May 2001 Volume 23 Number 2
In lower-tech areas of the world, the problem of eliminating insect infestations in artifacts is matched by the daunting problem of finding a viable disinfestation method. In Laos, where I recently worked at the Luang Prabang Museum, and where infestation is widespread and aggressive, nitrogen and even lower-tech carbon dioxide treatments are out of the question, since neither gas is locally available.
Freezing, though feasible, is expensive and would subject infested Buddha figures to a procedure unacceptable to religious Buddhists. Meanwhile, traditional deterrents like citronella oil, black peppercorns, and camphor, if not already discredited, require further testing not only as deterrents but also as insecticides before they can become mainstays of a disinfestation program.
At Luang Prabang, we used instead a method called Asolar bagging, developed by Tom Strang at the Canadian Conservation Institute, that uses sunlight to produce the 130°-140° F temperatures lethal to insects. Artifacts are wrapped in cotton to buffer moisture fluctuation and sealed in black-plastic bags that prevent moisture loss, maximize interior temperatures, and screen out UV and visible light. The object packets are then exposed to the sun. Inside, temperatures may rise as much as 40° to 70° F above the outdoor environment; adult insects, eggs, and the stages in between are killed in a matter of hours, victims of fatal dehydration and enzymatic dysfunction.
Tom developed solar bagging for rural Canadian museums and first presented it at the Third Biodeterioration of Cultural Property conference (1995) in Bangkok because of its obvious benefits for developing countries. It works. Tom has ascertained mortality rates for a gamut of museum pests at different temperatures, finding that 140° F (60° C) kills practically all of them in a few minutes.
He has determined the time needed for these lethal heats to penetrate different thicknesses of materials, times that may be as short as 40 minutes for textiles. And through tests and research into the literature on thermally-induced deterioration, he has determined that, with care, solar bagging does not place artifacts at risk of damage from condensation, mold growth, disruption of paint and other coatings, disturbance of glued assemblies, chemical deterioration from heat, or stress from shifts in relative humidity and equilibrium moisture content.
Still, Tom suggests caution with ethnographic leather, constrained textiles and objects with resins and adhesives that turn rubbery at 60° C. For details on these and other matters, refer to the references listed at the end of this article.
The procedure for solar bagging runs as follows. First, you need some sort of support furniture on which to place objects being solarized; the surface on which objects sit should allow for good air circulation and stand a hand's breadth or more off the ground. At Luang Prabang, we adapted two old benches that had lost their seats; we cut the legs down to 12 inches and created new seats by stringing nylon cord.
Next, you will need a small array of equipment and supplies. These include a solar-powered fan, an indoor-outdoor thermometer with probe, cotton cloth like broadcloth or light toweling for wrapping the artifacts, 6 mil black-plastic sheeting, packaging tape and 4 mil clear-plastic sheeting for enclosing the whole setup in a greenhouse envelope to exclude cooling breezes and help control moisture loss. Of these, only the cotton cloth and packaging tape were available in Laos. The rest, bought in the Bay Area, cost about $170.—largely a one-time cost.
Finally, you will have to make adjustments to the support so that you can attach the fan and thermometer and be able to maintain the clear-plastic envelope about a hand's breadth over the object packets. In Luang Prabang, for an envelope support, we lashed upright wooden slats to each of the four bench legs and could lash further extensions onto these slats, enabling us to adjust the height up and down as needed.
Once all this is readied, you need to measure the thickness of infested objects so as to calculate the heating times the artifacts will need. Then wrap the artifacts in cotton, seal them with minimum headroom in black plastic, and place the bags on the structure so that the most severely infested areas are facing upwards; this is best done indoors the day before you solarize.
On the morning of the treatment, place a mat on the ground if it's damp or cold and lay a large sheet of clear plastic over it. Bring out the loaded-up support and place it about two feet from one of the edges of the plastic. Lay out the solar panel for the fan and hook the fan onto the support so that it blows down the length of the support, below the objects. It should mix the cool air on the shade side of the objects with the warm air on the sun side.
Attach the thermometer where it will be in relative shade below the objects, yet visible; insert the probe in one of the object packets, on the sun side, sealing the puncture with tape, so you can monitor warmest and coolest temperatures within the system. Wrap the clear plastic up and around the support and close; in Laos, we used stones and clothes pins to secure the folded-together edges on the ground and also clothes-pinned the plastic to the tops of the slats so that it wouldn't slump. Make sure there's a hand's breadth of space everywhere between the object packets and the clear-plastic envelope so that air can circulate freely.
That's it. You monitor the temperature to ensure it has entered the lethal zone and to avoid warmest-coolest disparities of more than 25° F. You vent the envelope if temperatures climb above 140° F. Afterwards, wait 24 hours before opening the object packets to allow for moisture regain.
You can try alternatives. No black plastic? Use black fabric and clear plastic, or a black-lined box. No fan? Adopt a layout Tom has developed with an interior solar oven and chimney to keep the hot air circulating, or simply pump the outer bag like a bellows to mix the air. No dual temperature thermometer? Use two separate ones. (I'm planning to test a meat-roasting thermometer to replace the probe).
There are also bag-less variations. Relying on a published wood-mass-to-air-volume ratio for creating a chamber whose wooden walls buffer artifact's moisture loss during heating, Tom once helped a rural museum build an insulated plywood box in which to solarize its entire collection of farm equipment.
English Heritage has gone many steps further with this concept to develop a superlative plywood-paneled insulated chamber heated not by the sun, but with four wall-mounted, two-kilowatt electric heaters whose fans can operate independently of the heating element. These, along with heat/RH sensors in the chamber, allow English Heritage to obtain continuous spot-measurements of the environment and to control temperature and circulation of heat precisely. Project director Amber Xavier-Rowe reports that they may create another chamber inside a cargo container, which can be transported around the country.
Back to solar bagging. The principal drawback arises when objects are so thick, over three and a half inches, for example, that they require more than the eight to ten hours of 130° F-plus heat that a day's sunlight, even in June, can normally provide. This problem did not arise, however, in Luang Prabang; while there were infested objects five and six inches thick, infestation had reduced them to walls no more than two inches across. Another potential problem is oversize objects, though in Luang Prabang we were able to arrange our benches to accommodate infested furniture.
A final problem is security; in Luang Prabang, we considered the museum roof and sun balconies before settling upon a secure, unshaded lawn inside the museum staff's apartment compound. In short, there were no problems we could not solve, though it sometimes took time and ingenuity. And the signal advantages of speed, simplicity and low cost, combined with eminently well-tested effectiveness and safety, made solar bagging ideal for the museum's artifacts and operations. So easy was it, in fact, that as soon as the rainy season ended, we solarized every Tuesday until there were no infested artifacts left to treat.
Back in California, struck by the relative slowness, expense and elaborateness of the standard disinfestation methods, I feel that solar bagging—and heat treatment generally—has a vital role to play in developed nations too.
Strang, T.K. 1992. Published temperatures for the control of pest insects in museums. Collection Forum 8 (2).
----. 1995. The effect of thermal methods of pest control on museum collections. Biodeterioration of Cultural Property 3, ed. C. Aranyanak and C. Singhasiri. Bangkok: Thammasat University Press.
----. 1998. Another brick in the wall. Proceedings of the 3rd Nordic Symposium on Insect Pest Control in Museums, ed. M. Akerlund et al., Natur Historiska Rikmuseet.
----. 2001. Principles of heat disinfestation. Preprints of 2001, a Pest Odyssey, British Library Conference Centre, London, 1-2 October 2001.
Strang, T.K, et al. 2000. Low cost heating methods for insect pest control. Preprints of Tradition and Innovation: Advances in Conservation, IIC Melbourne Congress, 10-14 October 2000.
Xavier-Rowe, A., et al. 2000. Using heat to kill museum insect pests—is it practical and safe? Preprints of Tradition and Innovation: Advances in Conservation, IIC Melbourne Congress, 10-14 October 2000.