Retouching

Perry Hurt <phurt [at] ncmamail__dcr__state__nc__us> writes

>I'll attempt to put this issue in a nutshell from the point of view
>of a bench conservator.
>
>Oil paint chemically hardens with age. ...
>...
>
>In considering this one question comes to my mind.  From a
>conservation stand point the emphasis has long been on finding a
>varnish with the lowest possible solubility that also has useful
>optical and handling qualities.  We may have reached the limits in
>that search.  From a paint manufacturing standpoint, would it be
>more fruitful to explore acrylic paint formulations that are
>slightly less soluble, which could then safely be coated with the
>existing varnishes?

Greetings Perry, Thank so much for putting the problem into a
concise form.  Acrylic does not harden dramatically with age,
although in looking at some 25 year old acrylics they are slightly
stiffer and have a bit less elongation.  But in the scheme of things
I think you are absolutely right.  Acrylics are sensitive to most
solvents and do swell when in contact with water.

In doing research several years ago, we thought we would use
photomicrographs to show artists and conservators exactly what sort
of damage occurred when using water and mineral spirits to wash (and
I don't mean with a cotton swab) an acrylic film.  We had several
operators clean off our electronic scales with water and we kept
track of the pressure required to clean the scales as well as the
surface area of their fingers, obsessive, but I think it would be
very similar to what an artist might do in their studio.  In our
first pass in looking at failure of the acrylics, we were surprised
when we could not pick up signs of wear or spotting on the surface
and under 50 x magnification with bright field lighting source.  We
repeated under more vigorous washing and were able to show scratch
marks and some water spotting.  We also redid some of these samples
with washes of mineral spirits. Our sense from these early tests was
that a sufficiently dried acrylic film could withstand a certain
degree of water contact and mineral spirits without changing the
surface. Of course when we add pigment to the mix, we have observed
color pick up which can be profound in certain colors.  (Elizabeth
Jablonski et al. "The Conservation of Acrylic Dispersion Paintings:
2001 Paintings Specialty Postprints, AIC)

Under repeated washing and accelerated aging conditions, we would
continue to pick up color but after the fourth exposure this was
significantly diminished.  We realize that when a conservator is
picking up any color, many will simply stop the procedure.  So our
advice to artists was;  after your painting is finished, and if it
makes aesthetic sense to put on a varnish--first apply an isolation
coat.  We've recommended our soft gel gloss thinned with water
(mma/ba), as it produces the least amount of foam on the surface. On
some paintings this is very easy for the artist to accomplish... on
some it is quite difficult without a good deal of care and practice.
But when done well, it provides a clear acrylic surface that assists
in consolidating any pigment particles sitting on the surface of the
acrylic film.  We have also recommend that the artist wipe the
surface with a damp cloth to remove some of the residual surfactant.
We then recommend a final varnish of our MSA varnish: an n-butyl and
iso-butyl methacrylate mixture, or our polymer varnish, a solution
polymer of mostly acrylic and a small amount of styrene.

So far we have been quite successful in removing our MSA varnish
with mineral spirits, without damaging the paint film below.  Our
Polymer Varnish is removable in water at a slightly elevated pH. And
again we have not observed significant film change or color loss.  I
find this process has helped us with many a painting where the
varnish had to be removed by the artist.

We were quite interested in hearing the various papers offered up at
the MPU conference, that unless I dramatically misinterpreted their
finding, give some support to the idea that water may.... may given
certain limits... be used to clean an acrylic surface.  Many
conservators are already using wet techniques in their studios and I
think with significant success as well.   I think I'd have to ask
Bronwyn Ormsby for some clarity on her tests using mineral spirits
and the sort of damage that it may have caused, but I recall that
the mineral spirits didn't even remove the surfactant on the surface
of the acrylic film.

I realize much of this research is new and we may have to change our
recommendation when more information becomes available.  But I do
believe the multi coat application of an acrylic isolation barrier
followed by a mineral spirit or water resoluble acrylic varnish may
allow some greater opportunities for conservators to remove a
varnish without disturbing the layers underneath.

I look forward to your comments,

Mark Golden
Golden Artist Colors, Inc.


                                  ***
                  Conservation DistList Instance 19:60
                  Distributed: Thursday, June 15, 2006
                       Message Id: cdl-19-60-006
                                  ***