Responses to my posting about what precipitates "spue" prompted more
questions about "what is it" than it did about "when does it." Here's a
summary:
Spue is a white fine powdery odorless residue that appears on the
surface of leather bindings (not to mention saddles and other treated
leathers), looks similar to mold but shows a more even, uniform coating,
and is either salt or oxidized fat that exude from the leather under
"certain atmospheric conditions." Often, the salts and fats come either
from poor tanning [rarely] or, more likely, from inappropriately applied
leather dressings that contain things like Neatsfoot Oil or potassium
lactate--these were and are quite commonly applied, and their use is
highly controversial within the preservation community.
When the surface of the leather has more moisture than the interior mass
of the leather, the leather rejects the dressing (or the salts leach
out) and the result is lingering crystalline structures that remain on
the surface until they are physically removed. More subtle atmospheric
conditions, like pH and particulate contaminants, may also play a role.
The salt residues brush right off; the fatty residues are obviously oily
and tend to demand rubbing/buffing to improve the appearance.
Salt spue dissolves in water and has microscopic crystalline structure;
fatty spue melts when heated (no, I didn't try THAT test); molds exhibit
none of those characteristics or reactions. Nor does mold typically
present in low humidity exclusively on selected leathers (e.g., we have
a six-vol. set of which v.4-6 show the residue profoundly while v.1-3
are unaffected--apparently only half the set was treated).
Contamination of adjacent items is not an issue.
Recurrence is common. One librarian has actually established a regular
"wiping" program as part of his stacks maintenance operation because the
same volumes repeatedly "bloom" from time to time.
The literature is pretty anemic about this--see Abbey Newsletter, v.21,
no.2 (July 1997) for chemistry; http://www.hewit.com/sd2-bloo.htm for a
readable survey; discussion on Consdistlist listserv [all available on
CoOL site at: http://palimpsest.stanford.edu/]
In our case, the catalyst might have been a mid-December anomalous,
precipitous drop in our stacks air temp. (15 degrees within 24 hours),
which might have loaded sufficient extra moisture on the exposed binding
surfaces to trigger the process. But we still don't really know.
Richard Lindemann
Director, Special Collections & Archives
Bowdoin College Library
rlindema@bowdoin.edu