Freezing Collections to "Buy Time"
The longer a collection is wet, the more deformation and damage it will experience and the more likely it is to develop mold. This is true of virtually all types of collections - natural history specimens such as bird and mammal skins, ethnographic materials such as baskets or textiles, as well as books, photographs, or documents.
Mold can become established in as little as 48-72 hours, depending on temperature. If a large number of specimens or artifacts become wet, it is unlikely that there will be time and space available to dry everything before there is a risk of mold developing. In this situation, it is possible to freeze many kinds of specimens or artifacts to “buy time.”
It should be made clear that this kind of freezing is not a drying method, and does not kill bacteria or mold spores. However, fungal and bacterial activity will be dormant when a collection is frozen, so there is no longer a need to rush into decisions Once a wet collection is frozen, there is time to take advice, weigh options, and make thorough preparations for the drying and recovery stage.
Brief description of treatment
Small scale freezing can be carried out in a walk-in freezer or in an ordinary upright or chest freezer. For larger quantities, freezer containers can be brought to the Museum, space can be rented in a commercial freezer plant, or freezer trucks can be used to freeze a collection and then take it, frozen, to another place where it can be dried by any of various methods.
Ideally the freezing equipment should have the capacity to freeze very quickly. The reason for this is that fast freezing and low temperatures keep the ice crystals that form small, thereby reducing stress on the wet material. In practice, this means that items should be frozen with space around them rather than in a dense stack and that a blast freezer, if one is available, is the ideal choice.
What collections materials can be treated this way?
There has been little well-documented research on the effect of freezing collections that have been wetted, because almost all of our experience of freezing museum specimens and artifacts either concerns dry objects, which are being frozen for purposes of pest control, or waterlogged archaeological materials.Museum materials wetted in a disaster are different from either of these. Their cell structure is much stronger and more resilient than waterlogged archaeological material. However, they are wet beyond fiber saturation point, so that ice crystals will form within them when they are frozen, causing physical damage on a microscopic scale.
In assessing the effect of freezing on dry collections Florian (1997) and Carlee (2003) state that most organic materials and composite objects, including those with inorganic components, can be frozen. However, they warn that items should not be exposed to temperatures lower than 40 degrees C below zero, as thermal contraction can occur with some materials.
Experience shows that most wet collections materials, too, can be successfully frozen without damage, although several kinds of material should generally not be frozen.
The materials that are generally not appropriate for vacuum freeze drying include:
- Acrylic and oil paintings on canvas and panel paintings
- Materials with friable media (pastel, charcoal, some paints).
- Research collections, if it is a concern that DNA may be affected by freezing
- Artifacts with very complex structures incorporating many materials, such as musical instruments
- Japanese lacquer ware
- Wet ivory and bone
- Some plastics may not be appropriate to freeze as they may become embrittled
- Within photographic collections, if time allows, check for the following materials, which risk permanent damage if frozen: collodion wet plate negatives, cased photographs (daguerreotypes, ambrotypes, pannotypes, tintypes, ferrotypes], glass color transparencies (Autochromes), and lantern slides
- Although research is lacking, there is some feeling that magnetic media and audio grooved media (cylinders, discs) may be damaged by freezing
- Herbarium specimens prepared with high moisture acrylic adhesives may become tacky upon freezing and sheets may stick together
- Bones and wood should generally not be freeze-dried because splitting and cracking may occur.
Risks and benefits should always be weighed when considering freezing any wet collection. A major mold outbreak in a collection is so serious that in some situations freezing these materials may be the better option and should be considered. As always, consult a conservator before making these decisions.
When museums began disaster preparedness planning, staff should consider the vulnerability of these materials when planning storage, as the materials listed as “cannot be frozen” are particularly vulnerable to damage by water and are especially difficult to recover. Therefore, these materials should be given the greatest possible protection from water in the first place through choice of storage location and storage enclosures.
Wet materials are fragile and easily damaged, and should be handled as little as possible.
In some cases, it may make sense to freeze whole storage drawers with specimens or artifacts leftin situ. This method maintains order and identification as well as minimizing handling. If objects are to be shipped in this state, some sort of filler or padding can be placed in drawers to keep objects from slipping, and the entire drawer can be secured with shrink-wrap.
Because fast freezing promotes formation of smaller ice crystals, items should be frozen with space around them rather than in a dense stack. Take care in upright or chest freezers to leave room around objects. Walk-in freezers may be equipped with shelving units, which make spacing easy.
Note that this is somewhat different from packing dry objects to freeze for pest control purposes, because it is not necessary to seal wet objects in plastic bags unless bagging will help to pad and protect them physically during their time in the freezer. Objects are bagged for pest control largely to protect them from condensation when removed from the freezer. Since in this case objects are already wet when frozen, this precaution would serve no purpose.
Benefits of Freezing to Buy Time:
- Fungal and bacterial activity will become dormant when frozen
- The necessity to dry objects before mold becomes established is relieved
- Decisions on drying method can be made with deliberation
- Time is created to make preparations for the drying and recovery stages.