72F 50% RH is standard flatline conditions.  Almost no one can maintain
this outside of a sealed case with major mechanical attached.

72F +/- 5 & 50%RH +/-5% was standard for 50 years.  In UK and some other
European countries, 68F +/-5 & 45/55% RH +/-5% is common.

Storage of photographs has been moving to colder and less humidity
because of there sensitivity to water in fading and acetate base
degradation.  Big problem with acetate film now.

About 5 years ago Marion Mecklenburg and co-workers (at CAL, SI, now
SCMRE) stunned the Museum/Conservation world by saying one could go as
far as 72F +/- 20F & 50%RH +/-20%.  He was talking about storage of
airplanes at the proposed, SI, Dulles Airport, NASM building.  The
rational was that the moisture content changes, produced in the
materials by the 40F/40% range, did not exceed the plastic regime of the
mechanical properties, of "most" air/space museum materials.  Problem
was that this is a general statement and doesn't consider materials
which have tolerances inside these general ranges.  One, I know quite
well is albumen prints which fracture with a 5-10% RH change.  Already
flaking paint and very old paint, paper, leather, veneer, etc., which
have become brittle over time (lost their plastic component), fall into
the sensitive category.

So we are re-evaluating T/RH standards.  Mecklenburg will be taking
another shot at explaining his position at the annual AIC meeting
(Conservators) June 2000.  Most are skeptical.

I say +/-10-15% for non-critical "museum" materials and +/- 5% for
critical stuff.  RH set point can be slowly adjusted down to 40 %RH in
winter to protect building which reside in freezing conditions and up to
55% in summer for humid climates.  The key is slow adjustment (a month)
from one set point to another.  There is still significant debate and
many bruised professional egos on both sides.

Tim Vitale
Preservation Associates
510-594-8277
440-775-7331 until 9/26/99

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