Micro-climate and High Density Storage: Boxes for Archaeological Metals and Other Environmentally Sensitive Objects
Environmental conditions are difficult and expensive to maintain, particularly specialty environments such as the low relative humidity required for archaeological metals. A high density storage system in which the relative humidity and chemical deterioration ca be controlled is often needed in institutions that contain collections made from materials that have differing environmental tolerances.
At the Carnegie Museum of Natural History (CM), the old system for storing these objects was on shelves with minimal padding. Some of the fragile metal was encapsulated in plastic bags inside plastic boxes with silica gel. Others were placed on wooden shelves with degraded padding. The collection was difficult to see and difficult to handle. (See figure 1 – Original storage system.) Where present, the silica gel had not been reconditioned since for 30 years. The goal was to create a high density system, which was chemically inert, provided the possibility to maintain a micro-climate, and make the objects more accessible to researchers.
The CM Anthropology Department used National Endowment for the Humanities funding to improve storage conditions for small archaeological metals. The new system was designed to provide better physical support and organization for the collection, and to buffer against fluctuating relative humidity while protecting this sensitive collection from contamination. The new system is compact, simple and cost effective.
Deborah G. Harding
Carnegie Museum of Natural History
Edward O’Neil Research Center
5800 Baum Boulevard
Pittsburgh, PA 15206
Illustrator / Photo Credits
Deborah G. Harding
Gretchen E. Anderson
This article describes the storage solution designed for CM archaeological metals using materials that are easily acquired, simply sized and easy to cavity pack. A simple plastic box that could fit the designated storage area was utilized.
Commercially available plastic boxes were purchased, in a standard size that could fit the designated storage area and was of adequate size to accommodate its intended contents. Stacking trays with cavity mounts were constructed to layer small, flat items within. Each object was laid out in its own cavity so that it can be easily seen without handling, and each tray can be removed from the box with minimal handling. Each box has an inventory, with a photograph of each tray – making it easier to locate items. The environment inside the box is controlled by creating a passive micro-climate, using silica gel and a humidity indicator strip.
Materials, Tools & Supplies
Plastic box (polyethylene, polypropylene, polycarbonate) with a snap-on lid
Acid-free board (acid-free cardboard, foam core)
Acid-free /anti-tarnish tissue (alternatives include: Tyvek™, Teflon sheet™, Corrosion Intercept™, silver cloth)
Polyethylene sample bag (optional – as needed for small fragments)
Polyethylene or Polypropylene foam (either closed or open cell)
Cotton twill tape
Hot melt glue
Silica gel (dry conditioned)
cotton stockinet tube
Gasket material (closed cell polyethylene or silicone gasket)
Archival double stick adhesive (3M 415, 960 series)
Archival clear tape (example JLar™)
Stick for spreading glue
Relative humidity (Rh) indicator strip
Prepare the box:
- Choose a box that provides adequate space for your needs
- Clean interior and allow it to dry.
- Attach gasket to rim using double stick adhesive.
Prepare the mounts:
- Measure and cut a board to fit the space, leaving room for the silica gel desiccant. A second layer of cardboard can be added, increasing the strength by crossing the corrugations. Use hot melt adhesive to adhere the layers.
- Cut the foam (1/4”) to fit the board
- Arrange objects on the foam, allowing for each to have an individual cavity with a buffer space between. Mark and then remove the objects.
- Cut the cavities. Label each cavity with the object number so that it can be read when the object is situated within. An archival pen, specifically designed for plastics was used.
- Attach the bottom surface of the foam to the board using hot melt glue.
- Attach lifting loops made of cotton twill tape by gluing them to diagonal corners of the board with the hot melt adhesive. This allows the board to be easily removed from the box.
- Repeat above steps for each layer.
- Attach foam blocks to the top of the lower tray to raise the upper tray above the object. The blocks support and stabilize the upper tray.
- Line cavities with a liner (options include: Tyvek™, Teflon™, Corrosion Intercept™, silver cloth). The lining protects the objects’ fragile surfaces from abrasion when in contact with the foam and can also be used to lift the object from the cavity by providing a cradle.
Finish the box
- Stack trays into box, leaving enough room for the silica gel packet.
- Measure and prepare the silica gel. For metals, the silica gel should be as dry as possible, below 30% RH. Bulk silica gel can be dried in an oven that is set on low (50 ̊C/120 ̊F). There are alternative methods for drying gel that do not use an oven. Alternative ways of drying silica gel can be found by searching the internet.
- The conditioned silica gel is placed in a cotton stockinet tube. One end of the tube is stitched closed. The other is tied closed so that the gel can be removed for reconditioning.
- Place Relative Humidity (RH) strip into container so that it can be monitored.
- Attach inventory to lid (photographs can be added).
- Close lid and seal it with archival quality clear tape (JLar™).
There are a number of variations on the theme that can be used: alternative materials are listed in this article. Choose materials carefully, according to their availability and inherent properties, such as strength, or specific needs of the object. We used materials that we had available.
Since writing on foam can be problematic (smudged ink, concern with contamination from solvent) we have we have moved to a new method of labeling. The labels are now made by printing (by hand or printer) on to rag paper, cutting the label into a rectangle, and inserting the label into slits next to the cavity.
Silica gel: Choose the gel that is appropriate for your need.s Some gels come preconditioned, others have to be conditioned. Unconditioned gel can be heated to 50 deg C to drive off moisture for a dry application such as the one described here. If you are not used to working with silica gel, and the object’s environmental specifications require something different than very dry, you might consider using preconditioned gel.
This micro-climate system is based on a system that Anderson developed with help of conservator Rebecca Newberry and volunteers Ron Voelker and Verne Anderson at the Science Museum of Minnesota. The range of containers and methods were presented at the 2009 AIC Annual Meeting, Objects Specialty Group Tips Session and published in Objects Specialty Group Postprints (2009).
The original system using commercially available plastic boxes was similarly developed to protect archaeological metals from poor environmental conditions. These containers have maintained the desired climatic conditions for 15 years.
This box configuration was expanded at the Science Museum of Minnesota to adapt museum grade storage drawers for systematic storage of archaeological metals. It was also used to isolate bird study skins which tested positive for arsenic. The adapted drawers are accessible to researchers, while maintaining internal relative humidity conditions.
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Lafontaine, R. H. Technical Bulletin No. 10: Silica Gel, Canadian Conservation Institute 1984.
Tetreault, J. “Display Materials: the Good, the Bad and the Ugly”, pp. 79-87 in Exhibition and Conservation, Edinburgh, UK: Scottish Society for Conservation and Restoration. 1994.
Weintraub, S., “Demystifying Silica Gel”. Objects Specialty Group Postprints. Vol. 9. 2002.