Development of Archival Quality Leather
Work was done in the early 1980's by Betty Haines of the then British Leather Manufacturers Research
Association into resolving the long-term problem of identifying good quality bookbinding leathers. The
need for a standard for archival leather was considered necessary because of the lack of any reliable
guide by which a purchaser could be assured of obtaining a new binding leather with good long term
stability.
This arose because:
- Leather from a specific region could not necessarily be guaranteed to have been made using
tannins from that region
- The PIRA test had proved to be unreliable
In addition to this, it was known that the rate of ageing of Bookbinding leathers is increased by urban
pollution. The work proved that vegetable tanned leathers re-tanned with aluminium salts had a greatly
improved resistance to deterioration, and that chrome/vegetable tanned leathers have greatly reduced
durability, especially of the grain. The work resulted in the publication of a British Standard for Archival
Bookbinding Leather
BS 7451:1991 - Specification for Archival Bookbinding Leather. Unfortunately, although of high archival quality, the leather produced was too stretchy to
pare easily and was highly water-resistant. This was partly due to the direction of the research
(homing in on the archival properties), and also because of the lack of knowledge at the time regarding
semi-aluminium leather production. As a result the general reaction of bookbinders was to try the
material once and never come back! This therefore meant that concerned bookbinders and archivists
were no better off than they had been before the work was carried out in that they were not in a
position to tell a good long-lasting leather (pyrogallol tanned) from a non-lightfast leather (catechol
tanned).
The work of Betty Haines was followed by two further European Union funded research projects - the
STEP and ENVIRONMENT Leather projects. The result of these projects was an artificial ageing
regime, along with recommendations that vegetable tanned bookbinding leathers should be
manufactured using pyrogallol tannages, (already done at Hewits) and keeping the sulphate content of
the leather as low as possible. The artificial ageing system that was recommended involved heating
the leather to 1200C (later 1500C) for 24 hours, and then holding the leather in a polluting atmosphere
of 20ppm SO2 and 10ppm NO2 at 400C and 35% Relative Humidity (RH) for 6 days. The process
being carried out 4 times in total.
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:
- Evaluation of leathers currently used for bookbinding across Europe.
- To develop a leather quality testing scheme, that defined binders requirements in an objective
manner understood by both the tanner and the binder.
- 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.
- 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.
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