Freeze drying, or vacuum freeze drying--controlled drying in a sub-freezing vacuum--is coning to be the preferred method for wet books, because the water sublimes or goes straight from the solid phase (ice) to the vapor phase. In books that were only partly soaked before freezing, the moisture is never given a chance to spread, so swelling and other types of water damage are minimized. Coated paper dries without sticking together.
Vacuum drying, which is cheaper and more widely used, is a wet process. Like freeze drying, it has many variations, and can be done well or poorly. It is all right for business records, but not ideal for books because it exacerbates the problems associated with water. Books, unlike loose records, tend to warp and cockle as they dry, because they are restrained along one edge, and made of denser materials. They often contain coated paper which fuses into a block if not dried from the solid phase. Water-soluble adhesives and colors continue to spread during vacuum drying (just as they do during air drying) as long as enough moisture is present to carry them.
People who have wet books and want to freeze dry them find themselves facing four obstacles: finding someone who can do it, communicating with them, seeing that they do it right, and paying for it.
There are only four facilities in this country where large-scale freeze drying of books has been done: General
Electric Space Division in Valley Forge, Pennsylvania; Lock-hoed Missiles and Space Division in Sunnyvale, California; NASA in Houston, Texas; and Document Reprocessors of San Francisco. (BMS Cat, whose services were described in the June issue of AN on p. 34, has only a small freeze drying facility so far.) the method is not standardized or routine yet, even among these four.
The librarian, in dealing with smaller facilities, may find that work methods and word usage carried over from the food industry or related applications can interfere with communication. But freeze drying of green beans is not the sane as freeze drying of books. With books, the temperature must stay below freezing (about -20°F) all the time. It is better to break the concept down and talk about whether or not the material is thawed at any point during the drying process, and what temperature and pressure are used, and whether these conditions are varied at all as the books dry.
Freeze drying is not easy. Success depends just as much on the contractor's knowledge, experience and willingness to communicate as it does on his physical facilities. The contractor who knows how to do it right will be able to describe the method he uses to tell when the books are dry. This may involve weighing the books en masse at intervals, planting sensors inside or between selected books, or some other method. (If this is not done, the books may develop a delayed case of mold, which may not be detected for many months after they are returned to the library.) the contractor will know what the moisture content of paper ought to be (5%-10%), and how long it takes for it to equilibrate with the relative humidity of the surrounding air. He will appreciate the importance of keeping the books in order and keeping track of them (maintaining bibliographic control). He will handle them gently, especially if they have arrived in a nonfrozen state, and will not treat them like a bulk commodity. He will be frank and open about methods and facilities used, and willing to drop any method to which the librarian objects. He shows some familiarity with the librarian's concerns and is able to admit him or her to the decision-making process in critical decisions. It may be necessary for the librarian to travel to the drying facility in order to check on all these things.
As for cost, freeze drying costs the same or a little more than vacuum drying. If the superior results are taken into account, it is much cheaper: the cost of repairing the mechanical damage caused by swelling in vacuum dried books is greater than the difference in the prices of the two methods.
Some freeze dried books look entirely normal, except to the trained eye. Of course, if they were misshapen or mishandled when they went into the chamber, they will come out looking misshapen. the quality of the salvage and rehabilitation job rests partly on the handling and packing job done before the books go to the freezer.
Although there have been a number of articles published about how different libraries dried their books after a disaster, there has been very little research on freeze drying as such. There are signs now, though that this picture is changing as a result of so many conservators having been brought together by the Los Angeles Public Library disaster. It may have the same sort of effect, on a lesser scale, as the Florence Flood did in 1966: a stimulation of the field that will increase the number of books saved for the future.