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Subject: Paintings on glass

Paintings on glass

From: Niccolo Caldararo <caldararo>
Date: Monday, January 3, 2000
This is in response to the discussion of Aquazol by Deborah Parr:

Aquazol 50 and 500 is a case study on the introduction of new
materials into conservation and deserves some contemplation.

"They must consider the consequence of the employment of new
techniques with deliberate caution, avoiding blind conservatism as
well as impetuous approval." Richard D. Buck (Preface to the first
edition of On Picture Varnishes and Their Solvents)

Deborah Parr discussed the use of Aquazols on glass in an earlier
post. I had asked why Ms. Parr had used Aquazol in a treatment as it
is a new material and is untested in actual treatment on glass.
Also, I wanted to know why a conservator would use a material like
Aquazol in a treatment for which their are standard materials and
established treatment modalities which have proven to be stable and
compatible with glass and other similar substrates.  This stability
was demonstrated in my article in Studies in Conservation in 1997.
The choices conservators make, especially in situations like this
one, illuminates an important question for the relationship of
conservation science and conservation practice.  In her response,
Ms. Parr refers DistList readers to Richard Wolbers, et al. article
on Aquazol (1994).  Since I have long been concerned with the
process by which materials and methods become accepted in
conservation, this discussion provides a vehicle to examine this
most important subject.

Tony Werner stated in his Synthetic Materials Used in the
Conservation of Cultural Property (1963), that, "It cannot,
therefore, be too strongly emphasized that no synthetic material
should be used in conservation unless its properties are reasonably
well know.  This implies that the composition of a synthetic
material--as indeed also of a natural material--must be known in all
essential details so that its permanence and properties can be
accurately assessed."  Elsewhere (JAIC, 1987) I have discussed in
detail the history of this precept in conservation, but I would like
only to address here more recent generally accepted qualities
required for a material to be considered appropriate for
conservation use in this context.  The main point to be considered
here is not how well a particular material performs under
experimental conditions only.  Rather we are focused on what
experimental conditions are designed for and how well these can be
interpreted to predict the behavior of the material in contact with
an artifact.  And finally, of most importance is how the material
actually performs under normal conditions of exhibition, storage and
travel on an artifact.

Aquazol products are manufactured by Dow Chemical and were developed
in the 1970s as adhesive agents.  Chemically they are known as
Poly(2-ethyl-2-oxazoline) and their properties are summarized by
Chiu, et al. (1986).  Wolbers, et al., 1994, presented a series of
results from their experimental work on these products.  The value
of the results they provide is undermined  by the unique character
of the experiments, their design and type of data produced, which
depart in some important ways from earlier methods of testing
materials for conservation.   One might note, however, that in many
of these departures, the authors are trying to devise more effective
or efficient means of producing reliable data.

Wolbers, et al. present solubility data for the Aquazols but no data
on drying curves for solvent retention.  Such information
characterizes the first period of life of a film (Feller, 1959).
Other standard methods of determining changes in a material on aging
have been derived from Nathan Stolow's pioneering work (1959),
examples of these are Hedley (1980) and Michalski (1990).  A recent
adaptation of these methods by Alan Phenix (1998) benefits from new
instrumentation and research from other fields of polymer chemistry.
Nevertheless, this approach measures changes in solubility as
demonstrated by the swelling process of the film in various
solvents. Unfortunately, Wolbers et al. did not undertake similar
tests so their results are not comparable in this standard measure
of film analysis used ultimately for aging properties.

Wolbers, et al. 1994 utilized an accelerated aging regime to produce
the equivalent natural aging of only 24 years, usually the standard
for conservation stability is 100 years.  More important is the fact
that their design for aging varies significantly from that used by
most other researchers, making comparison difficult (see, e.g.,
Feller, 1976, 1987; Horton-James, et al. 1991).  Determination of
solubility was also performed in a novel fashion using size
exclusion chromatography.  Solubility was established by subjective
observation with no quantitative results and no reference data or
comparative samples  provided.  However, in tests with both Aquazols
chromatography  showed changes after aging.  Again, no comparative
data was available to interpret this response.  This was the case
with other tests reported, with thermogravimetric analysis as well
as differential thermal analysis.  The most useful test reported
involved the viscosity measurements to determine changes in
molecular weight after aging. Calculated molecular weights for
Aquazol 50 dropped from 50,000 to 44,000 and for Aquazol 500 from
300,000 to 210,000.  This most certainly must indicate a substantial
degree of degradation and changes are reported in the FT-IR data
after aging, but considered "essentially unchanged"  or "virtually
identical".  Remarkably, the authors interpret this change in
viscosity (and most assuredly DP) as "Aquazol preparations seemed to
decrease in size, rather than crosslink or increase in size."  They
then decide that in the case of Aquazol 500 the data can be
explained by chain scission.  Their resolubilization tests also
appear to indicate an increasing insolubility after aging.

I would agree with the authors' conclusions that Aquazols are only
weak consolidants and their tendency to depolymerize should caution
conservators to their use, especially in light of Tom Stone's long
term restudy of treatment (1996) relating to the performance of
methyl cellulose.  I am glad that Wolbers, et al. took the time to
investigate Aquazol, however, I think that it is unfortunate that
some conservators have taken the results of this study to support
the immediate and widespread use of them on objects.  The fact that
Wolbers, et al. caution that Aquazols should be studied further, has
apparently been overlooked.  Certainly there are many qualities the
Aquazols possess to make them attractive for use in conservation,
but we should wait until new developments in them arise or more
testing has been completed.

Niccolo Caldararo
Director and Chief Conservator
Conservation Art Service

                  Conservation DistList Instance 13:38
                Distributed: Wednesday, January 5, 2000
                       Message Id: cdl-13-38-001
Received on Monday, 3 January, 2000

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