mould growth. Prevention The following key points are recommended for leather or leathergoods for export: * Leathergoods should not be close wrapped in polythene, but a material which allows the leather to 'breathe' should be used, or adequate ventilation holes be provided in the polythene if used. * If possible, ensure that the leather is as dry as possible prior to packaging, ideally to below 20% moisture content. * Ideal storage conditions for leather are 20-25°C and 50-65% relative humidity.  

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