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Subject: Smithsonian press release

Smithsonian press release

From: William Real <realw>
Date: Tuesday, September 27, 1994
Regarding Mr. Druzik's posting on the new Smithsonian climate control

It seems to me there is some confusion here, and I think the conclusion
that we can save bundles of money on future HVAC treatments (or that we
have wasted bundles on past HVAC treatments) may be premature.  David
Earhardt, in his recent presentation at the IIC Congress in Ottawa, said
(if I heard him right) that the most sensitive material they studied was
rabbit skin glue, a material present in many works of art, and similar
to other materials very common in our collections.  Rabbit skin glue, he
suggested, could undergo elastic deformations (reversible) within RH
fluctuations of plus or minus 8%.  Beyond that range, rabbit skin glue
is permanently deformed.  My conclusion from this is that plus or minus
8% should be our target range for RH, even if other materials can
sustain elastic deformations at far wider fluctuations (e.g. cottonwood,
plus or minus 15%).

To achieve plus or minus 8% in most, if not all, climates, what are you
going to eliminate?  Certainly not humidification or dehumidification.
You might do with less expensive, less sensitive humidistats, which
would indeed be a capital savings, though not millions of dollars, I
would guess. Even if it is a capital savings, it does not appear, from
computer modelling studies commissioned by the Getty Conservation
Institute (Ayers/Lau, ca. 1991), that wider RH tolerances yield
savings in energy costs:  there was a negligible difference between plus
or minus 2% and plus or minus 7%.

The reference to protecting building materials from the climates
provided by "specialized" systems may also be similarly optimistic: many
modern buildings without vapor barriers cannot even tolerate 30% RH in
cold winter climates without some risk of damage from condensation (our
1974 building, by Edward Larabee Barnes, is just such a building). Even
if a building could tolerate 30%, say, but not 40%, again, what are you
going to eliminate from the HVAC system?  You're probably still going to
need the humidification system, even to achieve your 30%. Thus, capital
savings are not likely here either. Intuitively, holding 30% plus or
minus 8% would be less expensive, in a typical northern climate, than
holding 40% plus or minus 8% or 50% plus or minus 8%. But perhaps not:
to cite Ayers again, they found that in most climates, based on
computer modelling, maintaining 50% is cheaper than maintaining 40% or
60%.  They didn't look at seasonally-adjusted setpoints, though, so
there *is* probably some savings in applying the lower winter RH

I think many of our institutions, even with our specialized equipment,
have something like the environment now apparently recommended as a
result of the Smithsonian's research.  And most of us, I would hazard,
still see damages occurring.  We see flaking.  We see splitting of wood.
We see desiccation and maybe even mold.  Many of us are doing the best
we can, and we still have damages.

Perhaps what we need are more studies like the ones commissioned by the
Getty Conservation Institute, informing us of the true cost aspects of
these new guidelines.  It would be great if we could save that money for
our many other needs, wouldn't it?

Another aspect of climate control systems that is omitted in the
Smithsonian press release is ventilation--to prevent air stagnation and
to achieve particulate and gaseous filtration.  Regardless of the level
of relative humidity control and temperature control, we are always
going to need to move filtered air through our spaces.  We are always
going to need air handlers, fans, filter sections, and ducted air supply
and return.  I believe there are recommended air change rates for
library materials, in the order of 6 air changes per day, if I recall. I
wonder if commercial or residential systems can achieve that level of

At the recent IIC Congress, we also heard a paper by Jonathan P. Brown
on the difficulty of accurate RH measurement in museums due to
instrument accuracy, problems in calibration, and RH variations from
measurement point to measurement point.  One could argue that these
limitations in measurement invalidate our usual RH specifications.  One
could also argue that even with the tightest equipment and the most
diligent calibration, we are probably already getting significant
variations well beyond our tight specifications and well beyond what our
point-specific hygrothermographs are telling us.

William A. Real, Chief Conservator
The Carnegie Museum of Art
4400 Forbes Avenue
Pittsburgh PA 15213
Fax: 412-622-1979
Alt fax: 412-622-3112

                  Conservation DistList Instance 8:22
               Distributed: Wednesday, September 28, 1994
                        Message Id: cdl-8-22-001
Received on Tuesday, 27 September, 1994

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