Part 3 of The Manufacture of Leather described the various changes which take place during the conversion of raw pelt into leather and listed the categories of tanning agents which are commonly used. This instalment will look more closely at vegetable tanning materials and methods.

Vegetable Tanning

Types of Vegetable Tanning Material

Soluble components present in the bark, leaves or fruit of very many different species of plants have been found to have tanning properties. The process of leaching these compounds also yields a complex mixture of other materials (the "non-tans") which, while having no tanning power in their own right, contribute significantly in determining the properties of the resultant leather. The composition of the extract varies markedly between different plant species and is affected by the conditions during leaching (in particular, the temperature and pH). Good control is essential to ensure consistency.

Broadly, vegetable tans fall into one of two main classifications, namely the "Catechols" or "condensed tans" and the "Pyrogallols" or "hydrolysable tans".

Two commonly used examples of catechol tanning extracts are Mimosa, and Quebracho. The popularity of these tans stems from the fact that they bind very strongly and rapidly to pelt, (tanners refer to this as "high astringency"). The resultant leather has a distinctly reddish-brown colour with poor light-fastness, leading to a darkening and further reddening of the colour over time. In addition, the oxidation which takes place as the leather ages gives rise to acidic by-products which attack the structure internally, weakening the fibres and ultimately causing complete loss of strength. Nevertheless, this breakdown is usually fairly slow and products made with this type of leather can have many years of useful life.

Pyrogallol tans such as Sumac and Myrabolans, on the other hand, generally produce leather with a yellow or greenish-brown cast with good light-fastness - the colour will still darken but does not acquire the redness so typical of the catechol types. The pyrogallol materials tend to have a lower astringency which means that they are able to penetrate into the pelt very well before fixation takes place. This produces a deep tanning action and a mellow leather. The non-tans present in these extracts contain components which are able to act as buffer salts, further protecting the leather from attack by acidic atmospheric gases.

The tanner must choose the most appropriate tan or mixture of tans to produce the desired properties in the leather. At J. Hewit & Sons Ltd., for example, much of our light leather production is for bookbinding where light-fastness and longevity are of paramount importance so we use sumac, myrabolans and chestnut (another tanning material of the pyrogallol type which produces a rather firmer leather with a darker natural colour). We also manufacture leather for leather-goods which has a completely different set of requirements. We achieve these properties using a mimosa tannage.

Control of Tannage Conditions

The degree and speed of uptake of tan is influenced by a number of factors which may be controlled during tannage. The following conditions within the tanning liquor promote increased fixation:

• Lower pH, i.e. increased acidity
• Higher temperature
• Higher concentration of tanning material
• Higher astringency of tanning material

Conversely, good penetration is achieved by the opposite conditions but is also assisted by increased agitation or mechanical action (which has little effect on the rate of fixation).

Normally the aim is to achieve a uniform tannage throughout the thickness of the leather which is brought about by commencing tannage under conditions which favour penetration and allowing fixation to take place in the later stages. This avoids over-tannage of the surfaces leading to a rough grain which is weak and brittle. In the extreme, the build-up of tan at the surfaces may prevent further penetration into the pelt causing an under-tanned or raw streak in the centre.

There are circumstances where a non-uniform tan distribution may be desirable, although this is more relevant to modern rapid chrome tannages where careful control of the deliming operation can achieve a pH gradient which, in turn, influences the deposition of chrome throughout the thickness.

The tanner will also endeavour to reduce the variability across the area of the skin by setting up conditions which promote preferential tannage and filling of certain areas such as the bellies and flanks where the fibre structure is less dense.

Preparation of Tanning Extracts

In the earliest processes the crushed vegetable material was added directly to the raw hides in vats or pits, in a layered structure, and covered with water. The tanning components gradually leached out and were taken up by the pelt. This type of process is exceedingly slow due to the lack of mechanical action and the low concentration of tan in the liquor.

Although the above procedure may still be used during certain stages of the traditional pit-tanning of heavy leather it is much more convenient to extract the tan in a separate set of vessels dedicated to this purpose. A counter-current system is used with a series of vats containing vegetable material at various stages of "exhaustion". Water enters the vat containing material which has had nearly all of the soluble components leached out and after an appropriate time is pumped to the next vat and so on until it reaches a vat containing fresh material. The fresh material is, in effect, extracted by batches of liquor of progressively lower concentration until it is completely spent whereupon it is emptied out and the vat is refilled with fresh material. The liquor leaving the last vat is still relatively weak and is usually further concentrated by evaporation. It may be used in this form or spray dried to a powder but this uses a lot of energy and the cost must be weighed against the costs of storage and transportation.

Methods of Application

As mentioned above, the traditional method of tanning is by the use of pits. The tannage is started in a pit containing weak or nearly spent liquor and continues through progressively stronger liquors to achieve the necessary conditions for good penetration at the start and fixation towards the end. In its original form, this process entails a great deal of manual handling and, above all, it is slow, taking many months to complete. Hides and skins are an expensive commodity and very few tanners, nowadays, could afford to have so much money tied up as work-in-progress.

Improvements to the method have reduced the time considerably (although it is still slow compared to some other methods) and it remains in use for in certain cases where a firm and flat-grained heavy leather is required, e.g. for shoe soles.

The addition of a paddle wheel to stir the liquor in a pit was a major innovation and gave rise to the modern paddle which is usually semi-circular in cross-section. These devices are not suitable for heavier hides which are generally too inflexible to float around with the circulating liquor but they offer significant advantages over pits for smaller and lighter skin. The continuous flexing of the skins as they travel around brings about a large increase in the rate of penetration and at the same time, the concentration of the liquor is kept uniform throughout. For the majority of light leather production, drums have become the most important type of vessel. Skivers and wool-on sheepskins are still processed in paddles, however, as the mild mechanical action prevents damage to these skins.

The drum is by far the most prevalent type of processing vessel in the modern tanning industry. It is a cylindrical structure mounted horizontally on hollow axles with shelves or pegs inside and a sealed door. Liquors can be added and, in some cases, removed via the axles while the drum is rotating. The liquor may fill the drum to the axles (a "long float") giving a level of mechanical action similar to a paddle, alternatively the drum can be run virtually dry. The dry method is used to achieve a very rapid vegetable tannage of small skins and lighter hides. The only water present is that carried over in the skins from the previous processing stage. This means that the concentration of tan in the liquor is extremely high but the difficulties which this would cause in a pit tannage system, do not arise because the intense pummelling achieves full penetration in just a few hours. After drumming the skins need to be piled over a horse to allow fixation to proceed.

Drying to Crust

Whichever method of tannage has been used, it is normal for the leather to be dried out at this stage even if further wet processing is to take place later. This drying or "crusting out" is very important in that it assists in the final stages of tan fixation. This is also a convenient point at which to grade the leather and sort into batches for subsequent treatment. The dry leather may also be readily stored or transported in this state.