5.2 Deep freeze drying

There is considerable confusion over the use of terminology to describe the methods available for drying wetted archival and library materials. Therefore, at this point, it may be useful to review the terms normally used.

Vacuum freeze-drying is the process for drying frozen materials by sublimation at low pressure (high vacuum) and at temperatures below the freezing point. Possibly a less confusing term to describe vacuum freeze-drying is the single word "lyophilization." Both terms are interchangeable in the English language although the former is more in use. However, in other languages, French and Spanish, for example, lyophilization is the preferred word; technical dictionaries give it a clear, concise meaning: rapid freezing of a material at a very low temperature followed by rapid dehydration by sublimation in a high vacuum.

Vacuum-drying is the process used to dry wetted (non-frozen) materials by evaporation in a vacuum chamber at relatively higher pressures (low vacuum) and at temperatures above the freezing point.

Deep freeze drying requires only a freezer. It is a non-vacuum process which has been described by several names: simply freeze-drying, ambient freeze-drying, cold storage drying, non-vacuum drying. The deep freeze method dries a solid by dehydration. The same laws of physics that govern sublimation apply here also. The process is quite familiar to those who have placed unwrapped food in the freezer compartment of a domestic refrigerator. Over a period of time the food becomes dehydrated; it froze, acquired a vapor pressure higher than the point of coldest temperature (freezing unit), its water vapor molecules were drawn to the freezer unit and trapped there.

Deep freeze as a method of drying is relatively well-known and has bees used by a number of conservators in the past. However, the technique has not caught on perhaps because of the time required to dehydrate the frozen materials. But in a recently published article, Smith (16) describes the use of a commercial upright freezer which was modified to dry 200 to 300 wetted books in two to four weeks (and it can also be used to exterminate insects in books). The model is self-contained, has a vertical glass door freezer, and was chosen for availability of maintenance centers internationally, and its ability to cope adequately with minor disasters in libraries. The modification include the establishment of a temperature differential between book compartment and the evaporator (freezing unit), the elimination of unnecessary heat sources, installation of temperature controls, indicating thermocouple, signal devices and timers.