|Infobox on Leather|
|Example of Leather|
|Stowage factor (in m3/t)|
|Humidity / moisture||-|
|Risk factors||See text|
The leather may be in the form of wet chrome tanned material, or dried leather without a surface finish and requiring further finishing treatment (crust or rough leather) or leather with a surface finish requiring no further processing (finished leather). Then there are articles made of leather – gloves, footwear, bags, cases, etc.
The leather manufacturing process is divided into three fundamental sub-processes: preparatory stages, tanning, and crusting. All true leathers will undergo these sub-processes. A further sub-process, surface coating, can be added into the leather process sequence, but not all leathers receive surface treatment. Since many types of leather exist, it is difficult to create a list of operations that all leathers must undergo.
The preparatory stages are when the hide/skin is prepared for tanning. Preparatory stages may include: preservation, soaking, liming, unhairing, fleshing, splitting, reliming, deliming, bating, degreasing, frizing, bleaching, pickling, and depickling.
Tanning is the process which matches the protein of the raw hide or skin into a stable material which will not putrefy and is suitable for a wide variety of end applications. The principal difference between raw hides and tanned hides is that raw hides dry out to form a hard inflexible material that when re-wetted (or wetted back) putrefy, while tanned material dries out to a flexible form that does not become putrid when wetted back. Many different tanning methods and materials can be used; the choice is ultimately dependent on the end application of the leather. The most commonly used tanning material is chromium, which leaves the leather, once tanned, a pale blue colour (due to the chromium); this product is commonly called "wet blue". The hides once they have finished pickling will typically be between pH 2.8 and 3.2. At this point, the hides would be loaded in a drum and immersed in a float containing the tanning liquor. The hides are allowed to soak (while the drum slowly rotates about its axle) and the tanning liquor slowly penetrates through the full substance of the hide. Regular checks will be made to see the penetration by cutting the cross-section of a hide and observing the degree of penetration. Once an even degree of penetration exists, the pH of the float is slowly raised in a process called basification. This basification process fixes the tanning material to the leather and the more tanning material fixed, the higher the hydrothermal stability and increased shrinkage temperature resistance of the leather. The pH of the leather when chrome tanned would typically finish somewhere between 3.8 and 4.2.
Crusting is the process by which the hide/skin is thinned, retanned, and lubricated. Often a colouring operation is included in the crusting sub-process. The chemicals added during crusting must be fixed in place. The culmination of the crusting sub-process is the drying and softening operations. Crusting may include the following operations: wetting back, sammying, splitting, shaving, rechroming, neutralization, retanning, dyeing, fatliquoring, filling, stuffing, stripping, whitening, fixating, setting, drying, conditioning, milling, staking, and buffing.
For some leathers, a surface coating is applied. Tanners refer to this as finishing. Finishing operations may include: oiling, brushing, padding, impregnation, buffing, spraying, roller coating, curtain coating, polishing, plating, embossing, ironing, ironing/combing (for hair-on), glazing, and tumbling.
In general, leather is sold in four forms:
- Full-grain leather refers to hides that have not been sanded, buffed, or snuffed (as opposed to top-grain or corrected leather) to remove imperfections (or natural marks) on the surface of the hide. The grain remains allowing the fiber strength and durability. The grain also has breathability, resulting in less moisture from prolonged contact. Rather than wearing out, it will develop a patina over time. High quality leather furniture and footwear are often made from full-grain leather. Full-grain leathers are typically available in two finish types: aniline and semi-aniline.
- Top-grain leather (the most common type used in high-end leather products) is the second-highest quality. It has had the "split" layer separated away, making it thinner and more pliable than full-grain. Its surface has been sanded and a finish coat added to the surface which results in a colder, plastic feel with less breathability, and it will not develop a natural patina. It is typically less expensive and has greater resistance to stains than full-grain leather, so long as the finish remains unbroken.
- Corrected-grain leather is any leather that has had an artificial grain applied to its surface. The hides used to create corrected leather do not meet the standards for use in creating vegetable-tanned or aniline leather. The imperfections are corrected or sanded off, and an artificial grain impressed into the surface and dressed with stain or dyes. Most corrected-grain leather is used to make pigmented leather as the solid pigment helps hide the corrections or imperfections. Corrected grain leathers can mainly be bought as two finish types: semi-aniline and pigmented.
- Split leather is leather created from the fibrous part of the hide left once the top-grain of the rawhide has been separated from the hide. During the splitting operation, the top-grain and drop split are separated. The drop split can be further split (thickness allowing) into a middle split and a flesh split. In very thick hides, the middle split can be separated into multiple layers until the thickness prevents further splitting. Split leather then has an artificial layer applied to the surface of the split and is embossed with a leather grain (bycast leather). Splits are also used to create suede. The strongest suedes are usually made from grain splits (that have the grain completely removed) or from the flesh split that has been shaved to the correct thickness. Suede is "fuzzy" on both sides. Manufacturers use a variety of techniques to make suede from full-grain. A reversed suede is a grained leather that has been designed into the leather article with the grain facing away from the visible surface. It is not considered to be a true form of suede.
Shipment / Storage / Risk factors
Mould growth on leather
Generally, leather will contain a fungicide to protect against mould growth, however under adverse storage conditions and inadequate fungicidal protection mould may develop on the leather. Mould growth is favoured by the leather being made wet or by humid conditions ( a relative humidity greater than 70%) and warmth. Temperature changes moisture content and humidity. Leather at 20°C and normal humidity, 65% RH, will contain 14% moisture. If packed in polythene or sealed containers and the temperature rises to say 40°C, moisture is given off, can condense on the sides of the polythene or container and drip back on the leather giving localised high humidity/moisture sufficient to cause mould. Desiccants in such packages are only effective if the leather is well dried before packing. If temperatures can be maintained within reasonable limits, -10°C to + 10°C, then such problems should not arise. Moulds live on the water soluble component of vegetable tans, sugars and grease present in the leather. Because of this, vegetable tanned leathers are more susceptible to mould growth than chrome tanned leathers, as are also leathers with a high grease content. The physical properties of leather, such as tear strength, are usually unaffected by mould growth unless it is heavy and prolonged. Mould can be cleaned from the leather surface which may appear undamaged. Alternatively the surface can be frequently permanently discoloured by mould, and where the leather is to received further processing, such as dyeing and finishing, mould growth can give rise to subsequent faults such as irregular dyeing and finishing, or a fatty acid spue due to degradation of the fats. Mould will also grow on articles made from leather if the storage conditions are adverse. Gloves should be packed in dry waterproof containers to prevent water contamination as with other leather articles such as bags, cases etc.
Leather will absorb or lose moisture to the atmosphere depending on the relative humidity of the atmosphere. Shoes which have been packed in apparent good order and condition may be delivered at the destination with mould damage. Such damage may be caused either by an excess of moisture in the leather during the manufacture of the shoes or holding under damp conditions before packing for export. In some instances mould may be confined to the leather soles of the shoes, either on the inside or outside or both. Another cause of mould growth may be the high moisture content of the packing material, e.g. paper or cork. The natural moisture content of the leather, or cork or wooden components of the shoe, may cause the rusting of buckled and other metal trim, and care is necessary in attributing a cause in this connection. The adequate lacquering of metallic components reduces the likelihood of such problems.
Wet Chrome Tanned Leather
Whole hides or skins or splits are packed in the wet condition. Generally they contain fungicide to protect against mould growth and are enclosed by an impervious covering to prevent drying out. Damage to the covering, burst bales etc., will allow the leather to dry out and to lose weight. Whilst some drying can be overcome, thoroughly dried out ‘wet blue’ is unlikely to give a satisfactory finished leather.