Micro-Climate Control in the Museum Classic but New Difficulties
Prof. Emeritus Dr. Tsuneyuki Morita
Professor Emeritus, National Museum of Ethnology, Osaka, Japan
Abstract
N
ecessary and sufficient conditions of micro-climate
control in the museum are well known for us, including
even a conservator candidate, while the control practice is less
familiar than we expect. It is easy to say that the condition:
20±2 degree C of temperature and 50±5% of RH is ideal for
gallery and storage area in the museum, and that the more dry
condition is the better for the metal object. Then how can we
prepare a better condition to show a wooden piece at a side of
a bronze object in the same gallery. These types of work are
not a duty of air-conditioning equipment operator but of us,
conservation scientist. Their jobs are only to do an accurate
handle manipulation in accordance with the indication from
the conservation scientist, and operators have nothing to do
with no indication.
Half a century ago, when the actual museum microclimate standard was setup, insufficient development
of air-conditioning system waited us yet and humidifier,
dehumidifier, heater, fun, ventilator and other similar tools
weremain equipment to get better environmental condition
for almost of museums. Adding this, we have to understand
that the guidelines of micro-climate condition of the museum
were prepared mainly for developed European and American
museums. Conditions of other areas were few considered.
Just recently (the last modification on March, 2012), Canadian
Conservation Institute (CCI) proposed new environmental
guidelines, more moderate than the classic one, which
shows neither fixed areas of temperature nor of RH but only
maximum permissible fluctuation and gradients in short-term
and in seasons. The guide lines also presents six (6) classes
of control, from precision level to prevent dump level. This is
more practical than the classic one. And more, we have to note
Conservation In Museums (Present & Future)
that the both guidelines are prepared for general conditions
in the museum. It is too difficult to prepare a better condition
to each object.
Within the last some decades, introduction of highly sealed
show case and humidity control agents, such as active clay and
silica-gel, has led a dramatic change of the museum display.
At the same time, measuring room temperature and RH
has changed not to satisfy necessary conditions to facilitate
internally precise control of vitrine, because the inner space
is independent from the outside, if seal is perfect. When
condition of a well-sealed vitrine is kept, movement of the
internal moisture content is more important than that of RH.
Quantity rise in a sealed space shows that new moisture is
carried in from somewhere, i.e. either from an object or from
outside through a gap, and the souse should be possible to find
when we check a change of temperature. The most important
points of the micro-climate control for the vitrine are moisture
quantities and its volume change in a fixed area.
At the moment of transformation of water from liquid to
vapor, rapid expansion of water gives a strong tension to
the surroundings. One of the typical cases is momentary
movement of moisture from inside of an object to the outside
to cause possible accidents, which we call as drying damage
of object. That is, an enemy of fragile object is not changes of
relative humidity but movement of moisture itself.
As you know well, we call a moisture content in a given
quantity of dry air as “Absolute Humidity” or AH. However, our
care must be taken to check that there are two definitions of
AH: in the classic museum climatology and physics, as weight
of moisture contents in the one cubic meter (1 m3) of dry
air, of which unit is kg/m3, while in modern air-conditioning
engineering as those in one kilogram of dry air, by kg/kg
unit. Of course, “kg” unit can be replaced by “g” unit. Even
the famous book: “Museum Environment” by Garry Thomson
uses the both different standards to take careful readings.
It is simple why modern engineering uses “kg/kg” unit for
AH value, because volume of gas will vary with changes in
temperature while weight will not be affected. In the case of
the later, the conversion between volume and weigh of dry air
may be required, using “molar mass”.
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Air is not a homogeneous gas. As we look clouds in blue sky,
they float as some of group. If dry air and moisture make
homogeneous mixtures, pale white clouds should cover the
whole sky. Invisible group of moistures in the air always
migrate within a given space. RH is a suitable measurement
method for a large space to get an average value there,
consequentlyit is applicable for storage or exhibition hall.
We have paid much care to RH but less to AH, while new age
of the museum conservation should require more care for
the later. Silica-gel and other climate control agent will allow
reducing the rapid gradients within a limit, however, when it
exceeds the limit, what should we do?
This is a practical experienceof the author’s group. A vitrine,
which was sealed in a dry day, showed, for a few days, lower
value of RH than the standard. The content was a wooden
mosaic. More days of maintenance in this state can cause a
surface stripping. Sufficient quantities of silica gel (Art-sorb)
were set in the vitrine. However, results of our measurement
and calculation show insufficient quantities of moisture in the
case to be controlled by the gel. Then we inserted a sheet of wet
filter paper from a small gap of the vitrine. At a previous test,
approximate evaporation time to release necessary quantities
of moisture was checked. We have left a final adjustment to
silica gel to get a good result. On the contrary, we thought of
using hot and dry filter paper to remove it, while we lost a
chance to do a practical experience.
The first special frame box, which was perfectly sealed and
contained sufficient amount of silica gel on the back side of
panel painting inside, was that for “St. Catherine”(82.5 x 44.5
cm) by S. Martini (Coll. National Gallery of Canada, Ottawa).
It was designed on 1970 by N. Strow, Canadian conservator,
for the air-transportation between Canada and Japan and
for the exhibition display at Fine Arts Exhibition of Expo 70,
Osaka. The Second one was that for “Mona Lisa” by Leonardo,
which was designed by K. Toishi, Japanese, for an exhibition in
Japan. These both two cases has small size of space capacities,
and those perfect seals, combined with an action of silica gel,
protected sufficiently from disfiguration of wooden painted
panels. These were just exceptions. To make such a perfect
case is too expensive and practically difficult now.
Conservation In Museums (Present & Future)
However, today it is easy to find well-sealed vitrines. An
important point is not “perfect (as above two cases)” but “wellsealed”. If “perfect”, everyone may handle it with some cares,
while “well-sealed” requires always more cares by specialists.
Recent vitrines often use transparent acrylic panel for glass
one, and display designers and/or furniture makers make
design of them. They appreciate its high-transparency,
toughness, economy and others and show less interest on
physical properties other than its electrification. Even many
conservators ignore them. For example, thermal conductivity
of acrylic resin (PMMA) is 0.21 and glass1.03, the latter is
about five times easier to transmit heat than the former.
Also specific heat (or thermal capacity) of PMMA is 1.47kJ/
kg/K, and glass0.8kJ/kg/K. This means that the formeris
about twice harder both to get warm by outer heat radiation,
including spot lighting, and to get cold after heated. If acrylic
plate is radiated for long hours by heat, the heat effect should
be kept also for long hours.
For more than the last three quarter of centuries we discussed
micro-climate in the museum. Recent display innovation
requires us to discuss ultra-micro-climate as practice in the
museum.
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