Volume 15, Number 3, Sept 1993, pp.15-19

Eradication of Insect Pests in Museums Using Nitrogen

Introduction

In museums, the future for fumigation with traditional chemicals is uncertain. Studies have shown that they can cause harm to their operators, destroy the environment, and may damage rare antiquities and artifacts [1,2].

Surveys of collections in art and natural history museums indicate that beetles belonging to the families Anobiidae and Dermestidae and moths belonging to the family Tineidae are major pests. A list of commonly damaged materials in museums and their associated pests are listed by Zycherman and Schrock [3].

Considerable research has been conducted with the use of modified atmospheres to manage insect pests in stored grains and food. In most studies the lowest range of oxygen concentrations tested was 0.6-0.9 percent [4-11].

In experiments sponsored by The Getty Conservation Institute (GCI) at the University of California at Riverside, Rust et al. evaluated the mortality of all life stages of ten commonly found insect species (webbing clothes moths, furniture carpet beetles, firebrats, cabinet beetles, larder beetles, cigarette beetles, confused flour beetles, cockroaches, powderpost beetles, and western drywood termites) at 55% RH and 25.5°. C in a nitrogen atmosphere having less than 0.1% oxygen. The time required for 100% kill varied from three hours for the adult firebrats to 192 hours for the eggs of the cigarette beetle [12].

The GCI and the J. Paul Getty Museum have been testing a number of methods for maintaining a low oxygen atmosphere for treating infested museum objects. This paper describes methods that can maintain an oxygen concentration of less than 0.1% and the desired relative humidity.

Procedure

The basic procedure for producing and maintaining a reduced oxygen atmosphere for treating museum objects is to replace air with an inert gas in the bag that encapsulates an infested object. There are three variations in protocol:

1. The dynamic system. An inert gas is used to flush all air out of the bag by an initial high flow rate and then, when a level of less than 0.1% oxygen is reached, the flow is reduced to that required to maintain the low-oxygen atmosphere for the treatment period [13,15].
2. The dynamic-static system. The bag is purged with an inert gas, as in (1), and after the 0.1% oxygen level is reached, a quantity of Ageless(tm) oxygen scavenger (described below) is quickly inserted, the gas flow stopped, and the bag sealed for the required exposure period [13-15].
3. The static system. This method is ideal for treating small objects (<100 liters). No purging of air in the bag is necessary. A calculated amount of oxygen scavenger needed to absorb the oxygen in the bag and maintain the oxygen concentration at less than 0.1% for the fumigation period is inserted.

The main features of each procedure--which include bag construction, relative humidity control, and treatment procedure--are detailed below.

Bag Construction

Plastic films vary in their permeability to oxygen [16]. Principal criteria for selection are a low oxygen permeability, availability in convenient sizes, and heat sealability. For our treatments we have used Aclar, poly (chlorofluoroethylene) extensively. Cryovac, poly (vinyliadine chloride), which has a lower oxygen permeability, is another material that has been widely used by other conservators (it was not available in adequate width nor in the smaller quantities needed for our applications).

The bags were fabricated by joining large sheets of of plastic with a portable heat sealer to create a form conforming to the shape of the object. Precise circular holes were cut with a punch for the nitrogen supply tubes and sensor leads.

Relative Humidity Control

Figure 1.
Set-up for the anoxic fumigation of an uphostered chair. The schematic drawing below shows the system for the flow of nitrogen.

To avoid possible hygrometric shock to museum objects undergoing anoxic treatment in the plastic bag, the nitrogen from the cylinder was humidified to the object's RH before being injected into the bag. The humidification apparatus is shown in use in Figure 1, and Figure 2 provides a schematic. The system involved splitting the gas flow from the nitrogen cylinder into two valve-controlled lines via a tee. One line bubbled the nitrogen through water in a stout polypropylene bottle; exiting the bottle, the moist nitrogen joined the other (dry) flow of nitrogen in a mixing chamber, which flowed to a third bottle containing a RH sensor. By controlling the needle valves on the two lines, the ratio of wet to dry nitrogen was varied to achieve the desired RH in the combined nitrogen stream, which subsequently flowed into the plastic bag containing the object.

Figure 2.
Schematic of the humidification system for nitrogen as it flows from the nitrogen supply tank to the bag that surrounds an object undergoing treatment.

To minimize the influx of oxygen, all fitting from the nitrogen cylinder to the entrance of the bag used 1/2 inch brass O-ring sealed Swagelok fittings. Poly(propylene) tubing (1/2 inch) led to O-ring sealed fittings in holes that had been precisely drilled in the humidification bottles. A tee fitting with an on/off valve was attached between the sensor bottle and the bag. This allowed any nitrogen whose RH was being varied prior to achieving a final RH value to be exhausted to the room atmosphere.

Ageless^TM

Ageless(tm) is an oxygen scavenger patented and produced by the Mitsubishi Gas Chemical Company [17,18]. Stated by Mitsubishi to be a mixture of finely divided moist iron (ferrous) oxide and potassium chloride, it is available in several different compositions. Ageless-Z is the type suitable for application in conservation. Ageless-Z is designated as Z-100, Z-1000, etc., to indicate the milliliters of oxygen with which a single packet will react.

An oxygen scavenger such as Ageless can be used to extend the life of a low-oxygen sealed case or bag filled with an inert gas by absorbing any oxygen that leaks into it. To maintain a low concentration of oxygen in a sealed case or bag, any oxygen leaking into the case must immediately react with the Ageless; i.e., the leak rate cannot be greater than the rate of reaction of Ageless with oxygen. Based on the leak rate in ppm of oxygen per day, L, and the Ageless capacity, C (which is defined as the oxygen-absorbing capacity of Ageless in liters divided by the volume of the case or bag in liters), one can obtain the desired equilibrium concentration using the following equation:

[O2] = L ÷ 12.7 C

For example, 12 packets of Ageless Z-2000 can maintain an equilibrium oxygen concentration of less than 0.1% in a 1000-liter case leaking 300 ppm (.03%) per day.

Treatment Procedure

The bag encapsulating an object was initially flushed with a high flow rate of nitrogen conditioned to the desired relative humidity and the oxygen concentration, relative humidity, and temperature in the bag were monitored. During the flushing procedure, the relative humidity inside the bag should be constantly monitored. A 6- to 8-inch opening can be left in the bag seam in a corner opposite from the nitrogen inlet to allow efficient flushing without pressurizing the bag. This opening was heat sealed once the desired oxygen concentration was achieved.

In the dynamic protocol (1), after reaching an oxygen concentration of 0.1%, the nitrogen supply was decreased to a low flow rate to maintain the desired oxygen level. In the dynamic- static protocol (2), the nitrogen flow was turned off and the inlet and outlet valves to the bag closed. This setup was maintained for 2-3 days to measure the leak rate of the bag. After the measurement period, the bag was cut open on one edge, and with the nitrogen flowing at a high rate, a calculated quantity of Ageless was placed inside the bag, the opening heat sealed, and the nitrogen flow turned off.

During the fumigation of a 1500-liter bag enclosing an Italian arm chair at a constant slow flow rate, as well as during the fumigation of a 2000-liter bag enclosing a wooden table maintained at less than 0.1% oxygen using Ageless, there was 0.01% difference in oxygen concentration when the sensor was moved from a height of 80 cm to 30 cm from the floor; hence, there is no significant oxygen stratification inside the bag.

A minor modification of the dynamic system described in (1) above was used for treating infestation of the mixed-media contemporary sculpture by Edward Kienholz, "Back Seat Dodge '38," at the Los Angeles County Museum of Art. The estimated volume of the encapsulating bag was approximately 5,500 liters. Voids in the sculpture, such as the passenger compartment, trunk, and under the hood of the vehicle, were filled with nitrogen charged balloons to reduce the effective volume of the encapsulation. Two independent nitrogen supply systems were also used to reduce the required time for flushing the air from the encapsulating bag.

The procedure for fumigating a small object (volume <100 liters) would be identical to the above, except that the third protocol of inserting a calculated number of Ageless packets into the plastic bag enclosing the object (without the need for any initial flushing) can be used.

Commercial Fumigation Bubbles

Inquiries from a number of conservators in the United States who have large scale fumigation problems suggested that any procedure involving making a large number of bags will be troublesome. Hence, we tested the feasibility of a reusable commercial fumigation bubble [19], as well as conversion of fumigation chambers to use the nitrogen anoxia method [20].

The bubble manufactured by Rentokil Corporation, when fully inflated, measures 11.5 ft x 11.5 ft x 8.25 ft and is widely used by conservators who have large-scale fumigation problems. The bubble, designed for toxic gas fumigation, consists of a top cover attached via a zipper to a base sheet. In tests conducted at the Fine Arts Museum, Houston, we reduced the oxygen concentration to less than 0.1% by repeated flushing with nitrogen gas and evacuating the bubble, and we evaluated the leak rate to be 0.08%. The procedure for maintaining an atmosphere less than 0.1% oxygen necessary for treatment of infested objects required daily flushing of the bubble using about six 8000-liter tanks of relative humidity conditioned nitrogen. Hence it was concluded that use of this bubble will not be feasible until insect mortality rates at higher oxygen levels becomes available.

Fumigation Chambers

Many museums own fumigation chambers purchased many years ago. These chambers were designed to use gases such as methyl bromide, ethylene oxide, and sulfuryl fluoride. Due to increased environmental regulations on the use of these chemicals, these chambers are presently not being used. However, these chambers can be operated safely if the nitrogen anoxia method can be adopted.

A Vacudyne fumigation chamber (36 cubic feet) at the Los Angeles Museum of Art designed for ethylene oxide fumigation has been converted to use the nitrogen anoxia method [20]. The electrical changes involved modifications of control switches and manual over-ride of the automatic control cycle. The mechanical modification involved installation of oxygen, temperature, and relative humidity sensors inside the chamber, and installation of a relative humidity control unit to the inlet nitrogen supply.

The protocol for operating the chamber involved opening the 4- inch-diameter vent and flushing the chamber with relative humidity conditioned nitrogen. Once the oxygen concentration reaches less than 0.1%, the vent valve and the inlet nitrogen flow were closed and the oxygen concentration inside the chamber monitored.

Based on the evaluated leak rate of about 50 ppm/day, once the oxygen concentration inside the chamber is reduced to about 0.05%, the chamber needs to be reflushed every 8-10 days to maintain the <0.1% oxygen concentration.

Future Work

The study by Rust et al. in 1991 proved the efficacy of using nitrogen with less than 0.1% oxygen to kill all life stages of the ten commonly found museum insects studied. Although it is possible to maintain a 0.1% oxygen concentration, this may not be easily achievable in museums for large applications such as for the fumigation bubble described above. To improve the applicability of the nitrogen anoxia treatment, The Getty Conservation Institute is sponsoring an extension of the insect mortality study at higher oxygen concentrations at the University of California at Riverside. The effect of relative humidity on mortality of two common museum insects, cigarette beetle and furniture carpet beetle, will be evaluated at 33%, 55%, and 75% relative humidity in a nitrogen atmosphere with oxygen concentrations of 0.3%, 0.6%, and 1.0%.

Conclusions

The use of nitrogen gas to attain low oxygen atmospheres for eradicating insect infestation of museum objects is a feasible alternative to toxic gases. All insects commonly found in museums can be eradicated in a 0.1 % oxygen atmosphere. The methods described in this paper produced and maintained the relative humidity and oxygen concentration at the required level. Results of the extended insect mortality studies at higher oxygen concentration, which are presently being conducted at the University of California at Riverside, will make this non-toxic method of insect eradication even easier for museums to use.

Acknowledgments

We would like to acknowledge a number of people for their contribution at various stages of this project: Dr. Michael Rust and Janice Kennedy from the Department of Entomology, University of California at Riverside, for the mortality studies. Dr. Dusan Stulik, Dr. Frank Lambert, Dr. Frank Preusser, Mr. Jim Druzik, and Dr. Neville Agnew, all from The Getty Conservation Institute, for being part of every aspect of this project. We would also like to thank Mr. Brian Considine from the J. Paul Getty Museum, Dr. Pieter Meyers, Mr. Steve Colton, Mr. Don Menveg, and Mr. Neil Rhodes from the Los Angeles County Museum of Art for the practical implementation of this project.

References

1. Florian, M. L. "Ethylene oxide fumigation: a literature review of the problems and interactions with materials and substances in artifacts," Zycherman, L. A. and Schrock, J. R. (eds.), A guide to museum pest control (Assoc. of Syst. Collections. Washington D. C., 1988) 151-158.

2. Dawson, J. "The effects of insecticides on museum artifacts and materials," Zycherman, L. A. and Schrock, J. R. (eds.), A guide to museum pest control (Assoc. of Syst. Collections. Washington D. C., 1988) 135-150.

3. Schrock, J. R. "List of insect pests by material or apparent damage," A Guide to Museum Pest Control (Assoc. of Syst. Collections. Washington D. C., 1988) 113-120.

4. Bailey, S. W and Banks, H. J. A review of recent studies of the effects of controlled atmospheres on stored product pests, Controlled atmosphere storage of grains (Elsevier Scientific Publishing Co., Amsterdam, 1980) 101-118.

5. Zycherman, L. A. and Schrock, J. R. [eds.], A guide to museum pest control (Assoc. of Syst. Collections. Washington D.C., 1988).

6. Quek, L. C.; Razak, M. and Ballard, M. W., "Pest control for temperature vs. tropical museums: North America vs. Southeast Asia," 9th Triennial Mtg. of the Internat'l Council of Museums, Committee for Conservation (1990) 817-820.

7. Marzke, F. O., Press, A. F. Jr., and Pearman, G. C. Jr. "Mortality of the rice weevil, the Indian-meal moth, and Trogoderma glabrum exposed to mixtures of atmospheric gases at various temperatures." J. Econ. Entomol. 63 (1970) 570-574.

8. Navarro, S. "The effects of low oxygen tensions on three stored-product insect pests." Phytoparasitica. 6 (1978) 51-58.

9. Jay, E. G., Arbogast, R. T. and Pearman, G. C. Jr. "Relative humidity: its importance in the control of store-product insects with modified atmospheric gas concentrations" J. Stored Prod. Res. 6 (1971) 325-329.

10. Jay, E. G. and Cuff, W. "Weight loss and mortality of three life stages of Tribolium castaneum (Herbst) when exposed to four modified atmospheres," J. Stored Prod. Res. 17 (1981) 117- 124.

11. Gilberg, M. "Inert atmosphere fumigation of museum objects" Studies in Conservation. 34 (1989) 80-84.

12. Rust, M. K.; Kennedy, J. M.; Daniel, V.; Druzik, J. R. and Preusser, F. D. "The feasibility of using modified atmospheres to control insect pests in museums" (Submitted for publication to Studies in Conservation.)

13. Hanlon, G.; Daniel, V.; Ravenel, N. and Maekawa, S. "Dynamic system for nitrogen anoxia of large museum objects: a pest eradication case study" Second International Conference on Biodeterioration of Cultural Property, Yokohama, Japan (1992).

14. Daniel, V.; Hanlon, G.; Maekawa, S. and Preusser, F. "Nitrogen fumigation: a viable alternative" International Council of Museums, 14th triennial meeting, Wash. D.C,

15. Daniel, V., Hanlon, G. and Maekawa, S. "Non-toxic fumigation of large objects" 21st Annual Meeting of the American Institute of Conservation, Denver, Colorado, May 31-June 6, 1993.

16. Burke, John "Vapor Barrier Films" WAAC Newsletter, Vol 14, Number 2, May 1992, pages 13-17.

17. Lambert, F. L.; Daniel, V. and Preusser, F. D. "The rate of absorption of oxygen by Ageless; the utility of an oxygen scavenger in sealed cases" Studies in Conservation, Vol 37 (1992), pages 267-274.

18. Daniel, V. and Lambert, F. L. "Ageless oxygen scavenger: practical applications" WAAC Newsletter, Vol 15, Number 2, May 1993, pages 12-14.

19. Pine, S., Daniel, V. and Maekawa, S. "Large scale fumigation of organic materials using nitrogen" 21st Annual Meeting of the American Institute of Conservation, Denver, Colorado, May 31-Jun 6, 1993.

20. Daniel, V. & Maekawa, S. "Retrofitting of the fumigation chamber at the Los Angeles County Museum of Art to use the nitrogen anoxia method" Internal Report: GCI, 4503 Glencoe Ave., Marina del Rey, CA 90292.

List of Materials and Suppliers

Ageless oxygen scavenger:

Conservation Materials, Ltd., P.O.Box 2884, Sparks, Nevada 89432. Tel: (702) 331 0582.

Oxygen barrier film:

Aclar: Sealpak Industries, 13826 S. Prairie Ave, Hawthorne, CA 90250. Tel: (310) 973 1321.

Oxygen monitor:

Teledyne model 320: Teledyne Analytical Instruments, 16830 Chestnut Street, P.O.Box 1580, City of Industry, California 91749-1580 Tel: (213) 283 7181.

Heat Sealer:

Rolling wheel constant heat sealer: Century Chicago Inc., 301 Viola Lane, Prospect Heights, IL 60070. Tel: (708) 253 3619.

Relative Humidity Sensor:

Vaisala HMP133Y: Vaisala Inc.,100 Commerce Way, Woburn, MA 01801. (617) 933 4500

Nitrogen gas:

Contact your local supplier and ask for a nitrogen grade with an oxygen concentration less than 0.05%.

Commercial Fumigation Bubble:

Rentokil Ltd., Felcourt, East Grinstead, West Sussex, RHJ92JY. Tel: (0342) 833022.

Fumigation Chamber:

Vacudyne Altair, 375 East Joe Orr Road, Chicago Heights, Illinois 60411. Tel: (708) 757 5200.

Author Addresses: