If you work with museum objects in a natural history museum, “freezing” is a necessary part of the job. It may sound a bit odd to freeze a museum object of any sort, but it enables us to store specimens until they can be prepared and it is also an effective way of killing insect pests without the use of chemical pesticides. The use of pesticides leaves a residue on specimens each time they are used and this causes a build-up over time. By using the freeze-treatment method instead, we reduce this residual build-up and the related side effects that could possibly harm specimens over time and/or become a health hazard to the collections staff that care for them.
Most of the time we can “freeze-treat” our specimens in house, but for larger mounts, like a bison mount that is 6.5 feet tall by 8.5 feet long, we do not own a large enough freezer. Instead we had to seek out a facility that would not only have room to accommodate its size but was willing to tackle this project. Midwest Freeze-Dry in Skokie was up for the job and already had experience with the process.
To freeze-treat a natural history specimen, it first it needs to be enclosed in a protective barrier. In this case we used plastic sheeting because it came in a large enough size to cover the mount, but for smaller specimens plastic garbage bags or freezer grade zipper-closure bags work well. The purpose is twofold: (1) it creates a “sealed” enclosure so that insects cannot spread to other specimens and (2) it creates a moisture barrier so that any condensation that may form during the freeze-treatment forms on the exterior of the barrier and not on the specimen.
Once the bison mount was wrapped, in what resembled a plastic cocoon upon completion, we arranged a delivery date and then booked appropriate transportation with additional people to assist with the move. After the mount was carefully moved onto the truck and securely strapped in for the 6 mile trip north, the next step was to transfer it from our truck to the freezer truck, which was accomplished with a forklift and the help of many people.
Unlike freezing food items at home, to effectively kill insect pests, the specimen has to be exposed to at least two freeze cycles with a thaw in between. The method is pretty simple, the specimen is placed inside the freezer space and the temperature is dropped rapidly and held steady for a set time and then the temperature is gradually increased. After the space returns to room temperature, the temperature is dropped rapidly again for a second cycle and then gradually brought back up to room temperature again. This cycle ensures that any insect pests that went into a dormant phase during the first freeze cycle are “woken up” during the thaw period and then eradicated by quickly dropping the temperature again. The amount of time needed depends on the temperature of the freezer space, the size of the freezer space, what kind of insect pest you are dealing with, and the size and composition of the specimen.
When the allotted freezer time was completed, utilizing more people, two forklifts (one at each location) and a pallet jack, the Bison mount was transported to the museum for its final installation. You can see the Bison mount as well as other specimens from the collection in the exhibit, Food: The Nature of Eating, at the museum.
Amber K. King
Assistant Collections Manager