Ozone treatments

Sometimes I feel like Stefan Michalski and echo his sentiment--this
ozone thing has dogged me throughout the years of articles and
pollution seminars, and I simply want it to go away.  But it
doesn't. Maybe the question itself should be treated with ozone?
(Not the questioner--I like the questioner--just the question.)

Every time I encounter this issue it very nearly has the same
identical sentence, "It has been suggested that works be treated
with ozone to deodorize them."  One should immediately suspect the
treatment.  "Deodorization" bears a misleading association to human
armpits, where stopping perspiration or frustrating microbes are
operative strategies.  Ozone targets neither, it simply "nukes" the
whole area.  In fact, the non-specific effect of "deodorizing"
should tell you that ozone must be responsible for breaking up a lot
of chemically unrelated compounds caused by low and medium
temperature pyrolysis of disparate compounds.

There are several possible initiating steps for an attack on
aromatic compounds and the attack on unsaturated carbon-carbon seems
to usually follow from an electrophilic addition of ozone which
quickly divides into an RC=O and a Criegee biradical.  This rather
adroit move has a large rate of reaction where substituents on
olefins are electron-donating, breaking up the molecule and/or
conjugated system, hence causing fading.  It should follow that
substituents with electron-withdrawing properties have lower rates
of reaction with ozone.  And indeed that is often the case. Some of
these more stable colorants do end up finding their way into
contemporary artist's selection of materials but without knowing
that for a fact in specific cases, it's a risk, especially with
unorthodox use of materials.  To summarize, ozone used in this
manner could cause a number of unexpected damaging consequences.

The sensitivity to ozonolysis may not parallel sensitivity to light
in terms of degree however.  Colorants with lightfastness in the
6-25 million lux hours range may be very sensitive to the conditions
used during this "deodorizing" treatment. The presence and type of
binder may have a large or small role in limiting the rates of
fading.  Two different acrylic formulations--Grumbacher "Hyplar" and
Magna Acrylic--provided little protection to alizarin crimson
(although coatings generally provided much better protection).  As
far as the binder itself is concerned, I don't see much for ozone to
react with acrylics, but a painting's support, if of wood or natural
fiber, would certainly be vulnerable owing to the presence of lignin
(having both aromatic and olefinic character.)  At high humidity
other mechanisms begin to take over and even cellulose becomes
reactive.

In short, it's far better to consider odor from fire/smoke damaged
objects to be a self-correcting problem and an opportunity to remind
administrators whenever they smell it,  for both good fire
suppression and disaster preparedness, risk assessment, and chemical
filtration in air conditioning.

Jim Druzik
Senior Scientist
The Getty Conservation Institute

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                  Conservation DistList Instance 11:45
                 Distributed: Monday, November 17, 1997
                       Message Id: cdl-11-45-001
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