Gaskets: Sealing Exhibit Cases

Category:Exhibition
Category: Exhibit Technical Notes
Category: Exhibit Case Design
Category: Exhibit Fabrication
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Contributors: Members of the MWG's Materials Fact Sheet Working group, Jenifer Bosworth, Anne Ennes, Lisa Goldberg, Jamie Gleason, Jennifer Herrmann, Nancy Lev-Alexander, Patricia Silence, Catherine H. Stephens, and Theresa Voellinger
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=== How are gaskets used in exhibit cases? === 
Gaskets are a compressible material used to reduce gaps or create a seal between two surfaces that are not permanently closed, such as case access doors, removable panels or the space between a vitrine (bonnet) and its display deck. Gaskets are used to limit air exchange and to separate the object display chamber from other parts of the case. 
=== Why are gaskets a preservation concern? ===
Commercially available gaskets are made of a wide variety of materials, such as elastomers, silicone, Teflon, rubber, felt, or cork. Some of these materials are unstable or emit harmful volatile compounds. For example, sulfur emitting materials are a common source of pollutants that can cause damage. 
 
=== What types of gaskets can be recommended for inside exhibit cases? ===
A comparative list of gaskets can be found on Cameo.
Five important characteristics guide the selection of a gasket for use in an exhibit case: 
==== Material ====
Select a gasket made of a material proven to be stable. Even when a generally stable material, such as silicone, is used, testing is necessary to rule out gaskets containing harmful additives. 
==== Shape ====
Whether extruded or cut, the most useful shapes are rectangular or rounded pads and hollow "O" or "P" shapes. Diﬀerent applications require diﬀerent shapes; trial and error, including an examination of the product’s resilience, are required to choose a good gasket shape.
==== Size and thickness ====
An eﬀective seal depends on an appropriately sized gasket which allows the moveable surface to close and seat uniformly against the gasket. The objective is to ﬁll the space between the contacting surfaces. Excessive gasket thickness exerts stress on door and panel fasteners, causes hinges to bind, and deforms ﬂat panels. Undersized gaskets leave gaps at corners and between irregularities in the surfaces.
==== Density ====
The gasket must be soft enough to conform fully to the opposing surface, but have "memory" (ie, elasticity), enabling it to spring back into shape when not compressed. Softer foams are generally more appropriate; product literature speciﬁes the elastomer density (reported as Shore A and measured by a durometer). Consult Cameo to find out about the density values of various gasket materials for the application.
==== Attachment system ====
There are many options for attaching gaskets, including embedding the gasket into a channel or track, using mechanical fasteners, or an adhesive. Consider durability, ongoing maintenance, and chemical stability when selecting an attachment system. If pressure sensitive adhesive backing is desirable, select an acrylic adhesive system or apply an archival quality double-sided tape. See Adhesives for Use Inside Exhibit Cases - Wiki for more information about adhesive attachment systems. A comparative table of double sided tapes may be helpful in evaluating adhesive choice.
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=== Preservation Strategies === 
Evaluate the preservation strategy best suited to the needs of the objects being displayed. Very tightly sealed cases can expose collection objects on display to volatile offgassing from case fabrication materials, including the gasket, and/or the collections themselves. 
* Reduce air exchange, which can help to passively moderate interior relative humidity fluctuations. A gasket that performs successfully will slow the rate of air exchange between the gallery space and exhibit case. Well designed gasket systems can reduce air exchange rate to once every ten days. This degree of sealing is beneficial when the collections displayed require tight RH control. See Tech Note: Determining Exhibit Case Air Exchange Rates - Wiki 
* Reduce the transfer of dust, dirt, pollutants, and pests into the display chamber. All collection objects on display benefit from a dust free environment, as particulate debris can damage the surface and adversely affect the appearance of an exhibit. Note that small penetrations are often caulked with an appropriate material in conjunction with the use of gaskets. See Tech Note: Caulk Sealants - Wiki
* Safeguard against the accidental entry of water. A sealed case protects against liquid entering an exhibit case, whether due to a building water emergency event or a visitor spilling a beverage on a display case (if special events are held in the gallery). 
* Use gaskets of appropriate thickness and density to match the channel or trough and panel size. Proper density and thickness results in successful compression and seal for vitrine tops and doors. Work with a fabricator to determine appropriate gasket thickness and trough depth. 
* Make sure that gasket material allows clearance for any opening in a display case (doors and vitrine tops). To prevent air leakage, it is critical that access doors and removable panels:
***Include adequate fasteners to effectively compress the gasket material
*** Do not bow or deform under the pressure of the fasteners
*** Are seated evenly against a uniform gasket surface
*** Use a continuous gasket with no gap between sections
*** Do not bind against the gasket along the hinged side to ensure proper door operation
=== Additional Resources ===
:van Iperen, J., H van Keulen, K. Keune, K. Abdulah, and R. van Langh. 2021. "Crystalline deposits in new display cases at the Rijksmuseum: characterisation and origin". Studies in Conservation 66(5): 253-271.
:Alvarez-Martin, A., J. George, E. Kaplan, et al. 2020. "Identifying VOCs in exhibition cases and efflorescence on museum objects exhibited at Smithsonian’s National Museum of the American Indian-New York" Heritage Science 8: 115-127.
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