Desiccant Tubes for Storage of Unstable Metals
Dry conditions are needed year-round for archaeological iron because it can potentially corrode whenever the relative humidity (RH) rises above 15%. In a storage case one can achieve this extremely low RH requirement by adding a desiccant package. For practicality, the desiccant package design must also be:
- Inexpensive and easy to construct
- Include secure re-usable closures
- Use archival materials
- Facilitate the easy maintenance of the desiccant
- Fit into storage case spaces that are not needed for collections.
UMass Boston, Fiske Center for Archaeological Research
Department of Anthropology
University of Massachusetts at Boston
100 William T Morrissey Blvd
Boston, MA 02125-3300
Phone: (617) 287-6829
Photo Credits: Dennis Piechota
This design was developed at the Fiske Center for Archaeological Research of the University of Massachusetts, Boston campus (UMB) where we actively excavate artifacts from historic sites around the United States and the world. Archaeologists and graduate students recover large numbers of unstable ferrous artifacts during each excavation season. Even after stabilization (typically using tannic acid) these objects are best stored under desiccated conditions.
The university-wide heating and cooling system exacerbates the instability of archaeological iron. In the study/storage rooms, the relative humidity is often very high in the summer months when water is injected into the air handling system to provide basic comfort cooling. In the winter months, lower relative humidity is expected due to mechanical heating, but monitoring has shown that the RH can even spike up to 70% as moisture is sometimes injected into the system during unexpectedly warm winter days.
This article describes how to create a micro-environment inside a storage case so that the relative humidity can be maintained at or below 15%. A single museum storage cabinet, Delta Designs DDLX-G cabinet with exterior dimensions of 147 cm wide by 200 cm high and 80 cm deep, fitted with four pull-out shelves and one fixed shelf, was chosen as a container. Its internal air volume is approximately two cubic meters. Very little volume was unused by the artifacts stored within; only a 5-6 cm wide space just inside each of the two sidewalls, a space defined by the framing of the case, was not in use for collections storage. This space determined the shape of the desiccant packages; they were designed as 5 cm diameter tubes with a length of 70 cm related to the cabinet depth. The cabinet accommodated 12 tubes.
The tubes were constructed from sheets of needle-punched or ‘soft’ Tyvek® (Type 1622E) sealed with Tyvek® tape. Custom-cut Clip-n-Seal® brand bag-clasps allowed re-sealable access through one end of the tube, so that they can be emptied and re-filled with re-conditioned silica gel. Tyvek® straps were attached to each end of the tubes, to secure them to the sidewalls of the case. Bungee cords were attached to the existing shelf support stanchions of the case, to hang the tubes from their straps.
Note that Regular Density (desiccating) silica gel readily absorbs atmospheric water and must be periodically reconditioned to desiccate it for repeated use. After one year of installation and exposure to the university mechanical heating and cooling system plus approximately 20-25 cabinet openings, the silica gel needed replacement/recharging twice to maintain the target of 15% RH or less.
Materials, Tools & Supplies
DuPont Tyvek® Type 1622E
Lineco Tyvek® Tape, 1” x 150’
Clip-n-Seal Bag® Clips
Silica gel, regular grade (RD), Grade 03
10 inch, bungee cords
Testing sieve No. 14
- Cut the Tyvek® sheeting into an 8 ½” x 26” (22 cm x 66 cm) section.
- Fold it lengthwise and make two consecutive ½” wide folds in the mating edges
- Seal the fold with a continuous length of Lineco Tyvek® tape.
- To form the straps: cut two strips of strips of Tyvek® sheeting, each measuring 2” by 9” (5 cm x 23 cm). Triple-fold each strap lengthwise and staple each one near the ends of the tube. Over the seam of the Tyvek® tube, staple each strap at two points.
- Seal one end of the tube by making two consecutive ½” wide folds and finish the tube by sealing with a length of Lineco Tyvek® tape.
- For the re-closeable end of the tube cut a Clip-n-Seal® Bag Clip to 3 ½” (8.9 cm) and 4” (10.2 cm) lengths. Make sure that the Tyvek® tube portion is ½” (1.3 cm) longer than clasp to facilitate opening.
- Apply labeling to identify the tubes.
- Working inside a fume hood, complete the tube by filling with approximately 900 grams of RD silica gel and close. Note that silica dust is a respiratory hazard, and should be handled in a fume hood.
9. Within the cabinet, use the 10” (25.4 cm) miniature bungee cords to secure the desiccant tubes to the cabinet stanchions. Their hooked ends will conveniently grab the Tyvek® tube handles and facilitate easy installation and removal.
The storage case relative humidity should be continuously monitored, especially during periods of increased stored collection use. After fresh (dry or reconditioned) silica gel tubes are installed, the humidity inside the case will slowly rise as the silica gel absorbs moisture. We use dataloggers with a digital readout (Hobo UX-100-003, Onset Computer Corporation) to continuously monitor relative humidity.
Regular Density (desiccating) silica gel, must be periodically reconditioned by driving off its absorbed water.
- Silica gel is reconditioned by heating it for several hours in an oven at 180-200ºC.
- Working inside a fume hood (with the sash down to an appropriate height), remove the clip, open the end of the Tyvek® tube, and pour the silica gel onto a flat tray that fits inside a laboratory oven.
- The silica gel requires an extended period of cooling (overnight) before it can be safely placed into the Tyvek® tubes.
- Re-fill the Tyvek® tubes with dry RD silica gel, working inside a fume hood, with the sash down to the appropriate height.
- Note that the Tyvek® cannot sustain heat; do not put the tubes directly into the oven.
To facilitate the quick and easy replacement of spent for fresh silica gel, have twice as much silica gel on hand as the case will require and to store the excess in a sealed metal container.
It is a good practice to annually re-screen the silica gel with a #14 testing sieve or similar. This process removes the fine silica dust that can develop over time due to decrepitation. Be sure to screen the silica gel in a fume hood with its sash at the correct height for particulate capture. Note that silica dust is a respiratory hazard, and should be handled in a fume hood.