4 THE PREVENTIVE FREEZE PROCESS AT NMAI

Both the Research Branch and the Cultural Resources Center are equipped with large walkin freezers. All objects packed at the Research Branch for the move are lightly surface-cleaned with a vacuum and secured on a travel mount. Some are tied down with cotton twill or Teflon tape to corrugated pallets with polyethylene foam supports, while others are placed in small boxes with acid-free tissue, bubble wrap, or polyethylene foam padding. These housings are then bagged in clear polyethylene plastic sealed with tape and grouped in larger cardboard boxes. These large boxes are wrapped with an additional layer of polyethylene and securely taped. During the move process (but not during this study), the boxes are placed in the Research Branch freezer at approximately –20�C for five days. Boxes are then loaded on a climate-controlled truck for shipping to the Cultural Resources Center.

For the purposes of this study, freezing for pest management was done at the Cultural Resources Center instead of the Research Branch to eliminate possible damage in transit as a variable and allow condition after travel and before freezing to be assessed. The freezer at the Cultural Resource Center is a Bally pre-engineered walk-in freezer averaging a temperature of –40�C. From the perspective of insect mortality, there is no benefit to temperatures below –40�C, and lower temperatures may put the objects at greater risk of damage (Strang 1997). This temperature is lower than the –20�C routinely used at the Research Branch for the NMAI move process. The freezer used for the observational study is a two-stage 10HP refrigeration system with a 45-minute defrost cycle every 6 hours. Freezers with defrost cycles are not typically recommended for pest control because during the defrost cycle they tend to rise above temperatures required to kill pests (Florian 1990a). Two ACR Systems Smart Reader 2 dataloggers were placed in the freezer with the objects on two occasions. Data from this equipment indicate that the temperature in the freezer drops from approximately 20�C (room temperature) to approximately –40�C over a two-hour period. This rapid drop is key to preventing cold acclimation in insects (Florian 1986b; Strang 1997). Temperatures were recorded as low as –45.8�C. Defrost cycles were never warmer than –23�C, safely below the recommended temperatures for insect mortality. The air inside the boxes returns to room temperature over a period of three to six hours after removal from the freezer. Relative humidity below 0 degrees is difficult to monitor because of the reliance of the datalogger on nonfrozen moisture for accurate readings.