Factors that affect the area yield of leather in a tannery

As most tanners will know, the area or the yield of leather that is produced from the hides and skins that are processed in the tannery has a major affect on the profitability of the tannery. It is therefore important that tanners try to gain as much area of leather from the kilograms of hide processed. Most tanneries have a low return on investment, and in order to make tanneries sustainable, getting a good yield from the hides processed is essential.

There is no one single factor that determines an improved area yield in a tannery. Yield is influenced by a combination of both chemical and physical factors, and has to be built into the leather step-by-step through the various stages in a tannery. 

 

The method of curing has an affect on the area yield. Freshly slaughtered green hides produce the greatest area of leather, followed by brine cured hides, then wet salted hides, and lastly dried hides. Obviously, the tanner may not have a lot of control over the type of curing that takes place to the hides prior to tanning. 

The fibre structure of hides also plays a role in the area yield. Hides that have a very dense, compact fibre structure, such as some of the beef breeds, produce less area yield due to the compact, three-dimensional woven collagen fibre bundles. It is likely that the tanner will have even less control over the type of hides that enter the tannery for processing.

 

Soaking plays a more important role in yield than most tanners would think. It is critical that the inside of the fibre bundles are fully rehydrated, as well as the interfibrillary proteins. This is not always achieved in short soaking times. The use of soaking enzymes is an excellent way of removing unwanted skin components, and this has the effect of slackening the hide and increasing yield. Longer soaking times promote area yield. 

Good area yield cannot be achieved without a liming process that results in a relaxed, wrinkle free limed hide. The use of liming auxiliaries is essential, as well as a slow opening up of the hide structure. Liming chemicals must not all be added at the same time, and drum speed during the liming process must be kept very slow. Over-swelling must be avoided. The interfibrillary denatured globular proteins must have been removed from the hide on completion of the liming process. An extended liming process is desirable for achieving good area yield. 

The greater the cross-linking between the collagen and the chrome tanning chemical, the less the yield of the leather produced. This would seem to indicate that lower basification pH’s would be more desirable than higher pH’s. The same is true for syntans. The greater the leather is loaded with syntans, the more area is lost. Optimum offer of syntan and near complete exhaustion of the syntan from the float should be aimed for. The tanner should not force the leather to consume chemicals beyond its capacity. Higher offers of syntans can be used provided the hides have been limed for extended times as they will have a more open structure.

Fatliquors are important for leather yield as they affect the extent of lubrication in the interfibrillary spaces. The proper lubrication between the fibre bundles does not only affect the leather softness, but also the yield. Lubrication across the entire cross-section of the leather is effective at increasing the yield. Sulphited fatliquors give the best yield, followed by sulphonated fatliquors, and then sulphated fatliquors. 

The moisture content of the final leather is also important for leather yield. The tanner should aim for at least 14-16% final moisture content.  

 

 

The thinner the shaved wet blue, the greater the area or yield. Obviously, it must be remembered that it is not the tanner that decides on the thickness of the leather to be produced, but rather the customer.

Setting is an important operation for gaining area. The adjustment of pressure during setting is an important factor. The incorrect hardness of the rubber roller on the setting machine can reduce the area produced. Soft rubber rollers reduce the area produced.

Conditioning and staking also play a role. Vibrational staking does not provide as good an area yield as the old Slocomb staking machines. 

The most important factor affecting area in the dyehouse is the way in which the leather is dried. A slowly increasing temperature gradient is best to achieve leather area. The method of drying plays the greatest role in the dyehouse. Paste drying of corrected grain shoe upper leather gives the best yield, followed by toggle drying, then vacuum drying, then hang drying on a system that moves around the roof of the tannery. Very few tanners still use paste drying. Most have moved to the use of toggle drying. 

The finishing of the leather does not appear to impact on the leather area in any way. 

 

It is evident from the observations provided by the tanner in the original article that the area or yield of leather in a tannery is influenced by many factors during processing, and not just one factor. These include chemicals, mechanical operations, as well as things such as the type of hides and the type of curing. The tanner may have little control over these latter factors.