Category:Designing a Preservation-Grade Case or Enclosure

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Designing a Preservation-Grade Case or Enclosure[edit | edit source]

Preservation and Case Design[edit | edit source]

Displaying three-dimensional objects inside an exhibit case and framing two-dimensional materials provide protection that cannot be provided by open display. A sealed or ventilated enclosure creates a localized environment that can be designed to fulfill a variety of preservation criteria. The choice between various types of case enclosures depends on the risks presented by a particular exhibit design, the ambient exhibit environment, and the sensitivity of the objects. Creating a case or any preservation enclosure involves vetting of materials, design, fabrication, and performance testing. (See Figure 6)

Preservation-Grade Exhibit Cases, Risk Protection[edit | edit source]

An exhibit case or well-engineered frame can protect objects from:

The level of protection from these risks depends upon the design of the case. Additional preservation measures may be required depending on object sensitivity, specific preservation needs, and situational parameters, as discussed in Figure 6. Different types of case designs are described in the sections below.

Case Components[edit | edit source]

Preservation enclosures are designed to address the risks described above. The quality of materials used in case construction are critical in reducing risk to collection items. See Materials Choice section of this wiki. A preservation quality exhibit enclosure typically includes these components:

  • Display chambers or enclosures to house the objects. Objects are directly exposed to the conditions in this chamber, making the design and material composition of this chamber key to preservation-responsible design.
  • Lighting chamber may be a separate sealed compartment with its own access and venting. Other lighting configurations may use LED or fiber optic fixtures within the display chamber, provided the system electronics do not heat the objects. (See section on lighting)
  • Environmental maintenance chamber to provide environmental or chemical pollutant control. Depending on the quantity of control materials needed and design considerations, environmental or chemical pollutant control materials may be located inside the display chamber.
  • Particulate Filtration: Ambient air must be filtered as it passes through an exhibit case. Dust filtration at the vents is particularly important when the overall room HVAC filtration is ineffective or nonexistent.

Case Stability, Security and Access[edit | edit source]

Ensure collection and human safety by designing and constructing case/enclosure(s) that are stable, secure, and safely accessible.

  • Construct a physically stable, structurally secure case. Limit vibration by using movement-dampening devices. When floor or wall attachment is not possible, include space for ballast weight to prevent jarring and tipping.
  • Provide appropriate security features. Choose from security options to include the level of protection that the design team considers prudent. The case security devices and case construction should protect from vandalism and theft.
  • Ensure practical access. Incorporate doors or other practical access options in the case design. Ensure that a single person can enter the case and remove artifacts with ease and in a short amount of time.

Physical Stability[edit | edit source]

An exhibit case must be well fabricated and structurally stable to isolate and protect the objects from the physical forces of shock, vibration, and seismic activity. To limit the movement and vibration of case contents, do not connect cases to walls or ceilings that experience vibration from visitor movement, HVAC systems, or other equipment.

Anchor freestanding cases and provide additional interior ballast for top-heavy pedestal cases to prevent tipping. In earthquake-prone regions, secure exhibit cases with specialized anchoring systems engineered to absorb severe lateral movement. Design the case and floor interface so that visitors or cleaning equipment do not jar the case interior.

Security[edit | edit source]

Different levels of security against theft and vandalism can be designed into an exhibit case. Objects of moderate to high monetary value, objects that are likely targets of theft, or objects displayed in unattended locations can be protected through careful case design. On the basis of a security risk assessment conducted early in the project, the exhibit team should identify the device or design feature that meets the physical security need. Given the tremendous range and cost of security equipment, obtain estimates before proceeding with case design. Available security features include the following:

  • dual locking systems
  • movement sensors and alarms
  • shatterproof security glazing (see TechNotes Exhibit Case Glazing)
  • special glazing to protect against forced entry, ballistic attack, and bomb blast. (see TechNotes Exhibit Case Glazing)
  • concealed case screws and entry hardware.
  • tamper-resistant hardware
  • tamper-resistant hangers for frames.

In addition to providing security, an exhibit case also offers some protection during a natural or man-made disaster. Consider using fire- and water-resistant materials. Exhibit cases should protect their contents from fire exposure for at least 30 minutes and from the force of water from sprinklers heads and fire department hoses.

When the case environment is controlled by a mechanical system, include internal heat- and combustion-sensors. When several such cases are part of an exhibit, connecting them to an early warning air-sampling smoke detection system may be more cost-effective.

Access to Display Objects[edit | edit source]

While discouraging unofficial access is a performance feature of any case, reasonable access must be provided for a variety of curatorial and maintenance functions, including:

  • Cleaning the case interior
  • Maintenance of environmental control equipment located in the case
  • Periodic object inspection and routine collections care procedures
  • Access to objects for object rotation and emergency retrieval

When possible, ensure that each object is accessible without major disruption to adjacent objects. Provide sufficient access doors of adequate size.

Access Doors[edit | edit source]

Seek curatorial and conservation advice when determining the design of the access doors, their weight, and their locations. Rollers and temporary opening supports make oversized doors easier to open. In some walk-in cases, creating an interior pathway for access may be necessary.

Bonnets[edit | edit source]

Bonnet design should consider manageable size and weight to allow safe access. The vitrine top should never be so tall that it cannot be lifted safely over the objects inside the case. Be sure to utilize appropriate tools and methodology when lifting bonnets safely. See article on use of suction cups and other equipment.

Types of Enclosures and their Preservation Criteria[edit | edit source]

A case enclosure or display chamber separates the items contained within from the. Specialized case design enclosures can:


Case enclosures can provide a continuum of protective control from ambient room conditions to a more tightly controlled internal case environment. Design doors and panels to allow safe installation/deinstallation/maintenance for objects. Appropriate choices of security hardware will prevent tampering, vandalism, and theft. Damage to objects through vibration and/or building movement may be minimized through incorporation of appropriate design features; this can include case placement, use of a specialized base, or specific mount design. Glazing choice can protect objects from high light levels and ultraviolet light exposure; if using glass, only use tempered “safety” glass for glazing, interior shelves or exterior panels. Lighting illumination design will limit sources of heat exposure; choice of light placement and type of bulb will affect heat emission.

In situations where the relative humidity and temperature in the macro-environment (gallery) meets the preservation criteria, case enclosures may only be required to provide isolation and security. In situations where the intent is to provide a micro-environment inside the case, case enclosures can be designed to mitigate relative humidity, pollutants, oxygen content, or other factors. Cases including these conservation features will cost more to design and produce. Enclosures can be grouped into several categories.

Case types

  • Unsealed Case: Case enclosures that are not intentionally sealed or have an unspecified seal. The internal environment of these cases often equilibrates to the temperature and humidity of the external (gallery) environment; air exchange rates are difficult to predict and depend upon construction details.
  • Vented Case: Case enclosures that intentionally include gaps or vents to improve air exchange. Air exchange within these cases is passively controlled, and without careful design and testing, it is difficult to predict air exchange rates.
  • Internal Air Circulation Case: These cases use a fan control to enable air circulation while maintaining internal environmental conditions. These systems can also be used to move air from an environmental control compartment through to the display case compartment. Sometimes these cases are known as positive air pressure systems.
  • Tightly Sealed Exhibit Case: Case enclosures that provide a microenvironment different from the surrounding room through the use of sealants (Link to Tech Notes for gaskets and caulk) and buffering materials. These cases must be tested to determine the air exchange rate, which should be less than one per day to meet preservation criteria. Sometimes these cases are referred to as Airtight Cases. (add link to CCI TB 38)
  • Active RH Controlled Case: Case enclosures designed with a purpose-built microenvironment inside a case controlled through mechanical additions. These case enclosures can include fans to circulate or ventilate air, or to create positive or negative pressure.
  • Modified Atmosphere Environment Case: Case enclosures that provide a microenvironment that is filled with a gas other than air. (put this in a Sealed Exhibit case section and remove as bullet – specialized application)
  • Frame package: Enclosures that sandwich an object within a protective frame, including a back board, and often including a clear cover element (glazing). This type of exhibit enclosure is well covered in other sources (link to BPG wiki). This type of package is typically used for documents and photographs, but may be used for textiles or thin objects in shadow boxes.
graphic depiction of case types


All types of exhibit cases require maintenance to ensure optimal performance and longevity. Attention to security hardware, stability, cleanliness, aesthetic appearance are common to all case types.

Decision tree for factors to consider in choosing a type of case.

Unsealed Cases[edit | edit source]

Unsealed display cases include a range of case designs that do not utilize gaskets or other methods to deliberately make the case as air tight as possible. Unless a case is specifically designed to create a seal, the case will contain gaps that allow some measure of air exchange with the room environment and may also introduce particulates and pests. In institutions with environmental control of gallery conditions, cases can be intentionally designed with small gaps to allow for limited air exchange through diffusion. See the section 3.2 on Vented Unsealed Cases for more discussion about this design approach.

Preservation Considerations for Unsealed Cases[edit | edit source]

Depending upon construction details, the environment inside an unsealed display case offers limited protection from various risks; an unsealed case may reduce ingress of dust, external pollutants, and pests, and may concentrate internal pollutants inside. Design elements, including the use of gaskets, vents, and mechanical systems can reduce environmental risks; see other types of case enclosures described in Sections 3.1 - 3.7.

An unsealed exhibit case can still reduce changes from the ambient environment. Evaluate interior relative humidity and temperature parameters using data loggers and (when possible) use leak detection equipment to identify air leaks and determine air exchange rates. See TechNote: Using Ultrasonic Leak Detection Equipment to Test Exhibit Case Seal) and CCI TB 38

Consider sources of heat such as nearby mechanical system vents, electronic components, windows, or lighting. Internal lighting can produce significant heat gain inside a sealed case, depending upon the bulb choice or ballast location; see the section on case lighting. Temperature gradients may occur depending on the location of the heat source, which may also produce variations in humidity control due to stratification.

Relative humidity control inside an unsealed case will depend on the degree of air exchange. A case that is not well sealed may have some buffering capacity which can be enhanced through the use of humidity buffering media such as silica gel. However, be aware the buffering capability of an unsealed case is limited. If silica gel is used, it will need to be periodically reconditioned. See the section on Relative Humidity Control for more information.

Applications for Unsealed Cases[edit | edit source]

Unsealed display cases are appropriate for a wide range of objects, especially when physical protection is required, the exhibit space is well controlled, and the object sensitivity does not require a higher level of protection. Unsealed cases can offer some protection from the uncontrolled environment of an exhibit space. How these cases/enclosures protect against risks such as particulates, pollutants, pests, temperature and humidity fluctuations will depend upon design, institutional resources, room conditions, and choice of construction materials. Because unsealed cases have an unknown air exchange rate, humidity buffering media are not recommended for controlling the case environment. See Figure 6 for more information about design considerations.

Construction of Unsealed Cases[edit | edit source]

Unsealed cases can be constructed from a wide range of materials; using tested and stable materials inside the display chamber and/or using physical barriers such as a metal laminate to isolate a non-ideal fabrication material, such as wood, from the case interior will help reduce interior case pollutants. (See Exhibit Case Preservation Features and Controls). Cases built of metal or stable plastic components with glass or acrylic glazing can limit exposure to volatile organic compounds.
Design features can help limit risks such as physical access, ultraviolet light exposure, particulate infiltration:

  • Particulate infiltration and pest ingress can be reduced through the use of limited gaps, and filters that cover vents/ports
  • Choosing tested and stable materials for construction protects against degradation from off-gassing pollutants. See Products Used in Preventive Conservation - Technical Bulletin 32 - Canada.ca

Maintenance of Unsealed Cases[edit | edit source]

These cases can accumulate particulate pollutants and pest debris; their design must accommodate ease of access for regular inspection and cleaning as necessary.

Vented Cases[edit | edit source]

Vented cases will have larger gaps, or vents/ports to passively facilitate more air exchange in a room or gallery space in which environmental conditions are well controlled; however, the rate of air exchange through these intentional openings is often difficult to quantify or control. Vented cases are appropriate only when temperature and relative humidity conditions throughout the museum or exhibit space meet the established preservation criteria for the displayed collections.

Preservation Considerations for Vented Cases[edit | edit source]

Vented cases can be a strategic option when there are concerns about pollutant concentration within a display case or the objects on display are not at risk from ambient room conditions.
When using a case with intentional openings, evaluate whether the air exchange provided by vented openings or ports can provide enough dilution to improve air quality inside the case. However, gaps or vents allow dust and particulate infiltration, and may increase risk of water ingress in an emergency situation. If a filter is placed over a gap or vent, air exchange may be further reduced.

Applications for Vented Cases[edit | edit source]

Appropriate placement of vents can utilize convection or air circulation via heat differentials, resulting in higher air exchange rate. A vented case is useful for providing dilution under the following circumstances:

  • An object is constructed of a material that is especially sensitive to pollutants.
  • An object emits pollutants initially or over time.
  • The materials used in construction emit pollutants.

Construction of Vented Cases[edit | edit source]

Designing these openings to assure air dilution and circulation can be complex; increased air exchange is dependent on the number, size, and location of vents in the case walls and these factors will determine how air dilution and circulation are facilitated. As a note, mechanical control of air flow can be facilitated by drawing air into the case using fans; this is discussed under Internal Air Circulation cases.
The location and design of vents can promote air exchange throughout the case interior, especially when there is a heat differential that will foster a “stack effect,” resulting in air circulation within the case. However effective design is difficult to predict and measurement of air exchange rates within the enclosure can be crucial in evaluating its effectiveness.

  • Vents, or ports, can be unobtrusively placed within cabinetry decorative features, in recessed kick spaces, behind raised graphic panels, or on the backs and tops of cases.
  • To utilize the “stack effect,” employ vents at the top and bottom; this will encourage warmer air in the case to exit out the higher vents and cooler air to be drawn in at the lower ones. This placement can ensure sufficient circulation by increasing air exchange inside the case and preventing the formation of localized zones with different temperature, relative humidity, and amounts of volatile organic compounds.
  • Placing the gaps/vents on opposing case walls or at large intervals on the same wall will create diagonal air currents.
  • If pollution filtration media are used, one option is to place them at the bottom vent (intake) only.

Maintenance of Vented Cases[edit | edit source]

Filters can be used to prevent debris from entering a vented case. Low efficiency, or coarse, filters will allow ingress of fine particulates. High efficiency, or fine filters will affect the internal air circulation (stack effect).

Internal Air Circulation Cases[edit | edit source]

Internal Air Circulation cases use a fan control to enable air circulation while maintaining internal environmental conditions. Fan systems within the interior environment of a display case or enclosure provide ventilation by:

  • Introducing filtered air through a vent or port while omitting particulates and insects (See the section below on Pollutants for how to include filtration for these contaminants.)
  • Circulating air inside a sealed case environment
  • Supplying air from an environmental control compartment through to the display case compartment (see Active RH control section below).


Preservation Considerations for Internal Air Circulation Cases[edit | edit source]

Objects on display must be protected from vibration produced by the fan; this can be mitigated by design. Strategies to avoid unwanted fan noise include purchasing quiet fans, adding muffling features or running the fan outside visitor hours. For a sealed case that has internal air circulation, the decision to use humidity-buffering media, such as pre-conditioned silica gel, requires consideration of the number of vents, the air exchange rate, and the ambient room environment. In these situations, the case environment should be monitored to determine when the humidity-buffering media needs to be replaced or reconditioned. See section on Relative Humidity Control and Tech Note.

Applications for Internal Air Circulation Cases[edit | edit source]

Fan systems can reduce concentration of harmful volatiles by gently pushing air entering the case out into the room through intentional gaps or vents/ports. The filter (depending on its efficiency) will offer protection against the ingress of particulates and pests. Unless incoming air is conditioned, objects will experience any variation and fluctuation in temperature and humidity levels within the room. If a positive pressure environment is required, the exit openings or vents are designed to allow less air to leak out than that which enters. Fans used inside a case rather than drawing air from outside the case can be used to circulate conditioned air from a silica compartment throughout the artifact chamber; this design helps to avoid stratification and ensure evenly distributed relative humidity levels, particularly useful for very large display cases.
Fans are not usually necessary except for:

  • in situations where air must be flushed from the case due to interior pollution concerns
  • very large cases (more than 10 cubic yards per volume)
  • cases designed to maintain positive pressure


Construction of Internal Air Circulation Cases[edit | edit source]

Cases including a fan(s) and vents must be carefully designed and constructed. Preservation experts, the case designer, and fabricator should collaborate on design decisions such as choice and placement of equipment, electrical wiring, and procedures for mitigation against mechanical malfunction.
When an exhibit case includes a fan, its placement must allow for easy removal and replacement of the mechanical components. Be sure that a certified professional verifies that all electrical components are safely installed.
When constructing a case with internal air circulation or positive pressure exhibit case:

  • Seal joints tightly as needed
  • Select a filter for air entering the case from the surrounding room based on needed efficiency.
  • Use the smallest size fan with the lowest CFM to meet your needs. Design fan placement and air flow so that conditioned air is evenly distributed throughout the case environment.
  • Control air movement to create gentle turbulence that will ensure air currents do not blow directly onto objects. Baffles can offer additional protection.
  • Test fans before object installation because they can cause excessive airflow, unacceptable vibration, and distracting noise.


Maintenance of Internal Air Circulation Cases[edit | edit source]

Critical to the design of internal air circulation cases is easy access to fans and electrical systems for maintenance and repair as needed.

Tightly Sealed Exhibit Cases[edit | edit source]

Tightly sealed display cases include a range of case designs that utilize gaskets or mechanical systems to control the case environment. Specifically designed to create a seal that limits air exchange with the exhibit space environment and intrusion of particulates and pests, the construction of these cases utilizes design features and sealant materials (gaskets and/or caulk) to prevent diffusion of air from the surrounding environment. They are sometimes referred to as airtight. These cases may be critical for institutions with inadequate environmental control of room conditions. Consider the object and case material selection prior to specifying a sealed display case.


Note: For flat objects, such as paper documents, photographs, or textiles, see the section on Sealed Frame Packages.

Preservation Considerations for Tightly Sealed Cases[edit | edit source]

A tightly sealed exhibit case can be designed to provide a microclimate uniquely tailored to an object’s preservation criteria; the microclimate offered by a well-sealed case can be a practical alternative to controlling the entire exhibit space. Sealed cases are a particularly good choice when only a few exhibit objects are sensitive to humidity or when one or more objects require a different relative humidity. (see TechNotes: Environmental Control)
Enclosing papers, photographs, and textiles within tightly sealed frames can have the same benefits as putting them in sealed cases. A sealed frame incorporates an impermeable barrier as a backing board and uses a gasket material or tape to prevent air passage along the frame edges and glazing.
Relative humidity can be controlled passively in a well sealed case using silica gel in a drawer or panels within the display chamber. For silica gel to perform reliably the case should hold a seal of one air exchange per day or less. More tightly sealed cases can be specified, as air exchange rates of 1 exchange per 3 days or 1 exchange per 10 days which will require less silica gel to maintain proper humidity control. See Silica Gel Products and How to Use Them. While experienced fabricators can design and build cases with these more stringent air exchange rates, the seal must be tested after installation in the gallery or exhibit space. See TechNotes: Determining Exhibit Case Air Exchange Rates.
See the section on Relative Humidity Control for more information.

Applications for Tightly Sealed Cases[edit | edit source]

A tightly sealed exhibit case limits the rate of air exchange between the display chamber and the ambient environment. The climate inside a sealed exhibit case can be engineered to:

  • Protect against dust, external pollutants, and insects
  • Buffer changes in relative humidity, and can slow temperature change.
  • Maintain a specific relative humidity if used with humidity buffering media.
  • Maintain specific environmental parameters if used with fan or Active RH Control systems

off-gassing from construction or finishing materials may become concentrated within the case. Only use tested or stable materials inside the display and maintenance chambers (link to Choosing Materials Page and CCI TB 32). A physical barrier, such as a metal laminate, can isolate a less than ideal fabrication material, such as wood, from the case interior. See TechNotes: Monitoring Pollutants Inside an Exhibit Case and Using Pollutant Absorbers Inside an Exhibit Case.
Internal lighting can produce significant heat gain inside a sealed case, depending upon the bulb choice or ballast location; see the section on case lighting.
Relative humidity control can be maintained using humidity buffering media such as silica gel. See the section on Relative Humidity Control.

Construction of Tightly Sealed Cases[edit | edit source]

A tightly sealed case will incorporate these design features:


Tightly sealed cases can incorporate environmental maintenance components or chambers which utilize specific materials designed to maintain the interior environment at a specified relative humidity level. Design of these components must take into account the moisture capacity of the buffering material, case capacity for limited air exchange and air mixing, and the ability to maintain the humidity buffering media such as silica gel. See the section on Relative Humidity control.

Fans used to circulate conditioned air from a silica compartment throughout the artifact chamber to avoid stratification and encourage evenly distributed relative humidity levels, are particularly useful for very large display cases. See CCI's Airtightness Measurement of Display Cases and Other Enclosures – Technical Bulletin 38

For large cases, circulating air inside a case can homogenize the interior environment and allow for filtration to reduce pollutants; installing a system relies on carefully modulated fans and filters. See the details under Internal Air Circulation Cases for construction advice. See the section on for information about sorbents and other choices for appropriate filters.

Maintenance of Tightly Sealed Cases[edit | edit source]

Tightly sealed cases require monitoring of caulks and gaskets with their associated adhesives that can degrade or shrink over time. When the case seal begins to degrade it will be hard or impossible to maintain the internal RH. Testing air exchange rates can help determine when seals may need to be replaced. Careful visual inspection may find areas of concern, but ultrasonic equipment can identify the location of leaks or gaps. Humidity control media must be monitored and replaced to ensure proper humidity control.

Active RH Controlled Cases[edit | edit source]

Mechanical equipment can control RH within the display case effectively when used with a sealed case design. The RH can be controlled by either influencing the air entering the case or recirculating conditioned air internally. Note that the room temperature will affect the ability of mechanical equipment to control RH setpoints within the case. If the room temperature cannot be appropriately managed and the object requires a specific environment, temperature control inside the case may need to be evaluated. These cases require significant monitoring and maintenance. One larger unit may control multiple cases, if location, capacity, and the circulation route is factored into room, exhibit case and mechanical unit design. Very few active case systems are designed to control temperature; this equipment is more complex to adjust and maintain to ensure both temperature and RH targets are satisfied.

When relative humidity conditions are controlled by silica gel, saturated salts, or other humidity buffering materials, see the sections on Humidity Control. Additional information on incorporating passive relative humidity control into case construction can be found in Unsealed, Internal Air Circulation, and Sealed Cases.

Preservation Considerations for Active RH Controlled Cases[edit | edit source]

The challenges and resources needed to design, install, monitor, and maintain this equipment should all be thoroughly considered. The preservation expert, case designer, and the facilities manager should be involved in researching options, choosing, and designing an Active RH system early in the planning process.

Continual monitoring of the case environment is essential to prevent or respond quickly to extreme environmental fluctuations. It is critical to discuss and plan for mitigation against mechanical malfunction early in the design process, as equipment related to emergency power and remote monitoring may impact the budget and case design.

Humidity-buffering material such as silica gel can be installed as a back-up for Active RH control equipment. The amount of humidity-buffering material is dependent on scheduling and servicing the repair, case seal and size, room environment, and object tolerance. In these situations, the case environment should be monitored to determine when the media needs to be replaced or reconditioned. See section on Relative Humidity Control and Tech Note on Silica Gel.

Active RH systems featuring humidity control may require a water source or a way to control drainage. Some systems require more maintenance time from collections staff, because they use a water reservoir for humidification and a catch pan system for the condensate which must be filled and emptied. Filtered or purified water may be required to prevent equipment damage.

Active RH Control cases are useful in situations where there is a cost benefit to outfitting an entire bank of cases rather than maintaining environmental control using a large quantity of humidity buffering material. Cost benefit analysis will determine whether a larger microclimate generator should control several cases or if individual units would be more efficient.

Construction of Active RH Controlled Cases[edit | edit source]

Some exhibit case fabricators provide microclimate generators that can be incorporated into case design and construction to meet preservation requirements. In other situations the mechanical unit is purchased directly from a specialty vendor and coordinated into the exhibit design and construction drawings. Test the case immediately upon completion to make sure it meets specifications for humidity control. Testing should include environmental monitoring to ensure that the case can achieve and maintain the desired relative humidity.
The case fabricator and mechanical equipment vendor must work together to ensure adequate space and ventilation to expel heat generated by the unit, and to verify sufficient circulation between the unit and the display chamber. The location of utility connections and access to control panels, water reservoirs, and drain pans must be factored into the design. Ensure that water cannot damage objects or other case components (such as electrical wiring) if a leak occurs.
Ideally the mechanical unit will be installed and tested at the fabrication shop to ensure that the case can hold its specified conditions before it is installed onsite. Budget permitting, include representatives from the fabricator/mechanical unit vendor during installation. When an exhibit case includes an active RH control system, design to allow for easy removal if repair cannot be performed in-situ or if a replacement unit is needed.

Maintenance of Active RH Controlled Cases[edit | edit source]

Consider the costs for maintaining these systems to ensure reliable operation.

  • Complex maintenance or repair service will require an adequate budget and procurement plan.
  • Determine who is authorized to perform maintenance tasks.
  • Determine if compatible parts are readily available.
  • Maintenance may impact the display schedule if an object cannot be exhibited safely without an operational active system.

Modified Atmosphere Environment Case[edit | edit source]

Modified Atmospheric Environment Cases are required when a specific object must be displayed at conditions that differ from normal atmospheric conditions (temperature, humidity, or oxygen levels). When an object requires long-term or permanent display or storage in a reduced oxygen environment, the specialty housing is referred to as an “encasement.” As a note, pest control using anoxia is an entirely different application that is not addressed here. For information on anoxic pest control treatments, visit the MuseumPests website.

Research into the specific needs of the object to be displayed is required prior to determining if a modified environment is the appropriate preservation strategy. A modified environment can be either active (mechanically controlled) or passive (hermetically sealed), but both require specialized technical expertise and a commitment of resources, including a commitment to long-term maintenance and monitoring of the modified atmosphere environment display as well as the object itself as well as staff training.

Preservation Considerations of Modified Atmosphere Environmental Cases[edit | edit source]

Modified atmospheric display is not appropriate for all objects since some materials, like the pigment Prussian Blue, may be negatively affected by a low oxygen environment. See the Library of Congress's May 9, 202 Blog post Low-Oxygen Encasements for Long-Term Displays as well as the Studies In Conservation article Large-scale assessment of light-induced color change in air and anoxic environments (Beltran, et. al. 2013)

As a note, access to the object will be hindered by some types of modified atmosphere environmental cases, such as hermetically sealed low oxygen encasements. Deinstallation and resealing of the case will be complicated, so the object should be well studied and documented and any conservation treatment accomplished prior to installation.

Staff or contractor expertise must be dedicated to ensure that the maintenance and monitoring of the modified atmosphere environment cases remains within the specified parameters. Many modified atmosphere environmental cases require contracted maintenance and monitoring. This can include the mechanical, electrical, or security systems used to protect the object. Consider technical expertise and time commitments of staff prior to making a commitment to using a modified atmosphere, as transfer of knowledge will be required for long-term maintenance and monitoring of these specialized displays.

Applications for Modified Atmosphere Environment Cases[edit | edit source]

Modified atmosphere environment cases share some similarities with well-sealed cases or active RH controlled cases by providing an alternative environment from the general exhibit space. The use of modified atmospheric encasements allows flexibility in oxygen content to be set at levels that are different from the overall environment. Encasements have been used at several institutions as a preservation strategy for iconic objects when their mission mandates the need for permanent display, but these encasements are rare and difficult to design, fabricate and maintain in order to achieve the expected preservation benefits.

Construction of Modified Atmosphere Environmental Cases[edit | edit source]

Specialized technical knowledge is required when designing and constructing a modified atmosphere environmental case. The design and engineering required for these cases includes careful consideration of materials, such as shatterproof glass and handling for compressed gasses. As with any well-sealed display, off-gassing must be mitigated.

Modified atmosphere environmental cases are normally created from glass, powder coated metals, well-tested gaskets, and other stable materials to minimize further degradation of the object over the course of long-term display in a modified environment. An object that offgasses harmful volatiles should never be housed in a hermetically sealed display.

Maintenance of Modified Atmosphere Environment Cases[edit | edit source]

Modified Atmosphere Environment Cases require specialized sensors and equipment for long-term monitoring to keep the cases functioning properly. Sensors, data collection systems and any associated equipment require maintenance. Documentation and training must be compiled, updated, and passed on to staff over time.

Additional considerations when planning a modified atmosphere environment case are similar to Active RH controlled cases:

  • Complex maintenance or repair service require an adequate budget and procurement plan.
  • Determine who is authorized to perform maintenance tasks.
  • Determine if compatible parts are readily available.
  • Maintenance may impact the display schedule if an object cannot be exhibited safely without an operational active system.

Sealed Frame Package[edit | edit source]

Sealed framed packages are used to create safe storage and display environments. Resources are available about framing two-dimensional items such as paintings, photographs, and works of art on paper, as well as specialized resources for case design of items such as books or panel paintings.

Other types of objects, such as textiles may require specialized types of frame packages. These include textiles, three dimensional paper-based objects, shadow box constructions, and other items.

Applications for Sealed Frame Packages[edit | edit source]

Sealed frame packages provide enhanced protection from room pollutants, environmental fluctuations, and handling. Framed packages are an effective preservation strategy for transport or loan applications.

Preservation Concerns of Sealed Frame Packages[edit | edit source]

Sealed framed packages can be a cost effective and flexible preservation strategy for flat objects during transport, while on display, and in storage.
Consider whether harmful volatiles emitted from objects can be mitigated by placing buffering or sorbent media within the framed package. These materials may require replacement if the object will remain in the framed package during display and storage.
For most sealed frame packages and housings (excluding pressure mounts for some textiles), use of spacers will ensure that the surface of artwork does not touch the inside surface of the polyester, acrylic or glass window.

Keep sealed frame packages several inches away from exterior building walls that are susceptible to temperature change and condensation.

Construction of Sealed Frame Packages[edit | edit source]

Frame packages should be constructed from materials known to be stable, given their close proximity to the object. Ensure that any material exposed to the sealed interior of the package is confirmed to be stable. These surfaces may include frame edges, matboard, rigid supports including those with padding for textiles, fabrics, and adhesives. Hygroscopic materials used in the frame package may impact RH maintenance as they absorb and release moisture.

The simplest framed package involves encapsulating the object within a sealed polyester envelope often with a more rigid support and/or pollutant buffering material. Sandwiches, sometimes referred to as Plexi-Paks, protect the object between two acrylic sheets with the object mounted or attached via non-adhesive means to a support. Traditional frame packages can include pre-conditioned humidity buffering materials and/or pollutant sorbents. Barrier films and tapes are typically used to seal the edges and back of the package.

A comprehensive discussion on frame package designs and materials can be found in the BPG Matting and Framing wiki page.

For other types of objects, such as textiles, three dimensional paper objects, or shadow boxes, consult with a conservator to design and construct an appropriate frame enclosure.

Maintenance of Sealed Frame Packages[edit | edit source]

Inspect the edge and back seals of the frame package to ensure they remain intact, with no gaps or tears between exhibit rotations and periodically if the object remains on long-term display or in storage. Visually observe and monitor the object inside the package to ensure that the sealed package is providing the required environment. Replace humidity or pollutant control materials as needed.

Relative Humidity Inside Cases[edit | edit source]

The ambient moisture level in a room affects the relative humidity within an enclosure due to air diffusion, primarily through gaps in the case and secondarily through permeable exhibit case materials. These factors must be understood before designing an exhibit case that stabilizes or controls relative humidity. See the sections on Types of Enclosures and Preservation Criteria for information on how case construction affects the ability of the enclosure to stabilize the interior environment. See Exhibit Planning.

Moisture exchange between hygroscopic components inside a case, including case construction materials or even organic collection contents, also affects internal humidity levels as they release or absorb moisture within the case. Although wood construction materials used in cases release volatile organic compounds such as acids, the wood itself can contribute to buffering the case environment. Purposely adding other cellulosics such as paper liners and boards can serve a similar purpose.

Humidity buffering material such as silica gel can be intentionally used to create a target relative humidity range when the type of gel and quantity are calculated to reduce the rate and degree of RH change. Saturated salt solutions have also been used similarly, but they can present other challenges.

Designing a Case to Control Humidity[edit | edit source]

The following features are generally required in a case that includes humidity control:

  • A minimum air exchange between the case display chamber and the ambient environment. Air exchange can occur through case gaps and moisture permeation of case construction materials.
  • A means of stabilizing or controlling humidity inside the display chamber, either an active (mechanical) system using humidifying/dehumidifying equipment or a passive system using silica gel, hygroscopic materials, or saturated salts.
  • Separate access to the environmental and mechanical chambers to minimize changes to the collection environment.
  • Air circulation between the display and the buffering material. Options for providing air exchange include using a perforated material for the case deck or designing the deck with a perimeter gap.
  • Equipment to monitor internal relative humidity.

See the sections on Types of Enclosures and their Preservation Criteria for information on how case construction affects the ability of the enclosure to stabilize the interior environment when adding components to provide humidity control.

Relative Humidity Monitoring[edit | edit source]

Verify the case performance before enclosing objects; make sure that the case can maintain the desired environmental range using data loggers or indicator cards.

After verifying case performance, the relative humidity must be monitored to alert museum staff to environmental fluctuations or maintenance needs (including silica gel reconditioning).

Monitoring can be achieved via data loggers. Data loggers are available in standalone, wireless or wired configurations with various data retrieval methods. Data can be retrieved at regularly specified intervals or monitored remotely.

Humidity Buffering Materials[edit | edit source]

A variety of hygroscopic materials can be used within an exhibit case to maintain desired internal case relative humidity. The buffering capacities of these materials can vary, thereby influencing their efficacy. If the collection material itself is organic (such as paper, wood, and other plant materials), it will contribute to the buffering capacity of the case environment. To protect organic objects humidity buffering materials must be more efficient than the object itself. Methods for stabilizing relative humidity with known humidity control media include:

  • Silica gel with known absorption capacity can be used in a calculated quantity within a sealed case to give predicted results. See Tech Note
  • Cellulosics, such as cotton or paper, can be used to buffer framed packages or small enclosures. The buffering capacity of these materials is usually less than that of silica gel and is difficult to predict. See Tech Note
  • Saturated Salts are not commonly used for humidity buffering, but they have been used in the past, and information on their use can be found at: Humidity Fixed Points of Binary Saturated Aqueous Solutions. Some salts can increase other risks; for example magnesium chloride can leave harmful deposits on lead and glass. In acidic environments some salts release harmful volatiles. See Tech Note

Silica Gel[edit | edit source]

Silica gel is very effective at regulating relative humidity because of its composition and large surface area, which allows large quantities of moisture to be absorbed or adsorbed. Effective use involves appropriate choice of a specific type of silica gel and how it is conditioned, as well as design of a container that will allow it to affect the internal case environment. Selection of gel depends upon the humidity buffering requirements. Different types are commercially available:

  • Traditional silica gel works well as a desiccant.
  • Some gels are formulated to maintain an RH of between 40 and 60%.
  • Indicator gels include a mix of gel plus an indicator salt to help visually monitor moisture level.

Art Preservation Services' article Measuring the Effectiveness of Silica Gel: Moisture Buffering Capacity and MH Value covers information about different silica types. MFA’s CAMEO website provides information and a comparison chart about the different types of silica gel and their moisture capacities.

Conditioning[edit | edit source]

When using silica gel, it must be conditioned to a specific RH and enclosed within a sealed case, usually in a separate compartment. Case design should allow the gel to be reconditioned without opening the main case door. Silica gel can be conditioned to a specific relative humidity by adding moisture or drying the gel. Information on calculating the amount of silica required to condition can be found at these links:


Dataloggers can be used to collect information on case interior environments and to determine when the silica gel needs to be refreshed.

silica gel (section 5) provides information on reconditioning silica gel outside or within the case.

Containers[edit | edit source]

Silica gel function is improved by spreading the gel out over a large area which serves to increase the exposed surface area of the gel. Drawers, trays, or small glass containers of silica gel are often placed below the case deck or inside display supports. Avoiding loose silica will save time and reduce the release of fine dust, which is a hazard to breath and can damage collections. Containing silica gel is convenient and prevents the release of fine dust.

  • Silica gel beads or pellets can be contained in bags made of synthetic fiber fabrics, cassettes and panels. These can also be fabricated by museum staff.
  • Tiles of silica gel are also available, as are papers and foams impregnated with silica gels.


Silica Gel Products and How to Use Them provides additional information.

Cellulosics[edit | edit source]

Cellulosic materials used for buffering are most commonly used in frame packages, where experience shows they are very effective due to the limited air volume within the sealed package. Paper and woven cotton textiles can also be used to line mounts, case decks, and as other case components. The humidity effects of including these materials are difficult to predict and are dependent on both the case materials and the objects’ reaction to environmental fluctuations. Tech Note on buffering with cellulosics [Coming Soon]

Salt Solutions[edit | edit source]

Saturated salt solutions are successfully used in scientific applications to control RH within microclimates and to calibrate equipment. They have been used in specific circumstances, such as for archaeological materials, but their use is not common practice.

Salts with known deliquescence in solution can achieve a target RH. For information about the use of saturated salts see Humidity Control in Cases: Buffered Silica Gel versus Saturated Salt Solutions. For more current information about the use of salts and ongoing research see Saturated salt solutions in showcases: humidity control and pollutant absorption and Saturated salt solutions - CAMEO

A more common use of these salts as moisture indicators are seen in humidity monitoring cards.

Pollution Sorbents[edit | edit source]

The most important preventive strategy to protect collection objects from harmful pollutants when designing an exhibit case is to avoid (whenever possible) the use of case materials that emit harmful volatile chemical compounds. If the object itself emits harmful volatile chemicals, design the case with adequate ventilation to dilute the off-gassed materials. If ventilation is not an option, sorbents can be used to safeguard collections from harmful pollutants inside a well-sealed case.
Pollutant sorbents, such as absorbers/adsorbers and scavengers, can protect sensitive objects from harmful volatile chemical compounds. These compounds include acetic acid, formic acid, sulfur-containing compounds, ozone, nitrogen dioxide, and others. These compounds can be emitted from display case fabrication materials, from the collection objects themselves, such as unstable plastics, or originate from the outdoor environment/exhibit space.
The only way to evaluate the efficacy of pollutant sorbents requires monitoring of display case air, which requires sophisticated equipment. For this reason the use of pollutant sorbents is a supplemental strategy after avoiding the use of materials that off-gas.

For effective use of pollutant sorbents:


Commonly used pollution absorbers in exhibit cases include:

  • Activated charcoal, carbon-filtration media
  • Potassium permanganate based filtration media
  • Acid-free, alkaline reserve papers or boards included in a frame can provide similar filtering benefits because the paper will absorb pollutants as the air passes through the paper fibers. See Effect of Paper Alkaline Reserve on the Chemical Stability of Acetate Base Sheet Film
  • Zeolites
  • Anti-tarnish papers
  • Metal laminate films with embedded sacrificial metal particulates

Activated Charcoal (Activated Carbon)[edit | edit source]

Charcoal-based products are sold in two formats: loose pellets that can be contained in perforated panels or fabric mesh bags, and paper- or fabric-based sheets embedded with activated charcoal. Note activated charcoal is not the same as some common products like used for fish tanks or grills. See CAMEO's Activated carbon page

Often, activated charcoal does not react with pollutants but simply absorbs many of them. The charcoal becomes ineffective when it has taken up its maximum amount of pollutant. It is critical to replace charcoal-based sorbents before they become saturated, thereby ineffective. This can be difficult to determine, since, unlike potassium permanganate, activated carbon shows no visual change. Some products contain both activated charcoal and potassium permanganate to take advantage of both pollutant sorbents. Other activated charcoal products used as sorbents are impregnated with metal salts, potassium carbonate, or potassium hydroxide.

If the activated charcoal product is dusty, be sure to contain the sorbent to avoid spreading throughout a ventilated case.

Potassium Permanganate[edit | edit source]

Potassium permanganate is a pollutant scavenger commercially available in HVAC filters, sachets or as loose pellets. Potassium permanganate chemically reacts with pollutants. Replacement is easier to determine because potassium permanganate changes color from pink to brown as it becomes exhausted.
Note that potassium permanganate is highly reactive and powdery; the dust from this chemical can oxidize surfaces on contact. Take care to make sure this material does not come in contact with your collection items.

Acid-free alkaline reserve paper/boards[edit | edit source]

Acid-free alkaline reserve paper/boards can absorb and even neutralize volatile acids. They are frequently used in framed packages and encapsulations to provide a buffer to the internal environment. As for activated charcoal, there is no visual way to determine when the buffer capacity is reached; a safe practice is to replace the board during exhibit rotations.

Zeolites[edit | edit source]

Zeolites are molecules which have been engineered to collect and contain specific chemical compounds; they have been added to paper and board used for collection housing to provide molecular traps for compounds that damage many types of paper and other media. For more information, see Performance Evaluation of 4-Ply Rag Boards Containing Calcium Carbonate and Zeolites.

Anti-tarnish papers and cloths[edit | edit source]

Papers impregnated with reactive chemicals such as silver or copper fine particles can reduce pollution and corrosion in a sealed case.

Metal Laminate Films[edit | edit source]

Metal laminate films (such as Corrosion Intercept) incorporate finely divided metal particles that attract corrosion-causing volatiles. These opaque films are used in a wide range of industrial applications to reduce corrosion by capturing some pollutants. They are most effective when completely surrounding the object such as a special plastic bag, and are therefore most commonly used for storage protection. They are sometimes used on the deck or walls of a display case.

The Use of Pollutant Sorbents in Exhibit Cases[edit | edit source]

Whether using activated charcoal, potassium permanganate, or another material, a case built for pollution control is similar to one constructed for relative humidity control. The case must be as airtight as possible to minimize the amount of material needed and improve the efficiency of pollutant removal. Pollutant control materials may be placed in the case in one of several ways:

  • In a containment tray inside the maintenance chamber that is placed under or behind the display deck or in case panels, with a design that assures sufficient air circulation.
  • Hidden by objects, graphics, or object supports, and case furniture.
  • Used as an impregnated fabric or paper covering the case floor, walls, or shelves.
  • Enclosed in a frame package.


Use a sufficient quantity of the pollution-control substance as based on the size and design of the case, the suspected pollutant concentration, and the sensitivity of the objects. In all applications, greater surface area of the pollutant absorber increases the effectiveness of absorption.

Objects should never touch the pollutant sorbents. When these are used in cloth or paper form or enclosures, an isolating layer or an object mount is required to prevent direct contact between the objects and the absorber.

Design airflow to ensure adequate air circulation between the display chamber and the pollutant control material (See Technical Illustrations). Although a fan or another mechanical air circulation system achieves more effective circulation, passive convection is most often used, especially for smaller cases. Fan systems require regular maintenance but may be necessary in oversized cases or those with significant pollutant emissions.

The effectiveness of pollution control can be visually monitored using bright-polished silver or/and lead metal coupons that corrode in the presence of pollutants. This passive monitoring strategy is conducted in real time with the expectation that the coupons will corrode more quickly than the collection items. Other more specialized techniques to monitor for the presence of pollutants include the use of diffusion tubes, including Drager tubes, and other air sampling technologies borrowed from allied professions.

Lighting in Case Design[edit | edit source]

Thoughtful case lighting design choices can protect objects from exposure to excessive illumination, heat and temperature fluctuation without diminishing the visitor experience. Preservation considerations include type of lighting system or fixture, location of lamps, electrical requirements and lighting controls. Note that lighting technology advances rapidly. It is critical to stay abreast of the opportunities and potential challenges posed by these new products and systems.

  • Select lighting systems that avoid ultra-violet radiation and can control overall illumination to meet preservation criteria based on object sensitivity.
  • Ensure that heat generating components of the lighting system do not heat the artifact chamber by using separated compartments, fans or ventilation where needed.
  • Ensure that the location of lighting fixtures does not pose a threat to objects on display and is accessible for maintenance.
  • Consider the impact of the case lighting system materials for potential offgassing including wire coating and plastic panels.
  • Consider the need for flexibility as objects rotate, cases are reused for future exhibits, or objects within the same case have individual lighting requirements.

See section on Exhibit Design for detailed information on preservation considerations related to lighting.

Case Design, Prototype Production and Verification[edit | edit source]

For a case to fulfill preservation criteria, functional requirements must be clearly and specifically discussed and stated; this process includes collaboration between the designer and preservation professional. Preservation features should be reviewed with the exhibit team early in the design process.
While some risks may be addressed through gallery design, this section specifically deals with the construction of the case enclosure(s). Responsibility for case testing and verification of performance will depend upon whether the case was fabricated internally or by a vendor and through associated contractual agreements.
The following considerations are critical in evaluating design and construction details and in verifying case performance:

  • Define case performance requirements. Determine what preservation features will be built into each case, and clearly explain how the case will be evaluated to meet requirements.
  • Choose stable materials for construction.
  • Provide detailed drawings and specifications. Verify that fabricators follow specifications and construction tolerances.
  • When new designs or features are incorporated. Modify the case design until acceptable performance is achieved.
  • Verify performance of the fully assembled case. Test in the final location to ensure that preservation criteria have been met before object installation to allow for adjustments.

Prototype and Production Inspections[edit | edit source]

Once the design has been agreed upon, drawings and specifications must be detailed enough to ensure the case functions effectively and safely when fabricated. Allocate time prior to prototype production to complete material review and testing for materials suggested by the designer or fabricator. If the design warrants and funding allows, fabricate a prototype that can be fully tested and modified to correct deficiencies. Prototype testing, including the assessment of lighting and humidity control equipment, air circulation and case seal, must be completed prior to installation and exhibit opening deadlines.

Inspections are necessary during construction to guarantee that the specified preservation features are included. Ensure the case adheres to preservation requirements.

  • Access for object installation, rotations and maintenance
  • Access to lighting, environmental chambers, and electrical components
  • Specified furniture and supports to prevent object damage (deck, mount attachments, internal pedestals)
  • Vetted materials including isolation layers between construction/fabrication materials and display objects
  • Strategies to protect sensitive objects from pollutants such as filters or sorbents.
  • Approved light levels in the display chamber
  • Ventilation to remove potential heat caused by lighting or other electrical equipment
  • Adequate circulation between humidity buffering and artifact chambers
  • Rate of air exchange between case interior and exterior
  • Case seal criteria with proper gasket placement and joint integrity
  • Function of environmental control systems if applicable


Allow time in the installation schedule for final evaluation and testing on the assembled case in the exhibit space. Performance verification prior to installation is crucial to ensure that the case/enclosure functions as specified.

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