|Vacuum Freeze-Drying, a Method Used to Salvage Water-Damaged Archival and Library Materials: A RAMP Study with Guidelines (UNESCO, 1987)|
|5. Alternate methods of drying|
The difference between vacuum freeze-drying and vacuum-drying is, in reality, a matter of pumps. In a freeze-dry system the pump has the capacity to pull a vacuum of 4mm Hg and less; at temperatures below 0°C (32 °F) water as a solid (ice) can, under certain conditions of pressure and temperature, pees to water as a vapor (sublimation). On the other hand, in vacuum-drying the pump has the capacity of pulling a vacuum of 5mm Hg and above; at temperatures greater than O °C water as a liquid passes to water as a vapor (evaporation). A graphic representation of this phenomenon can be seen if an imaginary line is drawn in Figure 1 to represent the levels of pressures and temperatures cited.
As we have seen, the boiling point of water as a liquid can be lowered if the air pressure is decreased. The reduced boiling point at low pressure finds considerable practical application in the field of vacuum evaporation (evaporation under low pressure). This process is of primary importance in the auger industry. Boiling off the water from the syrup at normal atmospheric pressure would char the sugar. However, the pressure is kept so low that the water may be removed at rather low temperatures.
In the aerospace industry, thermal-vacuum chambers are used to test the behavior of spacecraft such as weather satellites under simulated space conditions. Such chambers, rather large in capacity, are very effective for drying large quantities of wetted (not frozen) archival materials. In one such chamber there is a purged air system, a method where vacuum pressure at 45mm Hg is applied for about one and a half hours. Then the chamber is purged with dry air of less than 1 percent humidity; the air passes over heaters at a temperature of 60 °C (140°F). This operation continues for about two and a half days then the vacuum is applied again. The cycle lasts for about five days. Steam injection is used to eliminate evaporation from the wetted materials (15). Other methods of eliminating or trapping are available, for example, through the use of refrigerated condensers.
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.
Natural freeze-drying is mentioned briefly as a matter of interest and to avoid confusion with the other methods in use for drying. It is a technique for a specific purpose and is limited to those parts of the worldwhere winter climates prevail et temperatures below freezing. Note how similar it is to the other methods already described.
Gratten and McCawley (17) write of the method; they call it "exterior freeze-drying'. in which waterlogged wood is freeze-dried without the need of a vacuum chamber. They take advantage of Canadian winter: the low temperature, the daily hours of sunshine to offset the latent heat of sublimation, and the wind conditions to help remove the water-saturated air from the frozen object. A tent-like housing with polyethylene walls protects the object undergoing treatment from undesired exterior conditions.
Conceivably, frozen documents and books could be dried under the same conditions as waterlogged wood. The process would be quite slow, but it would still be a form of freeze-drying "au nature!."