Originally developed for the food and grain industry, Carbon Dioxide (CO2) treatments (sometimes also referred to as either modified or controlled atmosphere treatments) have been safely adapted into the museum environment. Historic New England-Society for the Preservation of New England Antiquities (HNE-SPNEA) continues to use other forms of treatments, such as low temperature/freezing and low-tech heat treatments, but the use of CO2 remains the preferred choice for the following reasons:
• Existing in-house system.
• Unit is able to accommodate large objects, up to 9’ w x 11’ h.
• Gas is inert with no residue or carryover effects on collections.
• Lower cost, per large-volume operations and lesser need for auxiliary humidification compared to nitrogen.
• Objects can be safely and immediately returned to storage following treatment.
• Cost effective. Equipment and initial set up is expensive, but price of gas is quite reasonable and equipment needs little maintenance. In-house staff time is minimal during treatment cycle.
• With proper training, treatments can be done by staff without need for special permit or license depending on local and federal regulations.
HNE-SPNEA follows guidelines for using CO2 to treat pest infested collections based on material found in the Getty Conservation Institute’s 1998 publication Inert Cases in the Control of Museum Pests, which includes detailed technical data and information on mortality rates, personal research and studies, and 15 years experience using the system. Generally speaking, the length of treatment time inside of the unit is largely dependent upon temperature (25-30ºC), species vs. time (days), and a consistent oxygen level of 4.9- 8.4%, corresponding to a CO2 range of 75-60%. During the process, the CO2 levels drop to 60% over a 7 week period and then maintained at this level for an additional 21 days. This is to ensure mortality for more resistant species and heavy infestations.
Treating collections with carbon dioxide
HNE- SPNEA has over 37 primary historic house sites, and a collection of more than 80,000 oversized objects, for example, furniture, architectural fragments, and large rugs. The museum chose to purchase a standard Rentokil bubble unit in 1992, after a bad infestation of webbing clothes moths and furniture beetles at one site and a moth infestation in a stage area at the facility. A new membrane was installed in 2000 by the Maheu & Maheu Company. Purchasing an in-house CO2 bubble unit made it possible to treat oversized collections on a regular, monthly cycle. It also proved to be cost-efficient in terms of seeking alternative treatment methods and outside service vendors. The museum currently offers treatment services to outside clients including neighboring museums, galleries, and private clients. Once objects have been treated, they can be safely and immediately returned to storage areas.
The museum has treated many different types of collection items including: organic, inorganic, and composites. Thus far, there has been no damage to collections, as each treatment run is monitored daily for temperature, relative humidity (RH), oxygen and CO2 levels.
There has been some discussion about the risk of the formation of carbonic acid when carbon dioxide encounters water during treatment, particularly at higher relative humidity levels (Reichmuth 1987). However, the formation of carbonic acid is unlikely, as it requires liquid water, not moist air. Furthermore, the reaction is endothermic, meaning that an input of energy is required to break some stable CO2 bonds, so it does not occur spontaneously. Therefore there is little possibility of damage to objects with sensitive surfaces; however users should avoid treating anything that is wet or saturated.
Another reason for favoring CO2 for large-volume operations is its lesser need for auxiliary humidification. At a 60% CO2 level, 40% of the original water vapor remains, in addition to additional moisture buffering by wooden and paper collections in the treatment chamber (Selwitz, Maekawa 1998, chapter 8).
The unit consists of a large plastic membrane that is closed and sealed by a zip strip. An inner framework of wood supports the unit and acts as a ‘skeleton’ for the membrane. Auxiliary heating and humidification should be done prior to loading the unit, making sure to give collections ample time to slowly adjust to changing conditions. The room that the unit is in has an overhead heating system with a thermostat and a fan-driven humidification system. Overall conditions within the room and inside of the unit are monitored with LCD data loggers.
Collections are loaded into the unit and arranged for an even balance, allowing the CO2 gas to easily permeate through the objects within the unit. The unit is then zipped closed and a motorized vacuum system draws residual air out of the bubble creating a vacuum. The gas is now pumped into the unit at a rate of 5psi. Once the gas inside of the bubble reaches the maximum capacity volume, the unit is drawn creating another vacuum. The gas is once again pumped into the unit until the ideal oxygen/ CO2 levels are reached, within a 7-day period (25- 30ºC; 60% CO2). From this point on, the unit is maintained at the ideal levels and temperature and RH are regulated throughout the cycle, or about 21 days.
Once the cycle is complete the unit is vacuum drawn, the ventilation system is turned on, and the unit is opened. Once the CO2 levels within the room have returned to normal levels (.040%) the collections can be returned to their respective locations.
• The original unit was purchased through Rentokil in 1992 and a new membrane was purchased through Maheu & Maheu in 2000. The motorized vacuum unit was part of the original unit.
• There is a CEI Instruments CO2 analyzer, which is hard wired to a powerful ventilation system which automatically runs when CO2 levels exceed .10%. The monitor has a digital display reading and it monitors the level by taking a gas sample every two minutes.
• An overhead heating unit with digital thermostat allows the museum to run the unit through the colder winter months while maintaining the ideal temperature.
• An OXOR II oxygen monitor is used during the treatment cycle to monitor the O2 levels inside of the unit.
Integrated Pest Management Working Group
Treatment Subgroup March 2008