Volume 7 - Spring 1999


The Manufacture of Leather - part 7

by Roger Barlee

In earlier articles in "Skin Deep" we have discussed both the tanning and re-tanning of leathers. Whilst some mention has been made of the different types of vegetable tannages and their ageing properties, no discussions have as yet taken place on ageing properties of bookbinding leathers as a whole. Whilst I mentioned in the last edition that this subject would be the next topic in our progress through the manufacture of leather, recent discussions with some bookbinders has highlighted the subject. As a result I am going to use this article to put the tanners point of view on the ageing properties of bookbinding leathers.

Whilst nobody would question that the use of Alum Tawed leather or Vellum will result in the best ageing characteristics for bookbinding, their limitations make them unacceptable for general commercial bookbinding. Research over the last seventy years has been aimed at trying to emulate the long-term stability of Alum Tawed leather and Vellum in a more commercially acceptable product.

As was stated in part 4 of the Manufacture of Leather, there are two distinct types of vegetable tannin, 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. On top of this, the excellent buffering capacity from the high "non-tans" content of the product (approximately 40%) protects the leather against acid damage from the air. Nowadays bookbinders should ensure that their quality bookbinding leather has been tanned with one of several of these Pyrogallol tannins such as Sumac, Myrabolams or Tara. The disadvantage of these tannins is that they are generally expensive because supplies can be problematical, and they are difficult to harvest. The reason for this is that the majority of Pyrogallol tannins come from nuts and leaves.

Catechol tannins on the other hand, such as Mimosa and Quebracho, come from the barks of tropical hardwoods that are farmed for their wood. They are therefore far more widely used in the production of leather than the Pyrogallols. Catechol tanned leathers still have their uses in short-lived items such as shoes, bags etc., in time however red rot will appear. The reasons for the problems are firstly that the "Catechol" complex is not stable to oxidation, and secondly that these tannins have a very low buffering capacity as the non-tan content is much lower. Bookbinders should therefore be very wary of using cheap leather for restoring old valuable books, as these will generally have been Catechol tanned. A good rule is to steer clear of any natural leather that has a pink cast or poor lightfastness.

"Why do book restorers have to spend their time restoring 'modern' books, whilst those from the 17th and 18th centuries are undamaged?"

This question goes to the very heart of the bookbinder's problem. The statement is in fact very true, in that until the early 1800's, the leather used for the production of books generally came from local "European" woods such as Oak. Oak bark is a Pyrogallol tannin, and as such, oak tanned leather is long lasting. As far as the European bookbinding trade was concerned, the problems of the different types of tannage were not highlighted until the beginning of this century.

During the last century four important things happened. Firstly, as the general population became more affluent and educated, the demand for books increased. This was unfortunately also at the same time that, with the increasing trade with "the colonies", European tanners started using the cheaper leather and tannins coming in from Africa and India. The result was that the majority of leather bound books from the later half of the last century up until the 1920s were bound in Catechol tanned leather such as Mimosa and Quebracho. Another important factor was the use of sulphuric acid. Sulphuric acid is a strong acid, and as such it's presence increases the likelihood of acid damage to the leather. The reason for the introduction of sulphuric acid to the processing was the development of man-made dyes and mechanical shaving (please note I am not going to get involved with the pros and cons of shaving itself). Artificial dyes require to be fixed to the leather. This was initially done with sulphuric acid, although today formic acid is used (a weak acid). In the case of shaving, the problem this caused was iron staining. Nowadays tanners have a wide variety of sequestering agents available to allow the removal of iron particles, however in the 1800's the easiest way to remove iron was to bleach the leather using sulphuric acid. This bleaching not only left the iron in the skin but also the strong acid, both of which would have a detrimental effect on the future stability of the tannage. Finally, to add to the disaster, this was also a period of high atmospheric pollution, (SO2 and NOX), as the industrial revolution took off. These pollutants increased the amount of acid in the air, and therefore the speed of acid damage to the leather. This was more critical in Catechol tanned leathers where the amounts of buffer salts are negligible. On top of this there was the additional formation of red rot in Catechol tanned leathers as the tannins broke down from oxidation.

"Why did the work done in the early 20th Century with the introduction of the P.I.R.A. test not stop the problems?"

The problem of red rot in books was identified in Great Britain in the 1920s, and Innes carried out work over the next 10 years. It was as a result of his work that the first specifications for the production of bookbinding leather were first introduced. Innes highlighted the importance of using Pyrogallol tannages for bookbinding leather, and also the need for the use of additional buffer salts. A test was invented to check on the suitability of bookbinding leathers called the PIRA test. The PIRA test was designed to simulate the conditions that would result from general oxidation of the leather and acid damage. A piece of leather 2" square is soaked in a dilute Sulphuric Acid, and then for 10 days the leather has a measured amount of Hydrogen Peroxide dripped on. If the leather was suitable for bookbinding, the 2" piece would appear unharmed at the end of this experience.

Whilst this test was at least a good indicator of whether one leather is "better" than another it had one major problem. This was that a leather tanned with a Catechol tannin such as Mimosa could be made to pass the test by the addition of large quantities of buffer salts such as Potassium Lactate. As a result some of the more unscrupulous tanners did exactly this and sold totally unsuitable leather to bookbinders who were lulled into a false sense of security.

For the next 50 years, tanners of quality bookbinding leathers followed the research of Innes, and tanned their skins with Pyrogallol tannins. Since my Father joined J. Hewit & Sons Ltd in 1948, we have only used Pyrogallol tannins for our top quality craft bookbinding leathers, and have not used additional buffer salts. All our quality bookbinding leathers therefore carried a PIRA sticker until further developments in the 1980's made this test redundant.

"What has been done since the 1920's?"

Before talking about more recent developments, it must be emphasised that leather tanned properly with a Pyrogallol tan will last a considerable length of time. We have both Calf and Goatskin tanned with Sumac from the 1930's still as soft and as flexible as the day they were tanned.

The work that was carried out by the then British Leather Manufacturers Research Association at the request of the British Library was aimed at improving on a good leather, and also to find ways of stopping the rot occurring in already bound books of unknown origin. The BLMRA under the expert guidance of Betty Haines tested a large variety of different tannages, from fully chrome tanned leather through many different combinations of tannage. Eventually they identified a little used combination tannage as the one giving the both improved stability over Pyrogallol tanned leathers, while still keeping the important features necessary for hand binding. The particular tannage identified was SEMI-ALUMINIUM tanned leather.

Whilst it is a surprise to many people, vegetable tanned leather has a final pH in the range of 2.8-3.5. Higher pH's result in the stripping out of the vegetable tans, the oils and any dyes in the leather. In the case of the semi-aluminium leather, having been reduced down to around pH 2.5 to fix the vegetable tan, aluminium is then added and the pH is taken back up to around pH 4.0-4.5. This can occur because the aluminium forms very stable complexes with the vegetable tans and oils, and can be seen from the fact that a normal vegetable tanned leather shrinks in water above 70ºC, whilst the semi-alum leather will stand the boil.

We have produced this leather for over 10 years, however the problems caused by the

water-resistance of the leather - making staining difficult, and the large amount of stretch - making paring a nightmare, have resulted in little interest for semi-alum leather except with a few of the world's leading National Libraries.

"So what is the future for Archival Quality Leather and do tanneries care?"

Specialist tanneries involved in the manufacture of craft bookbinding leathers certainly do care. Work is just about to start, funded by bookbinding leather tanneries, along with a grant from the European Union. This work is designed to review the whole matter of archival tannages again under the title "Craft Project BES2-3432-Development of Archival Quality Leather". The aim of the research is to come up with an archivally sound leather that bookbinders can use easily. Tanneries, research bodies and bookbinders from Greece, Great Britain, Italy and Germany will carry out this work. J. Hewit & Sons Ltd are proud to be one of the partners in this work - watch this space to find out the results in 4-5 years when the research is completed!


Skin Deep - Volume 7 - Spring 1999

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