Craft Project BE-S2-3432 - "Development of Archival Quality Leather" was a pan-European project intending to have a fresh look at the whole archival leather position. The group was made up of tanners, bookbinders and research bodies from the UK, Germany, Italy and Greece, and the major objectives at the start were as follows:

1. Evaluation of leathers currently used for bookbinding across Europe.
2. To develop a leather quality testing scheme, that defined binders requirements in an objective manner understood by both the tanner and the binder.
3. Optimisation of semi-metal tannage, whereby a leather with all the required properties could be produced with a life expectance of twice the normal life of the vegetable tanned leather stored in polluted atmospheres.
4. To investigate the applicability of innovative "metal-free" tannery processes to consistently produce archival quality tannages.

During the first year of the project 72 modern bookbinding leathers representing the whole spectrum of European bookbinding and archival quality leather production were studied. The leathers were tested for their physical and chemical properties, and in addition the bookbinders graded the leathers on their feel and handle as well as on how easily they pared and could be used for binding. These 'organoleptic tests' (as they became called) graded the leathers from 0 (completely unsuitable) to 4 (ideal), with an overall result of 3 (or 4) needed for a "pass" - not surprisingly it was ONLY the leathers produced by specialist bookbinding tanners like J. Hewit & Sons Ltd that passed!

As one can imagine, this resulted in a mass of data from which it became apparent that there was correlation between some of the physical tests carried out on the leathers and the organoleptic tests. However the best correlation invariably resulted from new objective tests specifically aimed at mimicking the handling and working of the leather by the bookbinder. During the second year the physical tests were narrowed down to the following:

• Softness: a measure of how soft and elastic the leather felt
• Wettability: new test indicating the absorbency of the leather to paste and glaire
• Mouldability: new test correlating to how easily the leather may be moulded and retain its shape during bookbinding
• Notch Sensitivity Factor: new test to assess how easily a leather could be pared or shaved during bookbinding

In addition to the above, the Resistance to Flexing whilst under strain was measured as an indication to the strength of the leather on the joint. Together these tests resulted in a specification for a craft bookbinding leather (Appendix B). The methods for the 3 new tests can be found in Appendices D-F.

Whilst this work was been carried out, work was also under way into looking at the accelerated ageing of the 72 leathers using an artificial ageing chamber. Initially the leathers were tested using the procedure used from the STEP project, however it became clear that this test produced anomalies compared with known results of leathers over the last few hundred years - both alum tawed leather and vellums failed quite dramatically.

As was stated above, the original ageing process involved periods in the ageing chamber along with repeated periods at a temperature of up to 150ºC, and it soon became apparent that it was the heating of the leathers that was causing the problems. The process was then refined without the heating stages, and leathers of known tannage were tested over increasing periods. The ageing regime that was finally implemented involved holding the leather in a polluted atmosphere containing 25ppm SO₂ and 5ppm CO₂ at 40ºC and 35% RH for 12 weeks. After the 12 weeks the residual shrinkage temperatures (Ts) were measured in order to look for leathers with either a high residual Ts, or a low drop in Ts. The results in line with known natural ageing - Chrome tanned leathers being the best, followed by alum tawed and vellum, with vegetable tanned leather being the most degraded.