"Bloom" on leather and many other objects can arise from at least two
different sources and has correspondingly different chemical composition
and should be treated differently.  One source is mold, a biological
source, probably arising from a sufficiently long exposure to relatively
humidity high enough to promote mold growth.  A second source is migration
of fatty materials, typically fatty acids like palmitic and stearic acid,
from the bulk of the leather to its surface where crystallization of the
acids occurs.  These two types of bloom can be readily distinguished by low
power stereomicroscopical examination.  Molds shows fluffy and filamentary
structures that are now smeared by wiping with a needle, whereas fatty
acids are waxy, non-filamentary, perhaps fuzzy crystals, easily smeared by
wiping with a needle.

Different methods for removal of the two types of bloom are required.
Fatty acids are insoluble in water but are soluble in mineral spirits and
other non-polar or very slightly polar hydrocarbon solvents and may be
removed with very slightly moistened swabs.  Unfortunately, since the
source of the fatty acid bloom is fatty materials in the leather (either
natural in particularly high fat content leathers, or added by such
products are saddle soap and leather dressings) these fatty acid blooms are
likely to redevelop over time.

Fatty acids blooms apparently develop because there is an incompatibility
between the fatty acids and the materials in the leather in which the fatty
acids were dissolved initially.  This incompatibility may have always been
present and just becomes noticeable after sufficiently time passes for the
fatty acids to migrate from the body to the surface.  Or, incompatibility
may develop as the chemical composition of the object changes with time,
primarily by oxidation, hydrolysis, and crosslinking of the compounds in
the object.  Oxidation may increase the polarity of the object,
crosslinking may reduce the molecular free volume of the fatty phases, and
hydrolysis of soaps and fatty oils may produce higher concentrations of
free fatty acids that can migrate.  These reactions are all affect in a
very complicated way by environmental conditions, particularly relative
humidity.  Decreased air circulation may contribute to fatty acid bloom
production.  It has been observed that bloom forms more often on oil
paintings and prints that are framed behind glass, than on paintings that
are not glazed (Williams, R.Scott, "Blooms, blushes, transferred images and
mouldy surfaces: What are these distracting accretions on art works"
Proceedings of the 14th Annual IIC-CG Conference, May 27-30, 1988, Toronto,
Canada, pp. 65-84).

The advisability of trying to remove fatty acid blooms should be
considered.  Aside from the unsightly appearance of the fatty bloom, there
is probably not much fear that the leather object will be further damaged
just because this fatty acid bloom is on the surface.  The entire object is
probably saturated with the fatty acids or compounds that break down to
produce the fatty acids (such as neatsfoot oil and soaps so commonly found
in leather dressings) so that removing the fatty acids from the surface
will have little effect on the overall fatty acid content of the object.
Since there could be damage to other objects that come into contact with
this bloom, direct contact between this and other objects should be
avoided.  Also there may be reaction of the fatty acid bloom with other
components of the object such as metal fittings or fastenings, that may be
corroded by the fatty acids, especially copper based alloys.  Such a
situation would definitely warrant removing as much fatty acid bloom as
possible, and the physical isolation of the fitting from the fatty leather
surface (perhaps for example by polyester or polyethylene plastic sheet
separators between fitting and leather).

Mold is not soluble in water or mineral spirits.  If the mold is embedded
in a layer of fatty material, then dissolution of the fatty material may
also remove the mold, but this is a secondary effect.  In general mold must
be removed by some sort of physical or mechanical process perhaps aided by
cleaning solutions.  Also, since mold is biological in origin and will grow
if conditions are right, some process to kill the mold might be considered.
 Such treatments are beyond my experience and information for this must be
obtained from someone else.


______________________________ Reply Separator _________________________________
Subject: Treatment of leather
Author:  Museum discussion list <[log in to unmask]> at INTERNET
Date:    3/15/96 9:52 PM


We have in our collection leather bridles, halters, reins, saddles,
saddlebags and so forth that have a white "bloom," which I'm assuming is
mold. It wipes clean, and I've used basic saddle soap which also removes
the film. The items are stored in a climate controlled HVAC building, but
I'm wondering if there isn't enough air circulation. Has anyone else had
this problem?
Please, no lectures about calling conservators. This is just a topic for
conservation and feedback.
Thanks.

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