Standardized Packaging Containers for Silica Gel
Moisture absorbing materials such as silica gel are commonly used within museum cabinetry to eliminate or reduce the fluctuation of relative humidity (RH) and to condition the enclosed space to a desired level of relative humidity.
When silica gel is employed inside cabinetry, it is recommended that museum staff use systematic procedures and standardized containers.
The gel can be conveniently contained in a standardized enclosure or container to assist in quantity control, facilitate maintenance and improve handling efficiency.
Economical containers can easily be fabricated to suit the needs of most institution cabinetry and curatorial circumstances.
Division of Conservation
National Park Service
Harpers Ferry, WV 25425 USA
Tel (304) 535-6139
Fax (304) 535-6055
Photographs: Hali Taylor
Silica gel is a nontoxic, regeneratable adsorbent. It is a hard, inert crystalline material which is available in a variety of particle sizes (2-5mm diameter) and it can absorb up to 40% of its weight in water (Thomson, 1986).
Low cost, chemically non-reactive construction materials for the fabrication of silica gel containers are commercially available. Custom containers can be made in a range of forms, such as panels, tiles, cylinders, bags and trays (Figs. 1, 2).
Figure 1. Standard bag after completion.
The specific design and fabrication technique for the containers will vary according to the physical requirements of the museum cabinetry (i.e., how much room is available, whether the containers will be installed vertically or horizontally).
Two container designs for silica gel can be adapted to most routine needs: soft nylon bags (Fig. 1), and rigid plastic panels (Fig. 2).
Figure 2. Standard panel under construction.
Standard Bag Design Specifications
- size: 12in square (filled size)
- capacity: 3lbs gel
- bag fabric: woven nylon screening or spunbonded polyethylene
- edging: cotton twill tape 1/4in wide
- closure system: hook and loop tape
- stitching: sewing machine zigzag and straight stitch
- stitching thread: quilting weight cotton/polyester blend
- channeling: 5 rows of stitched channels (21/2in wide)
Standard Panel Design Specifications
- size: square foot increments
- capacity: 11/2lbs gel per square foot
- structure: white acrylic grid of 1/2in cells
- cover fabric: nylon screening or spunbonded polyethylene
- cover fabric adhesive: heat-set acrylic adhesive
Materials Tools Supplies
- 100% cotton twill tape, 3/4in wide
- Acrylic grid, 2ft x 4ft x 1/2in panel
- Acrylic heat-set adhesive, approximately. 150ºF sealing point
- Cotton/polyester blend quilting thread
- Hook and loop tape, 1/2in or 3/4in wide
- Woven nylon screening:
- 6mil, 52 x 52 picks or greater per inch, plain weave
- 6mil, 52 x 52 picks or greater per inch, plain weave
- Cut a piece of fabric 13in wide by 32in long.
- Attach hook and loop tape. Stitch the hook side of the tape to the bottom edge of the 13in square, and the loop side of the tape to the top edge of the 13in square. Use a zigzag (or overcast) stitch on the top edge and a straight stitch on the bottom edge.
- Fold fabric in half so that the fasteners will attach.
- Stitch sides together using a straight stitch.
- Fold the twill tape in half around side seams.
- Sew all side seam layers together using a zigzag stitch.
- Mark channels 21/2in apart with a pencil line.
- Sew channels using a zigzag stitch, reinforcing the stitching at the hook and loop fastener.
12-inch Standard Panel
- Cut the acrylic grid panel into 12in squares using a band saw, table saw or hand saw.
- Grind or sand away any partial cell walls that remain.
- With a roller, apply a minimum of two coats of adhesive to both sides of the panel. Allow the adhesive to dry between coats.
- Stand panels on edge and allow them to dry.
- Cut two pieces of nylon screening or spunbonded high density polyethylene cover fabric 1in larger than the panel (e.g., 13in x 13in).
- Attach one cover fabric (before filling with gel) by heat sealing it in place. Use an iron adjusted to the medium heat setting.
- Turn panel over and fill cells with 11/2lbs of silica gel.
- Attach cover fabric to second side as before.
- Trim away excess fabric with a sharp knife or adhere around sides to panel edges if additional strength is required.
The overall quantity of silica gel required in a cabinet must be carefully determined from several factors: cabinet dimensions, permeability of cabinet to air and water vapor, its air leakage rate, the pattern of relative humidity in the ambient room, the desired interior RH and the length of time between maintenance, Weintraub, in press.
The moisture vapor permeability of the container construction materials is an important factor; humidity must be able to pass through the container in an unobstructed manner.
Air circulation around the gel packages also should be maximized, and it is best if the greatest surfaces of the container are not flush against the cabinetry. Rigid panels or tiles can be placed on end, bags can be suspended. The quilted channels in the bags increase access of air to the gel. The gel itself should not be deeper than 11/2in.
The size of the container can vary but should be standardized, as much as possible, to contain pre-established gel quantities. In this way, the gel can be more easily conditioned to the required RH levels, and the amount of time to do so per container size can be conveniently calculated.
The silica gel content can be measured in convenient amounts for handling purposes and to fit certain cabinet sizes. One, three, and five pound containers have proven to be most useful.
Gel should always be weighed after being conditioned to the same RH level (usually in a totally “dry” condition, or at 50% RH). The gel weight should be indicated on each container.
The design of the container should facilitate its loading with silica gel, and the packages/enclosures can be permanently closed, or a temporary fastening system may be used.
Before installation the gel packages should be thoroughly blown off with compressed air (using 50lbs pressure) in order to remove all the dust and damaged gel that is normally present.