Heat & Humidity
This article originally appeared in Exchange,
a newsletter published by the Wisconsin Historical
Society. (Volume
24, Number 2, March/April 1982) It is the third in
a series of articles titled Conservation Corner.
The series features information about maintaining
an adequate environment for the storage and exhibition
of historical collections, employing proper collection
care techniques, and recording and cataloging historical
collections. This article was written by Tom McKay,
retired local history coordinator for the Wisconsin
Historical Society.
I saw a sad thing one day last fall. On exhibit in a small museum
was a beautiful little 19th-century sampler with two
dark stains running from top to bottom. How did this happen? How could
it be prevented?
The damage to the sampler can be attributed to two factors. Years ago, the sampler's original owner placed it in a frame. The method used was common at the time, but it turned out to be destructive. The owner mounted the sampler against a pine board and framed it in a manner that allowed the sampler to touch the front glass. The sampler eventually came to the local museum which hung it on the wall in a period room. Unfortunately, this museum, like many in our state, could afford no heat in the winter. In an extreme temperature drop, frost formed on the inside of the glass directly over the sampler. When the frost melted, the sampler absorbed water where it touched the glass. Wooden boards are highly acidic, and the water from the frost created an acidic solution. The dark stains appeared later, evidence of an acidic solution actually eating away at the sampler.
The damaged sampler needed two kinds of preventive care. A future column will discuss the first: proper framing. For now, let's turn our attention to heat and humidity control.
Fluctuations in temperature and relative humidity cause dimensional change in all objects. The more extreme the fluctuation, the greater the change. Worse yet are rapid fluctuations which cause rapid dimensional change. These fluctuations weaken all objects, but their effects are most dramatic with objects such as furniture where glue joints can break and wood can split from dimensional change.
Ideally, historic objects should be kept at a constant temperature
and relative humidity year round. While optimal conditions
vary with different types of objects, for many the
conditions would be approximately 68 degrees Fahrenheit
and 50-percent relative humidity. Many local museums
find themselves so far from these ideal conditions
that they lose all hope of dealing with heat and humidity
control. Rather than throwing up their hands in despair,
small museums must begin first by taking evasive actions
against extremes of temperature and humidity.
Many local historical societies do not have the money to heat their
museums. Societies faced with this situation should
not regard heating the museum in the same way as they
regard heating a private home. A constant temperature
of 45 degrees Fahrenheit will prevent frost and condensation
and will eliminate the most extreme and rapid dimensional
change. In fact, when the heating system has no humidification
device, a temperature of 45 degrees Fahrenheit is likely
to be accompanied by approximately 30-percent relative
humidity. This would be better than heating to a temperature
of 68 degrees Fahrenheit which could drive the relative
humidity as low as 10 percent. Perhaps a heating system
that operates economically, such as a heat pump, would
put a low heat level within the museum's means. If
a constant low temperature is achieved, it is best
to schedule winter meetings and events in another building
rather than put objects through the rapid fluctuations
of heating up and cooling down the museum.
When heating is completely prohibited by the budget, make changes for winter. To lessen the risk of frost formation, move objects hung or placed against outside walls. Drain or empty all pipes and vessels to avoid having them freeze and burst. Construct storage shelves of wood sealed with shellac rather than metal, and inspect metal objects in the collection during the cold for the formation of frost or condensation.
The worst heat and humidity problems are most often encountered in
basements and attics. Many museums use such areas for
collection storage. A relative humidity level greater
than 65 percent is an invitation to mold growth. This
means virtually every basement used to store collections
needs a dehumidifier. Attics, of course, present the
problem of extreme heat. A ventilating fan and sheets
of foam insulation under the rafters can reduce attic
temperatures 15-20 degrees. However, insulating an
attic must be carefully planned. Check on the insulation's
R-value, flammability and proper installation.
More than any other element of the museum environment, heat and humidity control demands substantial expenditures. When looking at the budgets that constrain the operations of their museums, local historical societies must ask if they can afford to provide heat and humidity control. When looking at the collections that uniquely document the history of their communities, local historical societies must ask if they can afford not to provide heat and humidity control.
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