Painting installed on marble wall

Carol Clausen <carol_clausen<-a t->occshost< . >nlm< . >nih< . >gov> writes:

>We are thinking of hanging a framed painting (oil on canvas) on a
>marble panel. The marble slab is an outside wall

The problem you posted on the Conservation DistList is a common one
and congratulations on recognising the potential danger.  You have
already received advice from George Adams regarding this and his
advice is sound.  I add to it because in my experience the actual
potential for damage is probably understated. Adams also attributes
the damage to mortar to high humidity due to reduced air
circulation. I think of this in another way.

In my opinion, the fundamental problem is that relative humidity is
dependent to a large extent on temperature.  If your room air is 50%
at 20 degrees C (I don't know if you are a celsius or fahrenheit
person) and the marble is less than that, a microclimate will be
created behind your picture with a higher r.h.  You can estimate
this by measuring your temperature difference and consulting a
psychometric table.  Noting also that mold is likely to grow above
67% rh, that the canvas dimensions are determined to a large extent
by the available moisture, and that there is a phenomenon called the
J curve (where a sized canvas will continually slacken with
increasing moisture to a certain point where it will suddenly and
spectacularly shrink) the prospects are downright scary.

>From many years of looking at pictures hung on external walls in
homes, historic houses and even some museums, I am convinced it is a
really bad place for pictures.

There are several things you can do to reduce this effect, but the
easiest is to hang on an interior wall.  We back all paintings with
1/4" Fomecor board which acts not only as a dust barrier and
provides physical protection during handling, but provides a fair
bit of thermal lag. I've been using this for (gulp) 30 years and
still feel on balance it is the best backing board and backing is
the first best step for deterioration control in a paintings
collection.

Adams mentions creation of a "standoff" for the picture if it must
be displayed on the exterior marble wall.  My approach would be to
create a false display wall and insulate it.  This could take the
form of a piece of 3/4" plywood bolted to the marble with 4" spacers
and incorporating 2" of styrofoam insulation on the back.  We would
probably cover this with fabric to make it a feature--if you can't
hide it, dress it up!

To see if this will work in your situation, and refine the
thicknesses, obtain a representative measure your temperatures and
room rh over a wide range of outside conditions and consult your
psychometric table. Again, congratulations on recognition of a
problem very widely not recognised and which causes pretty
significant deterioration.

Thomas Dixon
Chief Conservator
National Gallery of Victoria
Melbourne Australia

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                  Conservation DistList Instance 11:50
                 Distributed: Monday, December 8, 1997
                       Message Id: cdl-11-50-002
                                  ***