Painting installed on marble wall

>Thomas Dixon  <ngvcons<-a t->vicnet< . >net< . >au> writes:

>Adams also attributes the damage to mortar to high humidity due to
>reduced air circulation. I think of
>this in another way.

Dixon and I do not disagree at all; as I said in my earlier note,
the high humidity is "caused by the reduced air circulation and
resulting depressed temperature."   The significant moisture does
not come through the wall, but from the room; and the r.h. locally
rises because of the local chilling.  This chilling doesn't occur
elsewhere on the wall because there is adequate air circulation to
prevent the chilling.

This phenomenon is an excellent demonstration of the small
dimensional scale of, and the importance of, "microclimate"  in
conservation; and indeed it was this phenomenon and its great power
for causing damage that made me recognize the importance of using
small electronic sensors to measure climate, not mechanical
hygrothermographs.  The severe climate problems are rarely at the
center of a room, they are at odd locations such as behind a
painting on an exterior wall, or over a ventilation discharge, or
between the face and reverse of a painting.

As an added note, we have found that it is far more accurate in such
cases to compute the local r.h. by the process Dixon describes
(although we do it electronically) than to attempt to measure it
directly, particularly because all but the most expensive, and
physically largest, types of r.h. sensors lose accuracy at the most
important extremes of the scale.  This calculation assumes that the
moisture ratio (grams of water per gram of dry air) of the air in
the room is essentially uniform; and experience has confirmed that,
provided that local condensation has not locally reduced the
airborne water.  Thus, we generally measure the temperature and r.h.
at a moderate region of the room where the r.h. will be 40%-70%, and
measure only the temperature at the regions where we expect more
extreme temperature. (The exception to this is cases where we
suspect a water source or sink may exist.)

We also agree with Dixon regarding backing paintings, with a minor
addition.  We remove a lower corner of the backing to permit some
exchange between the ambient air and the air at the painting
reverse. We have never tested this rigorously; but our theory is
that it takes very little obstruction to introduce the desirable
thermal lag Dixon mentions (e.g., common stretcher marks) and this
relatively small opening prevents building up big differences,
particularly of r.h.

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                  Conservation DistList Instance 11:50
                 Distributed: Monday, December 8, 1997
                       Message Id: cdl-11-50-003
                                  ***