The first four parts of The Manufacture of Leather covered the processes involved in obtaining the skins, and the conversion of these skins into leather. Part five will discuss the sorting of the crust leather and the beginning of the subsequent wet-end processing of the skins.

Drying

In the final section of part four, it was mentioned that the skins are dried out following their initial tannage to form crust leather, as this is a convenient point at which to grade the skins. The method, and degree of drying is dependant on the type of leather being produced, with any future transportation of the skins also an important issue.

In the production of vegetable tanned leather, after the skins have been tanned, they have to be "horsed up" for several days to allow for the fixation of the tannin to proceed. The reason for this is that the bonds formed between the fibres and the vegetable tannins, as mentioned in the previous discussions, are not as strong as those produced with chromium, the hydrogen bonds that do form take several days to stabilises. Following on from this initial horsing up, the skins are then mechanically de-watered using a sammying machine. A sammying machine is basically an industrial version of the mangle that would have been used to wring water out of clothes in most households until fairly recently and consists of two felt rollers through which the skins are squeezed. The resulting leather is still fairly moist and the skins have now to be hung to dry in a cool room for around a week to ten days - if the temperature is too hot, the loose tannins would be drawn to the surfaces and cause cracking of the grain.

Chromium tanned leather is treated somewhat differently – see Note 1.

The ‘Crust’ Warehouse

Once the skins have been dried, they are taken to the Crust Warehouse for sorting. The name crust coming from the fact the skins are very hard at this time having only had a basic tannage and minimal oiling. It is worth noting at this stage that skins are often bought or sold in the crust. This happens with many different types of skin from all over the world, but especially in the case of skins from the tropical areas of the world. The reason for this is that in hot humid climates, tanning the leather is the safest method of preserving the skins from rotting. In the case of bookbinding leathers, the two main sources of goatskins come from India and Nigeria, and are both sold in the crust state. Whilst this does give that advantage of skins generally free from the problems of putrefaction, the buyer is limited in the type of tannage offered, and therefore in the end product that can be produced. Alum tawed goatskins, for instance, cannot be produced from vegetable tanned crust leather as this is a specific tannage in its own right.

Sorting

The sorting of the skins in the crust state is very important for the profitability of any tannery. In all cases, premium prices can charged for the top-quality aniline leathers, whilst the poorest embossing grades are generally sold at or below cost price. The grader looks for many different types of flaws in the skin, as were mentioned in our first article – insect damage, scratches, flay damage, grain rot etc. The majority of these flaws have been in the skin since it's arrival in the tannery, however it is only when the skin has been unhaired, tanned and dried that the grading can effectively take place with any degree of accuracy. Grading of leather is very subjective since skins are very rarely perfect, and the skill of the grader comes into play in deciding whether particular fault(s) warrant the skin being downgraded or not. On top of sorting for faults, the sorter may also be grading for size, length, and substance (thickness).

Having been sorted, the skins will then be allocated to specific orders for customers, after which they will be shaved to substance, retanned and dyed, and finally finished to the customer's requirements.

Shaving

The first of these processes is shaving. This process is generally carried out on wet stock. The skins are therefore dipped in water, normally the day before to allow thorough penetration of the water. The skins are then "set out" to remove any creases and pleats that had dried into the skins. This process is carried out by a setting out machine that consists of two rollers, and a blunt spreading helical blade that stretches the skin.

Shaving of skins has two important objectives, firstly to level out the substance (thickness) of the skin, and secondly to bring the substance to a precise figure.

Hides and skins all have areas where the substance is naturally heavier - the spine, the butt and the neck, and areas where it is noticeably thinner - the bellies. The act of shaving the skin reduces the variation that occurs, although in many cases, the substance will still be less in the belly edges. The shaving machines can shave to a surprising degree of accuracy - down to ± 0.05mm.

As mentioned above, one of the tasks that have to be carried out by the sorter is to grade the skins for substance. The reason for this is that it is important that too much of the skin's substance is not removed. If a skin that could make an end substance of say 2.0mm was shaved down to 0.6mm the leather produced would be equivalent to a skiver with very little of the corium being left, and would result in a very weak skin. Care therefore has to be taken to ensure that only a minimal amount of shaving takes place wherever possible.

Fig.1 illustrates a side view of a typical shaving machine. The skin (D) is fed past a feed roller (E), and then between the thicknessing feed roller (C) and the shaving cylinder (F). The thicknessing feed roller is engaged by pressing the foot pedal (A) and can be adjusted towards or away from the shaving knife depending on the final substance that is required. The shaving cylinder is kept sharp by regularly sharpening the blade using a grind stone (H), and the impeller (I) spins anti-clockwise to ensure that the skin does not become wrapped around the shaving knife.

Fig.2 shows the shaving cylinder, you will note the helical nature of the blade. There are two helixes meeting in the middle of the cylinder. As the blade spins, the blades stretch the skin both lengthways and sideways, and in the process remove the majority of creases that are present in the skin.

Note 1

In the production of chrome tanned skins and hides, the chromium sulphate binds very strongly to the fibres. The resulting complexes are very stable, and this enables the tanners to remove the water both mechanically, and then still further by the use of vacuum drying. In vacuum drying, the skins are flattened out on a heated bed, a cover is then placed over the skin, and the air pumped out. The subsequent partial vacuum that is formed allows the water to be "boiled off" at a substantially lower temperature in a matter of a few seconds. The resulting chrome tanned leather has a strong blue colour, is damp to the touch, but certainly dry enough for sorting with a good degree of accuracy. Chrome tanned hides and skins can therefore be dried, sorted and back into production within one day of tanning. This allows a very fast throughput of skins, minimising the amount of stock necessary to allow the tannery to operate.

Following the shaving operation, the leather is then processed through the dyehouse, where the leather is retanned, dyed and re-oiled. We will cover this part of the process in the next edition of Skin Deep.

The images in this article are taken from the Leather Technician's Handbook by J.H. Sharphouse and are reproduced with the kind permission of the Leather Producers' Association.