Controlled Microenvironment Container System
This storage system was developed to house large objects that need to be stored in a controlled micro-environment. The system may be adapted for use with smaller objects that can be stored in standard pH neutral or alkaline buffered boxes, by constructing two to four stacking trays to a box.
Charles O. Diesen
Minnesota Historical Society
345 Kellogg Boulevard W.
St. Paul, MN 55102 USA
Tel (612) 297-5774
Illustration: Karen Ackoff after Charles O. Diesen
Custom trays for large objects are constructed from pH neutral, corrugated cardboard. The trays are designed to accommodate the objects and a packet of silica gel which is used to maintain a suitable level of relative humidity inside the assembly (Fig. 1).
Figure 1. Microenvironment created in tray with polyethylene sheeting and silica gel.
The trays are enclosed in a sleeve made from 6mil polyethylene tubular sheeting which is heat sealed to provide a barrier to reduce the rate of moisture gain and loss. Because the tray is covered with transparent plastic, it is possible to visually inspect the object as well as a relative humidity indicating card (or a small container of indicating silica gel), to monitor the condition of the object and the internal environment of the container.
Materials Tools Supplies
- 100% cotton twill tape, 1/4in
- Alkaline buffered paper board, corrugated
- Cotton muslin, unbleached
- Crochet hook
- Glue gun and hot melt adhesive
- Hook and loop tape
- Humidity indicator strips, cobalt-salt or humidity indicator cards
- Polyester sheeting, 2mil
- Silica gel (use color indicating for level of relative humidity if indicator strips are not used)
- Tubular polyethylene sheeting, 6mil or polyester/polyolefin heat sealable tubular roll
- Construct a tray to house the objects. (See Davis, “Tray and Box Construction,” this volume.) Make sure the tray sides are higher than the height of the objects.
- Position the objects in the tray so that they are not touching. Mark the tray bottom in at least two places per object to insert the twill tape that will secure the objects and the silica gel packet.
- Remove the objects, pierce the board on the marks, and pull the cotton twill tape through the bottom with a crochet hook. If hook and loop tape is used, cut slits with a knife, and pull the tape through.
- If the object is friable or is metallic, prevent contact between the object, the twill, or hook and loop tape by placing a strip of polyethylene or polyester sheeting around the object where the ties would make contact.
- Secure the object in place with the twill tape or hook and loop material.
- Construct silica gel packets by cutting and sewing a five-inch square from unbleached muslin. Fill the packets with approximately 250g of silica gel for each 2ft3 of volume. Alternatively, packets for color indicator silica gel can be made according to instructions by Raphael, “Standardized Packing Con-tainers for Silica Gel,” this volume.
- For objects requiring drier environmental conditions, condition the silica gel by heating the packet in an oven at 150°C for two hours to remove the moisture.
Alternatively, packets for specimens requiring higher relative humidity should be conditioned according to instructions provided by Raphael.
- Tie the silica gel packets in position inside the tray and include a humidity indicating card.
- Enclose the tray in heavy tubular polyethylene.
- Heat seal the ends of the poly-ethylene with a heat sealer or using a standard iron, shielded with a piece of polyester film.
A good alternative material is a polyester, heat-sealable, tubular roll stock that consists of polyolefin, on the inside surface, fused to polyester on the outside surface. This material forms a tube that is stronger and may provide a better moisture barrier than the polyethylene alone.