Controlling Temperature Inside Exhibit Cases
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Elevated and unstable temperatures inside an exhibit case accelerate deterioration of a wide range of collection materials. Inappropriate case and lighting design can cause such elevated temperatures.
Why is temperature control inside an exhibit case important?
Temperature stability is a preservation concern for the planning and design of exhibit cases. Temperature extremes (above 75°F) and wide fluctuations in temperature within an exhibit case are responsible for considerable damage to display collections.
- Many museum objects contain thermally-sensitive materials that expand and contract at different rates; this material response can result in structural and surface cracking and the consequent lifting of paint, and separation of varnish and decorative layers.
- Increased temperatures speed up many chemical degradation processes and biological activity, as a rule, increases.
- When heated, some materials soften causing surface tackiness, and sagging or drooping of the structure.
- Temperature change directly affects the moisture content of organic materials; as the air inside a case heats up and cools, the relative humidity fluctuates. This causes moisture changes in the object (a process which causes dimensional stress and age-embrittlement). Radient heating can cause damage even if the RH is kept constant.
- Heating and cooling creates convection air currents within an enclosure which increase air exchange through small holes or gaps. This can in turn exposes objects to higher amounts of dust and more rapid fluctuations in temperature and relative humidity.
What causes an exhibit case to overheat?
There are three basic forms of heat transfer: Conduction, radiation (infrared), and convection.
- Conduction: Heat can move through case construction materials from an exterior heat source by passing from molecule to neighboring molecule. Denser materials usually conduct heat more quickly; use of insulating materials limits this form of heat transfer.
- Radiation: Heat is also transmitted by electromagnetic wave motion similar to light. It transfer from one warm object such as a light bulb outside the case or in a lighting attic through the air to a cooler material, such as an exhibit shelf or display object.
- Convection: Heat is transferred by air movement from warmer to cooler surfaces. Air movement is typically caused by expanding warm air that rises, and cooler air which sinks.
How can the case design limit heat build-up?
Some key design features will prevent heat gain in exhibit cases. Use insulating materials in the construction of the display chamber where necessary. Because the principle cause of elevate temperatures is the heat generated by lighting fixtures and lamps, special attention is required when designing integral case lighting.
- Construct the lighting chamber from a conductive material: Use metals such as aluminum or sheet metal as opposed to wood. These materials dissipate heat out to the exterior.
- Isolate the lighting chamber: The lighting chamber air must be physically separated from the display chamber air so that heat transfer can be controlled. Any gap between the two chambers destabilizes the temperature due to convection and conduction heating.
- Vent the lighting chamber: Venting the top or the back of the lighting chamber to the ambient space allows heat from lighting equipment to dissipate through convection currents. A fan may be required in large cases or constricted locations.
- Consider using an insulating glass between the lighting and display chambers: Because conduction causes most heat gain in the case, locating an insulating material between the lighting and display chambers is a practical method for reducing heat gain where high temperatures cannot be otherwise reduced.
How does one identify an insulating material?
Commercially available products will have R value or a U value rating.
- R values: Measure the thermal resistance to the conduction. Higher numbers indicate better insulating properties.
- U values: Measure the thermal conductivity of the material. They are measured as thermal units transmitted through one square foot per hour per degree Fahrenheit temperature difference between the two sides. The lower the number, the higher the insulating properties.
|FOR GLAZING||TO LINE CASE WALLS|
|Thermal-pane glazing||polyethylene foam|
|Triple-pane glazing||batting insulation (fiberglass)|
|Heat-reflective glass||metal foil reflective insulation|
Products, Manufacturers, and Suppliers
Mention of a product, manufacturer, or supplier by name here for information only and does not constitute an endorsement of that product or supplier. Listed materials have been used successfully in past applications. It is suggested that readers also seek alternate product and vendor information to assess the full range of available.
- Metallic Barriers
- Marvelseal 360 or 1177, Ludlow Corp, Laminating and Coating Division, Homer, LA, 710040
- Metallic Reflective Insulation Laminate
- Reflectix Aluminum Foil Insulation, Reflectix Inc. Marketville, IN, 46056