A collection of pre-1800 books were recently transferred to Library and Archives Canada’s Rare Books Collection. A census of the collection revealed that the majority of the books had various levels of leather deterioration. In some cases, the leather was cracked and flaking, and in other, more extreme cases, the leather was powdering and crumbling. This is an inherent and common issue seen in manufactured skins from this period. Leather deterioration takes place by two processes: Reaction of tannins used in leather manufacture to environmental pollutants (hydrolysis) and exposure of leather to light, heat and oxygen (oxidation). Both hydrolysis and oxidation result in the gradual disintegration of the leather fibre network and weaken its structural integrity. The by-product of leather disintegration is an acidic powder, often orange or red in colour. Not only does this deterioration cause an immediate threat to the individual book structure, but it also threatens the rest of the collection through the contamination of leather dust and particulate. In many cases, the leather leaves visible residue on surfaces and surrounding books. For these reasons, the conservators at Library and Archives Canada (LAC) developed and carried out a remedial treatment to stabilize the leather.

Deteriorated leather: The first step in the process was to assess the level of deterioration and carry out an initial surface cleaning of the books to remove as much leather dust as possible. This was carried out in the rare books vault, on all 500 books, using a gentle vacuum and small brush.

The most effective method of stabilizing deteriorated leather is through the use of a surface consolidant. A consolidant is a solution which is applied directly to the leather to seal the surface. Although it can’t stop or reverse the chemical instability in degraded leather, it creates a barrier that protects the leather from airborne pollutants and reduces the flaking and powdering. Handling affected leather books after surface consolidation is a much cleaner experience as well.

Testing

A series of tests were performed to determine the sensitivity of the leather to water and solvents. Based on these findings, we were able to reach a conclusion on the most suitable consolidant recipe to use.

The testing carried out was called Shrink Temperature Test, where small samples of leather removed from the books were heated in water until a reaction occurred. The lower the reaction temperature the less stable the leather. The tests concluded that some leathers were quite unstable and that they could be easily damaged by the application of consolidants containing water and solvents.

Spot testing: Four consolidant recipes were made up and tested on three volumes representing the identified species of leather found in the pre-1800 books, that of goat, sheep, and calf. The test relied on visual examination to determine the likelihood of discolouration by staining or residue deposit by the various surface consolidants.

The tests conclusively revealed that one particular consolidant exhibited no visible signs of staining or residue on the leather; Hydroxypropylcellulose dissolved in one solvent, then diluted in another. It was decided to use this recipe to treat the collection.

Applying the consolidant under a fume hood.

The consolidant was applied to localized areas using a small brush. The treatment was carried out in a fume hood, due to the solvents used in the consolidant recipe. The books were then left to off-gas for 24 hours in the fume hood before being returned to permanent storage.

Now that the leather surface of the book has been stabilized, we can determine, with the help of the census information, what other, if any, treatments are necessary to make these books more resilient and available for future generations to access.