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Leather Tanning

Posted: 2016-08-07 12:07


There are two types of vegetable tannin for leather, the ' Pyrogallols , or ' hydrolysable tans ', and the , ' Catechols ', or ' condensed tans '.

Pyrogallol tannins are used in the production of the main quality bookbinding leathers because the Pyrogallol complex is stable to oxidation. This can be seen by the good lightfastness of the leather. Additionally, the excellent buffering capacity from the high, ' non-tans , content of the product (approximately 40%) protects the leather against acid damage from the air. For the production of quality fine binding bookbinders should purchase leathers which have been tanned with one of the following Pyrogallol tannins namely Sumac, Myrabolams or Tara. However, the disadvantage of these tannins is that they are generally expensive as obtaining supplies is problematical, and they are also difficult to harvest. The reason is that the majority of Pyrogallol tannins are extracted from nuts and leaves.

Catechol tannins such as Mimosa and Quebracho, come from the barks of tropical hardwoods. They are more widely used than Pyrogallols. Catechol tanned leathers still have their uses in short-lived items such as shoes, bags etc.,in time, however, red rot will begin to occur. The reasons for the problems are firstly that the ' Catechol ' complex is not stable to oxidation, and secondly these tannins have a very low buffering capacity as the non-tan content is much lower. It is vital, therefore, bookbinders should be very wary of using cheap leather for the restoration of rare and valuable, antiquarian editions as these will have generally been Catechol tanned. A sound general rule is to avoid any natural leather which has the appearance of a pinkish cast or poor lightfastness.

In the early 1800's book production leather was manufactured in the main with tanninis from the local ' Euopean ' woods such as Oak. Oak bark is a Pyrogallol tannin, and as such, oak tanned leather has longevity. It was not until the beginning of the 20c. until the issues with various types of tannage for bookbinding leather was brought to the attention of the trade.

During the 19c. we can look to four circumstances that lead to an improvement in the standard of bookbinding leather production. Firstly, as the general population became more affluent and educated, the demand for books increased. Coincidentally, trade with, ' the colonies ', gained momentum meaning that the use of cheaper leather was more prevalent as tannins began to be imported from countries such as Africa and India. This resulted with the majority of leather bound books from the latter decades of the century being bound in a Catechol tanned skins such as Mimosa and Quebracho. Another important aspect was the introduction of Sulphuric Acid into the leather manufacturing process. This strong acid was used in the development of man-made dyes and mechanical shaving. Artificial dyes were required to be fixed into the leather. The simplest method back in the 1800s for iron removal was Sulphuric Acid bleaching. The bleaching not only left an residual of iron in the skin, but the bleaching action would also continue to degrade the future stability of the tannage overtime. The final aspect was the beginning of the Industrial Revolution and the increasing, highly atmospheric pollution of the time being caused by a combination of Sulphuric and Nitogen Oxide. These pollutants could be readily absorbed by leather. The reaction was more critical in Catechol tanned leathers in which the ratio of buffer salts is negligible. The additional formation of ' Red Rot ' in this category of tanned leather was also commonly found as they were more prone to oxidation. Nowadays, a much weaker formic acid is used with a numerous other varieties of non-detrimental, sequestering agents available for the removal of harmful iron particles.

The problem of ' Red Rot ' was initially identified in Great Britain in the 1920's, with Innes beginning his research into this phenomenon over the next decade or so. It was as a result of his work the original specifications for bookbinding leather would be eventually introduced. Innes raised the profile of using Pyrogallol tannages and buffering salts. A test was invented to check on the suitability of bookbinding leathers which came to be known as the PIRA (Printing Industries Research Association) Test. This simple test was designed to simulate conditions resulting from general oxidation and acidity. A small cutting of leather is soaked in a dilute Sulphuric Acid solution, followed by a 10 day period where a Hydrogen Peroxide solution is repetitively, dripped onto the sample. If the cutting appeared unharmed that particular skin could be deemed suitable for bookbinding use.

While many established tanneries considered this test was a good reference it was not without controversy. Leather tanned with a Catechol tannin such as Mimosa, could be artificially made to pass the test by the addition of large quantities of buffer salts during processing such as Potassium Lactate. Purchasers could be easily fooled by these false results from unscrupulous suppliers, being deceived into a false sense of security about the robust qualities of their skin.

Since the 1920s, the then British Leather Manufacturers Research Association; ostensibly at the request of the British Library, have undertaken further research to improve ' good leather ', and to also find ways of stopping rot occurring in books bound in a leather of unknown source.

The BLMRA, under the expert guidance of Betty Haines, tested a large variety of tannages, ranging from Chrome tanned to lesser known combinations. Eventually they identified a little used Semi-Aluminium combination tannage ,as the most effective in providing improved long term, stability for Pyrogallol tanned skins; which would at the same time maintain the necessary qualities for hand binding.

It may be a surprise to many binders to know that vegetable tanned leather has a parameter pH reading range of approximately 2.8 - 3.5 alkalinity. Higher pH readings often result in the stripping out of the vegetable tans, the oils and any inherent dye colouring present. In Semi-Aluminium leather the readings are reduced down to around 2.5 which fixes the vegetable tans. If more Aluminium is then added the pH is then returned to a more stable reading in the 4.0 - 4.5 range. This occurs because the aluminium forms very stable complexes with the vegetable tans and oils. This reaction can be proven by the experiment of boiling a Pyrogallol tanned leather in water above 70 degrees centigrade which will shrink, as opposed to a Semi-Aluminium tanned skin which will remain unaltered in size.

Reference:'' The Manufacture of Leather - Part VII ''. Roger Barlee.
'' Skin Deep '' (The Biannual Newsletter from J.Hewit & Sons Limited. No.7 - Spring 1999.

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