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Leather Tanning

Posted: 2016-08-07 13:07


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.


The Fairfax Bible (1290)

Posted: 2016-05-28 07:59


Recently, one of the most historically relevant and archaic books I have ever worked upon passed through the workshop. The " Biblia Sacra Latina Editionis Vulgatae ", (more commonly referred to as, " The Fairfax Bible ), came to me with it's cover in a very dry, worn and degraded condition. Drawn from the Fairfax Archive, it was very evident following my initial assessment, interventive treatment would be required. The text block was compiled of vellum leaves dating from the late 13c., with hand inscribed Latin scriptures displayed in the ' double column ' presentation. As is commonly found in liturgical books of this period the text had also been augmented throughout with numerous illuminated, Historiated Capitals and representative Marginalia. Although, for whatever reason a number seemed to have been deliberately ' cut out '. After consultation with the owner it was decided, that due to the problems which can be encountered in controlling the reactive properties of vellum, it was my client's choice that no treatment was to be undertaken on the folios.

The forwarding methods used in the binding's structure indicated the book had been rebound sometime in the mid 19c. It had been gathered as single bi-fold sections and sewn onto 5 x raised, either Jute or Hemp cords, with two colour thread, single core ' rolled ' parchment, Headbands. Endpapers were of the simple ' tipped-on ' variety. The binding was given a full covering and a ' tight back ' spine finish. Both Headbands were in need of repair and reattachment.




Initially, a light soap wash of the leather was administered to remove any ingrained, surface dirt. Following this my attention was given to the preliminary and vellum leaves by brushing away, mechanically all traces of abrasive particles which may have harmed the permanence of the inscriptions. The location of the gutter margins are particularly prone to such accumulations.


The split front endpaper inner-joints were repaired with a reinforcing lining of Tosa Washi Japanese repair tissue. Lacunae located at the foot of the fly leaf was infilled with matching ' wove mould ' paper using standard western conservation techniques.

Treating the cover all loose fragments were detached, and a more defined profiling of the infilling areas were traced. Utilising Bank paper, profile template patterns were traced from the upper board and Headcap areas, and then subsequently transferred to the Fair Calf repair leather. The repair leather was cut marginally oversize, pared to the appropriate thickness for working overall and then further edge pared. The working sequence was to repair and fashion the Headcap first, to be followed by the split outer-joints, then the foredge of the upper board, and lastly the underlaying of the corners was carried out.

The following photographs depict, in sequence, the restoration treatments in progress. The application of the period dye staining treatment, using a combination of spirit/aniline dye solution is also shown. Following a 24 hour drying period the cover was lightly, fed and dressed with a conservation compound of Beeswax, Lanolin and Hexane to act as a long term humectant fin the leather to prevent further drying.

A third party agent had been previously commissioned by the owner to construct a detachable lid, archival box to house the repaired manuscript. Being critical here, the concept of a loose detachable lid is not ideally the most the most secure way to protect an item of such antiquity as this. One also has to bear in mind ,when a book is being treated during repair; sometimes minor structural and dimensional changes may occur. If a box is made pre-treatment, there is always a chance a custom fit may not be achieved. Therefore, as a general rule it is advised that bespoke storage boxes/containers are constructed once restoration is complete.


Split ' outer-joint ' on the upper board. The condition of the dry, degraded leather can be clearly seen in this photograph. The Headband is insecure in it's anchorage, and could easily detach and become lost.


The underlaid, repaired and reshaped Headcap.


The above and following image show the profile contour, and infilling for the upper board repair.



Localised repair of the split ' outer-joint ' on the upper board.


Lining and reinforcing of the front endpaper ' inner-joint '. Lacunae infilled with matching, weight and texture ' wove mould ' repair paper.


Dye staining of the cover to ' blend-in ' the repairs.


The structural function of the refurbished manuscript.


The cover after the dye application.


Minimal spine decoration with two, single gilt rules either side of the raised bands. The paper label is an identifying MS Shelf Mark for the purposes of cataloging the collection.


A Pressing matter....

Posted: 2016-03-06 13:16

Many bookbinders use old Victorian copying presses in their bindery and you may find it of interest to know for what purpose they were originally used. In the late nineteenth and early twentieth centuries, copies of documents were often kept, not on separate sheets, but in special copy books which had leaves of tissue paper.


The document to be copied had to be written in copying ink or typed with a special ribbon. A leaf, which was protected from those underneath by an oiled board, was dampened with water, the document laid on it and then another board placed on top. the book was then closed and nipped in the press for a few minutes, transferring the ink to the moistened leaf, which could then be read.

So now you know...

Man vs. Machine. Discuss...

Posted: 2016-02-26 09:39


Here is a letter submitted by one David Frean Esq., (The Society of Bookbinders Newsletter. No.2 August, 2005), which I found very interesting reading, reference the seemingly ever present discourse on the comparison of quality between hand and machine production.

Dear Ed,
David Pye, in his book, " The Nature and Art of Workmanship " (CUP 1968, Facsimile Herbert Press 1995), has some interesting things to say on why hand wrought precision is preferred to machine perfection. He makes comparison between the workmanship of risk and the workmanship of certainty. Machines are used to minimise risk. Once machines are properly set up, the workmanship is consistent, and relatively failure free. In hand bookbinding, however, the risk rises exponentially as the final stages of finishing take place, and the risk is even greater when the material being worked upon is of intrinsic value. The pleasure in seeing and handling a well-crafted, handwrought article arises not only from the merits of the item itself, but also from the knowledge it represents a skilled performance. It is the same pleasure one can get from listening to a top musician, or from watching a top athlete in action.

David Pye also says that it is often the very small irregularities in good handwrought work that add richness and pleasing diversity that is lacking in a bland machine-made object. I once watched a skilled bricklayer building a wall with blue engineering bricks, and was amazed to see how carefully he chose and placed each brick. He explained that the so called " blue " bricks incorporated random patches of other colours - reds, yellows, browns and blacks - and that it was only by careful matching that a pleasing result could be achieved where the touches of other colours added richness to the overall blue tone of the wall. Similarly, it is the minute variations in gold tooling by hand that makes it more appealing to the eye, rather than the regulated appearance of machine blocked gold work.

These days, mechanised book production seems to be so driven by cost factors, that machine perfection has largely been supplanted by mere machine uniformity. New books often do not open flat, rapidly disintegrate if subjected to much use; whereas handwrought precision creates volumes which are not just visually pleasing, but are a pleasure to handle with an innate durability.


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