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Hermetically Sealed Case for the Storage of Objects in a Stable, Inert Atmosphere

Purpose

This special storage case is designed for the preservation of sensitive organic materials by statically maintaining low levels of oxygen and a pre-determined relative humidity. The case is useful in preventing deterioration caused by gaseous and particulate air pollutants, fluctuations in relative humidity levels, biological infestations (microorganisms and insects) and thermally and photolytically induced oxidation.

Author(s)

Shin Maekawa
Frank Lambert
Frank Preusser
Eric Hansen
The Getty Conservation Institute
4503 Glencoe Avenue
Marina del Rey, CA 90292 USA
Tel (310) 822-2299
Fax (310) 822-9409


Photographs: Tom Moon

 

Description

The case, which consists of a display section and a pillow-shaped bellows, is filled with humidity-conditioned nitrogen and hermetically sealed from the surrounding atmosphere (Fig. 1). The sealed, nitrogen-filled case, maintains equilibrium pressure with the surrounding atmosphere by inflating and deflating of bellows. The bellows respond to changes in barometric pressure and to changes in temperature thus preventing air leaks in the case. The action of the bellows limits the oxygen leakage to a low rate of 5ppm/day.

Figure 1. Hermetically sealed display/storage case.

The case contains sensors for oxygen and humidity, a humidity buffer, an oxygen scavenger and a pollution sorbent. Under normal conditions the case can be operated without any major maintenance for approximately 10 years. After that time, the case has to be flushed with humidity-conditioned nitrogen, and the buffers and sorbents must be replaced. The low oxygen atmospheric condition can be maintained in the case for more than ten years by using an oxygen scavenger. Silica gel conditioned to a specific relative humidity level is placed in the case to remove excess moisture released by the oxygen scavenger, and to maintain a stable relative humidity in the case. Activated carbon packets are placed in the case as a sorbent for volatile gases released by the object, as well as internally released and infiltrated pollutants.


Materials Tools Supplies

  • Activated carbon
  • Aluminum base plate, frame & corner elements, pressure retaining strips
  • Bellows
  • Gas-tight valves and tube fittings
  • Glass plates
  • Hermetically sealed electric connector
  • Oxygen analyzer & scavenger
  • Polychloroprene O-rings
  • Polypropylene/polyethylene/aluminum/polyethylene foil
  • Pressure retaining strips
  • Relative humidity sensor
  • Silica gel


Construction

The display section consists of five glass panels and an aluminum alloy base plate which are mounted on a specially designed aluminum extrusion frame. The aluminum extrusion frame has two grooves, one for placement of O-ring seals, and another one for ease in handling the glass during installation. The frame also has a flat area where the pressure-retaining strip plates are mounted (Fig. 2).

   

Figure 2. Cross section of aluminum extrusion         Figure 3. Detail of corner assembly of aluminum extrusion frame.
frame and hermetic seal.

The aluminum extrusion sections are fitted with screws to corner elements that also are made of aluminum alloy (Fig. 3). The aluminum retaining strips, mounted onto the frame with screws, exert pressure on O-rings producing an airtight seal between the frame and the plates. All aluminum components are anodized for surface resistance to moisture and atmospheric pollutants.

Six small ports and one large hole are drilled or machined on the base plate. These small ports, fitted with valves, are connected to tubing for nitrogen supply, purging, recirculation, and connection to the bellows. The large hole is covered by an O-ring sealed aluminum disk and serves as an access for insertion and replacement of the oxygen scavenger, silica gel, pollutant sorbent and sensors.

The pillow-shaped bellows is connected to the base plate of the case with a short tube and fittings. The bellows is made by heat sealing two oxygen barrier sheets, such as polypropylene/poly- ethylene/aluminum/ polyethylene foil. The capacity of the bellows should be at least 10% of the display case’s volume.

Sensors for oxygen and relative humidity are positioned inside the display section of the case and their electrical leads are soldered to the hermetically sealed connector mounted on the base plate. The micro-environmental conditions in the case are measured from the outside by connecting the monitors to the electrical leads on the hermetically sealed connector.

During installation the nitrogen gas supply system is connected to simultaneously purge the air from the case and to condition its contents to a pre-determined level of relative humidity. The oxygen level measurement system also is attached to the case for determination of the oxygen leak rate.


Comments

A prototype of the case has been in use in the Cairo Museum, Egypt for the display of a mummy. Oxygen levels of less than 2% and a relative humidity of 35% have been maintained for over two years. A detailed description, including engineering parameters, is available by requesting: Final Report, Mummy Display Case, Joint Project of the Egyptian Antiquities Organization and the Getty Conservation Institute, May 1989.

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