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Re: [BKARTS] Leather conditioning



It would be very nice if a conservation lab somewhere set
a chemist to studying the question of leather deterioration,
leather dressing, and leather tannage for permanence.

Some years ago such an attempt was made.  At the request of
the British Library the British Leather Manufacturers Research
Association (BLMRA) designed a serious research program and
sent a prospectus to national libraries around the world, and
to major research libraries, seeking monetary support.

In the end, only the British Library was willing to contribute
money to the project.  The project went ahead, but the research
goals were greatly narrowed, and resulted in a publication which
recommended vegetable tannage with aluminum triformate re-tannage.

That produced a leather with good prospects for permanence, but
bookbinders did not much care for it at the time.

So, we have to fall back on published literature from a variety
of sources, integrating what we read with the deteriorated books
in hand, and making the best decisions we are capable of at the
time, realizing that sometimes we will be wrong.

So.

The old British Museum leather dressing was originally developed
for highly humid areas, such as parts of India, etc. where the
British Empire had records offices, and sometimes the formula
could contain a small amount of DDT to help control bugs; the use of
thymol sprays was developed to fight mold in those same areas.

The lanolin-neatsfoot oil formula was originally developed as a
leather dressing for harness and saddles, and was published as a
U.S. Dept. of Agriculture circular.

Acid, esp. sulfuric acid, has been blamed for most of the problems
afflicting leather.

Whether added to the tan bath (from about mid-19th century) to
accelerate tannage, or as a contaminant in illuminating gas, when
gas lights were in homes, businesses, and libraries.

This information was made clear in a 1901 (reprinted in 1905) _Report
of the Committee on Leather for Bookbinding_, a British committee.

They also identified ammonia fumes as part of the problem, but in
general, only acid has been remembered.

A few years later, in the Technologic Papers of the [U.S.] Bureau of
Standards there were two articles: "Determination of Sulphur in
Illuminating Gas," by R.S. McBride and E.R. Weaver.  No. 20, March 7, 1913.

The following year J. D. Edwards published "Determination of Ammonia in
Illuminating Gas,"  No. 34, March 2, 1914.

The effect of acid is to harden and embrittle leather, through removing
moisture which is required to maintain flexibility.

The effect of alkali (such as ammonia) is to soften leather and make
it powdery.

When an acid and an alkali come into contact they neutralize each other
(more or less) and the result is a salt.  Salts are deliquescent (they
dissolve under humid conditions and crystalize under dry conditions)
and wherever they crystalize they can weaken or break leather fibers.

Leather dressings have one thing in common; they are hydrophobic.

If a leather is flexible it contains enough moisture to stay supple
and a dressing will act to retain that moisture and prevent additional
moisture from being absorbed.  A balanced condition.

If a leather has become inflexible or powdery, it lacks moisture (among
all its other problems) and if any leather dressing would help, it should
be one which adds moisture to the leather.

But leather in that condition can easily be destroyed by moisture.

No clear answers, but maybe a little (non-gas) illumination of the problem.

Jack

Thompson Conservation Lab.
7549 N. Fenwick
Portland, Oregon  97217
USA

503/735-3942 (phone)
503/289-8723 (fax)


http://www.teleport.com/~tcl

"The lyfe so short; the craft so long to lerne."
Chaucer  _Parlement of Foules_ 1386

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