Conquering the Dreaded Red Rot

 

Throughout the Past Caring Project we’ve often mentioned the term red rot as a major part of the collection’s conservation issues. As I’ve recently completed consolidating one hundred degraded leather bindings of the Poor Law Union and Workhouse collection, it feels like a great time to write a post on the topic.

However foreboding the name, red rot is a fairly common type of leather degradation that most often occurs in bindings from the late industrial revolution onwards. Leather, like all organic material, breaks down and degrades over time. However, due to changes in the manufacturing process of leathers during the 19th century, the material from this time period is often in poorer condition than earlier leathers. On red rotted bindings, the leather is fragile and weak, often splitting, cracking, and coming off the binding at the joints, corners and top and bottom of books. The leather is also easily abraded and scratched. In its poorest states, simply placing the book on a work surface causes powdery leather fibres to fall off. Over half of the books of the Poor Law Union and Workhouse have this kind of degradation, with 300 large Minute Books and Ledger  bindings that are damaged enough to require a conservation treatment.

Fun with a microscope!

For a visual comparison, I took two small fibre samples from new and red rotted leather to photograph under a microscope. These fibres were gently scraped with the back of a scalpel from the flesh side of the leathers (the side without the grain).

Leather fibre samples

New leather sample (left) and historic sample (right).

The sample on the left was taken from a new piece of archival quality leather; the fibres were reluctant to come apart during sampling. The sample on the right was taken from the inside of an 1846 Minute Book and was noticeably easier to tease apart.

Samples at 55x magnification: new leather (L), red rotted leather (R).

Samples at 200x magnification

Under magnification, the fibres from the new leather are longer and more cohesive, which gives strength and durability to the leather. The shorter, less cohesive fibres of the Minute Book is why the leather on the latter binding is fragile, easily abraded and ends up all over hands and clothes during handling.

What changed during the industrial revolution? Why is it called red rot?

To transform animal skin (most commonly calf, sheep, goat, and pig in the UK) into bookbinding leather, the skin must go through a chemical process called tanning, which makes the skins resistant to putrefaction and decay. During this process, the animal skin (which is previously  prepared by removing hair and flesh) is submerged in the tanning liquor- an astringent liquid made from leaves, twigs, wood and bark of plants (two examples are sumac and chestnut). The tanning liquor attaches and coats the fibres of the skin, preserving it in a way that creates flexibility and malleability while resisting biological attack. After this, the leather is dyed and ‘finished’ to create the final product. This process can take several months or more.

Leather tanning from Diderot's Encyclopaedia

Engraving of leather tanning from Diderot’s Encyclopaedia. (Link for more images of leather processes here). Image: public domain

As demand for leathers increased in the late 18th and 19th C, leather makers began using more astringent plant matter (such as Hemlock and Gambier) to speed up the tanning process. These quicker tans have a slightly different chemical makeup and result in leathers more susceptible to absorbing pollutants from the atmosphere- which plays a large role in its deterioration. Once absorbed, the pollutants react with the tannin and become acidic, which increases the rate that the leather fibres break and degrade, reducing its strength. These speedier tannins also react with light to create the characteristic red and oranges tones of red rot, while poor storage conditions exacerbates deterioration.

What are we doing to help?

While there is no way to fully halt or reverse red rot, one of the main goals of the Past Caring project is to slow the rate of degradation and enable readers to access the volumes without causing further physical damage to the items. We also aim to reduce the amount of powdery leather deposits that gets on the users, tracks through the bindings, and are potentially inhaled during use.

With the above goals in mind, we chose a two step approach: first treating the leather with a very light consolidant (adhesive) to prevent the leather fibres from coming off the binding, then creating a user friendly cover to minimise the amount of abrasion and direct contact with the leather during use.

Consolidant testing

Initial testing of the consolidant at different concentrations.

The leather consolidant is a mixture of Klucel G© (a light adhesive with proven longevity) which is dissolved in an isopropanol (a purified version of rubbing alcohol). The consolidant mixture was chosen because it did not blacken or significantly darken the leather, which retained the binding’s original appearance, while being a relatively benign treatment in the long term. The consolidant is gently applied with a large brush and after it is dried, a bespoke cover made from archival polyester is created to protect the binding. We chose polyester as it is smoother and less abrasive than a card cover, while enabling the spine or any label markings to be viewed on the binding. The bindings are then housed horizontally in low-acid archival boxes which will help to protect the bindings from any environmental or physical damage, while absorbing excess acidity.

Phew, only 200 more bindings to go!

Thank you for reading! Please feel free to leave a message below or email me at tiffany.moore[@]exploreyork.org.uk for with any questions or comments.

Tiffany

 

 

Sources

Kite, M., & Thompson, R. (2008). Conservation of leather and related materials. Oxford, Burlington, MA.

Larsen, R., & Vest, M. (1994). STEP leather project: evaluation of the correlation between natural and artificial ageing of vegetable tanned leather and determination of parameters for standardization of an artificial ageing method. København, Royal Danish Academy of Fine Arts, School of Conservation.

Roberts, M. T., & Etherington, D. Vegetable Tannins. Bookbinding and the conservation of books: a dictionary of descriptive terminology. Online edition. http://cool.conservation-us.org/don//. [Accessed 05.08.2017].

Vidler, K. (2014). ‘Collagens and Tannins in Bookbinding Leather’. Conservation of Leather Bindings. West Dean College. Unpublished.

Vidler, K. (2014). ‘Common Types of Bookbinding Leather Deterioration’. Conservation of Leather Bindings. West Dean College. Unpublished.

 

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