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16. An open forum on freeze-drying
16.1 Questions and answers
In this imaginary open forum, the questions posed (to which answers are provided) were taken from a number of actual conferences on conservation of books and documents, seminars on disaster preparedness, and panels discussions on freeze-drying. The "Questions and Answers" have been placed in this study to expand on a number of points related to freeze-drying which may have been touched upon or briefly mentioned in the text.
16.1.1 In simple terms, what is the difference between freeze-drying and plain vacuum-drying?
Actually, the difference is in the amount of pressure developed by the pumping system. With the capacity required to reach levels of relatively high vacuum, freeze-drying becomes possible. The frozen materials in the chamber are sublimated; they pass from water as a solid to water as a vapor without going through the liquid stage. In vacuum-drying, the pumping system produces lower values of vacuum. Here, the materials are placed in the chamber in their wet condition and are evaporated, that is, water as a liquid passes to water as a vapor. In freeze-drying, the water vapor is normally trapped by a refrigerated condenser; in vacuum-drying the water vapor can be trapped by this method or others such as steam ejection.
16.1.2 Is freeze-drying expensive?
Yes and no. Freeze-drying becomes expensive when the vacuum chamber used for drying is large and sophisticated, and the amount of water-damaged materials is small. If the quantity is relatively large, freeze-drying becomes inexpensive when you calculate in terms of time, personnel, space, materials and sap- plies required to the dry the materials by traditional methods. Freeze-drying in a smaller vacuum chamber is, of course, less costly although capacity is limited and drying takes longer. In either case, if it appears that your materials will require expensive restoration, but can be replaced by some other format, there is little to gain by freeze-drying.
16.1.3 What about the cost of vacuum-drying?
Vacuum-drying is somewhat less expensive than freeze-drying; one reason is that you do not freeze wetted materials beforehand; the chamber is less sophisticated. However, what is gained in economy is lost in what freeze-drying supplies: stabilization, time, the advantages of sublimation over evaporation. The conditions of costs that apply to freeze-drying - large quantities of materials in large chambers are less expensive - also apply to vacuum drying.
16.1.4 Can wet (non-frozen) materials be freeze-dried, and conversely, can frozen materials be vacuum-dried?
Yes, to both parts of the question. However, when wet (non-frozen) materials are placed in a freeze-dry chamber, as the initial water "rushes" off, they will freeze. When this happens there is a possibility that bound books may come apart and heavy calendered paper puff or split. It is best to freeze the wet materials first. In regard to frozen materials placed in a vacuum-dry chamber, during the drying cycle the ice will melt, become liquid, and carry the risk of possible migration of colors, stains, and inks (62).
16.1.5 Which is the better system, freeze-drying or vacuum-drying?
Remember that in freeze-drying the materials are frozen first, then sublimated in a freeze-dry chamber. In vacuum-drying the materials are placed in the chamber in a wet condition for drying by evaporation. In general terms, the tendency seems to favor vacuum-drying for office records when there is no imminent danger of mold attack and a chamber is available immediately. On the other hand, for books and their special structure, freezing followed by sublimation is the preferred method. Books printed on coated stock cannot be salvaged by the evaporation process; sublimation in a freeze-dry chamber is the only way.
16.1.6 Shouldn't time be taken to sort or weed out wet materials prior to wrapping and crating for freezing?
A moot question. Much depends on the situation and a variety of factors: Amount and importance of the items, degree of damage, climatic conditions, help available, time, and so on A case in point, where the research value of water-damaged materials outweighed a decision to weed out in order to save time and money, is the action taken at the Meyer Library flood, Stanford University (4). Involved were some 50,000 volumes of a general research collection in humanities and social science. The materials had been collected for ten years as part of a collection development program that was a prime resource for faculty and students; their value weighed heavily in the decision not to sort or weed out.
In another case, water-damaged items in a library basement after a fire were exposed to conditions favorable for rapid mold attack. So, some 30,000 volumes and other vulnerable materials were frozen and placed in cold storage without sorting or weeding out (31). Later, it was found that some items could have been replaced at a fraction of the cost. However, this wee not considered during recovery operations because of the prevailing conditions.
16.1.7 Shouldn't wet materials be cleaned of soilage prior to freezing?
It would be desirable, but not it requires a loss of precious time and risk of mold infection to organize crews and setup cleaning arrangements. However, there will be occasions when there is no choice but to go ahead with some cleaning. Each case will have to be Judged by its own merits.
16.1.8 What is the best way to wrap wet materials for freezing?
The primary reason for wrapping wet materials is to keep them from freezing into large, unwieldy blocks of ice. Some people use freezer paper on books or bundles as if it were arm sling; DO need to fold over. In other cases, if the wet materials are placed in boxes or crates, a few at a time, and will not be removed for either the freezing or freeze-dry operation, there is no need to wrap or separate with freezer paper.
16.1.9 At what temperature should water-damaged materials be frozen and stored?
Freezing and storage temperatures which have been used vary from about -20 °C (-4 °F) in a cheat freezer for a few soaked books to -30°C (-22 °F) in cold storage for larger quantities of wetted materials. Waters (5) recommends a freezing temperature of -20 °F (-29°C); lower temperatures will do no harm. In Stanford University's recovery operation, that temperature was used in a cold storage facility (63).
16.1.10 Vet books and documents undoubtedly swell and expend with freezing. doesn't that cause damage?
Saturated materials do swell even more on freezing but this additional thickness does not add to the damage already canoed by water. In studies conducted by the Research and Testing Office of the Library of Congress, there was DO evidence found that freezing causes damage of cellulosic and proteinaceous materials (5).
16.1.11 Do you deed a special chamber for-freeze-drying books and documents?
No. Generally, the design, size and original purpose of the chamber makes DO difference. Huge vacuum chambers made to test gear and equipment used in space programs have successfully freeze-dried large quantities of water-damaged books and documents. Small chambers, such as those used by taxidermists, can freeze-dry the same materials; the quantity is smaller because the capacity of the chamber is much leas.
16.1.12 Bow do you know when the materials in a chamber are dry?
Methods for determining the level of dryness vary. When heating strips are used, temperature probes are inserted in several of the frozen materials ant the temperature is plotted. As the dry state of the materials is reached, the temperature curve moves to the temperature of the heater strips (63). Another method is to plot weight lose. The frozen item is weighed before it is placed in the vacuum chamber; at several intervals of time during the drying cycle the test sample is weighed. A state of dryness is reached when the weight lose reaches a plateau (36).
The Aquaboy, a book probe manufactured by K.P.Mundiger, Wartenburg, West Germany, is a useful instrument for measuring water content of paper. This probe has two metal prongs, resembling a tuning fork, that are placed net on the exposed page of a book; the book is then closed. A dial on an instrument to which the prongs are attached registers the absolute humidity inside the book (35). A reading of five to seven percent indicates normal moisture content. Note that this percentage is Dot "relative humidify" as some people think. In technical Jargon, the moisture content of paper is called "absolutes humidity" which is the actual weight of water vapor in a unit of moist air in grams per cubic centimeter.
And a final method, though not very accurate, is to open the chamber from time to time and touch the materials to test for dryness.
16.1.13 Is there a risk of overdrying and, as a consequence, run the risk of damage to the materials?
Yes, to both parts of the question. Books that come out of a vacuum chamber in this condition, which is usually the case, must not be opened and closed because the pages and structure is so fragile. Documents must also be handled with care The solution to this problem is to get some moisture back into the paper. This can be done by placing the dry materials in a room with normal conditions of relative humidity; about 55 to 60 percent. It can take up to three or four weeks for the paper to equilibrate with the atmosphere.
One research center solves the overdrying problem with a series of humidity chambers of varying humidities and temperatures for the introduction of moisture into the books. Pressure. is applied to reach the approximate water content they had originally (13).
Paper is like a sponge. Water content under normal conditions is between five and seven percent by weight. However, this level varies proportionally with the amount of relative humidity in the air. For example, at 80 percent relative humidity, paper absorbs between 9 and 14 percent water (leather absorbs 18 to 20 percent); at 65 percent the various parts of a book can vary 6 to 9.5 percent of water content (6).
16.1.14 Can parchment and leather be freeze-dried?
Both can be stabilized by freezing then vacuum freeze-dried. However, at some point, particularly with rare or valuable items, the advice of an experienced conservator should be sought. This is especially important where parchment is concerned since this material is altered structurally when wet. The best moment for seeking advice may be after stabilization of the materials, but Dot after the freeze-dry cycle. Only an expert should carry out the recovery work.
In relation to the question, French conservation scientist conducted some freeze-drying tests under laboratory conditions on paper, leather, and parchment. These were immersed in water, frozen, then freeze-dried in a laboratory vacuum chamber. One of the observations made was that freezing can be used as a method of storage for the three materials. The results of freeze-drying each material follows: The technique is wholly indicated for drying paper. [Bather can be freeze-dried without great risk. On the other hand, parchment poses distinct problems since water causes internal changes that result in thickening and a decrease in elasticity. The scientists point out that these changes are reversible. After freeze-drying, the parchment sampler were placed in the hands of expert restorers at the Bibliotheque Nationale who used moisture and pressing to recuperate suppleness, color and thickness (64).
16.1.15 In the freeze-drying process, heat is sometimes applied to the frozen materials. first of all, why is it done and doesn't the heat harm the materials?
Beat is applied to the frozen materials in order to hasten the sublimation of the ice; it is normally done when large quantities of materials must be dried. The temperatures applied by flexible rubber heaters on shelves in the chamber are Dot excessively high. In one case (32), the temperature starts out at 32 °F (O °C) on books and will not get warmer than 80 to 85 °F (26.5 to 29.4 °C). In another case (14), with frozen materials heated by aluminum foil heaters, the book temperature wag never higher than 100°F (37.8°C). One research center uses 40 °C (104°F) as the maximum shelf temperature for documents.
16.1.16 Once the materials come out of the freeze-drying chamber can they go straight back to their shelves?
The reply must be qualified. In the first place, the materials that come out of the chamber might be overdried and require rehydrations to a normal level before they can be handled safely. Secondly, some materials may have gone into the chamber in a damaged condition; they will come out the same way and require repairs or restoration. Thirdly, the stacks where the materials were water-damaged may Dot be in any condition to receive them and may need rehabilitation.
An ideal situation would be to take the materials to a place where they can return to normal humidity content, where they can be pressed, repaired, or restored, and be under constant vigilance for mold infection (some books and documents come out of the chamber partially dry).
16.1.17 Would it not be less costly to replace water-damaged materials than freeze-dry?
Generally, yes. It books, for example, have been soaked in water to the point where they hare swelled, split their component parts, warped their boards, it would be much cheaper to replace, if such can be done, than to restore.
16.1.18 It is generally knows that freeze-drying will not destroy mold spores, but what about insects?
Actually, freezing is deadly for some destructive insects. An infestation of beetles, the Anobiidae, at Yale University's Beincke Library, was controlled by freezing the affected book's. The university biologist who recommended this method had been using it to kill insects, their eggs, and larvae for exhibits and laboratory purposes. As far as known, the method had Dot been used for infested library books (65).
16.1.19 How do you go about freezing insects?
Simply put the infested material in a freezer. At Tale, tests were first run in domestic freezers. When they proved successful, a 368 cubic foot (10..3 cubic meters) blast freezer (a type of rapid freezing model was obtained in which the infested books were frozen at -30 °C (-22 °F). First, the books were sealed in polyethylene plastic and put in the freezer for 72 hours. They were taken out and permitted to dry overnight. The only moisture noted was on the outside of the plastic envelope (65).
16.1.20 Why weren't the infested books fumigated?
They could have been. Nearly 40,000 volumes were involved. Tale personnel opted for the cleaner, safer, although slower method of freezing (the freezer had capacity for about 400 books at a time). Fumigation would have required that the entire building be vacated, sealed, and be left that way for several days. Spraying wee ruled out because of possible harm to the books; vacuum fumigation was also considered, but also ruled out because of high costs (65).
16.1.21 What should you do ii mold is widespread before the wet materials can be frozen?
Fumigation is the fires thing that comes to mind. However, there is no clear-cut answer at the moment; opinions vary. The controversy centers On health hazards, and the long-term effects of the toxic products used (ethylene oxide, among others) to destroy effectively fungal growth and spores (also insects and eggs). And for deep, penetrating destruction a vacuum chamber, fixed or portable, must be used. Many countries do not have these sophisticated machines, nor the professionals required for the use of fumigants that are highly toxic to people.
Nevertheless, there are lower scaled, albeit less effective, steps recommended in the literature: fogging with thymol crystals dissolved in a suitable solvent for use in specific cases. For example, when the damage is extensive in order to destroy spores already present, and prevent growth of new ones (3). Or, in cases of large collections where access to them has not bees possible for several days, the same fogging technique is recommended (5). Because of health hazards, fogging should be done by a professional fumigator.
But what action can you take ii fumigation and fogging facilities are Dot immediately available? There seems to be agreement among a number of conservators (based on informal talks) that in such a case, and faced with widespread growth on wet materials, they would do the following: First, not try to brush or rub off the growth; it will leave a difficult-to-remove smear. Next, prepare the materials for freezing to stabilize them, then, at the appropriate moment, freeze-dry. At the end of the drying cycle, the mold can be brushed off (use masks for people; brush outdoors) with no problem.
To take the problem a step further, the same conservators were asked to recommend a course of action if neither fumigation or freezing facilities (vacuum chambers included) were available. They were unanimous in suggesting that the wet materials be removed from their damp venue and placed in a cool, dry place for air-drying in the traditional manner. They would use any and all electrical materials possible to speed up drying and maintain strict control of the environment. Their final recommendation is, at some point, i! at all possible, to fog or fumigate.
16.1.22 Can mold-infected materials be fumigated in the same chamber where freeze-drying takes place?
Yes A professional who knows the safety and health requirements should do the Job since the fumigant used is toxic, and most places have strict air pollution standards when the moment for venting take e place.
Waters (5) suggests, as an additional preventive measure, that after sterilization the chamber be fogged with a 12 percent solution of thymol crystals in trichloroethylene. This treatment is a fungicidal butter of temporary duration' but offers a high degree of resistance to further attack by mold.
16.1.23 It freeze-drying seems to destroy the visible growth caused by mold spores, why is it 80 necessary to sterilize and fog with a buffers? Why not put the dry materials in proper storage?
For what it is worth the following case is cited. After the Military Records Center fire in St. Louis, Missouri, further recovery work wee done with materials that had remained damp for four months. Mold had inevitably set in. However, instead of fumigating the infected materials, they were placed in the vacuum chamber facilities of the McDonnell-Douglas Space System Laboratory. The drying treatment arrested the development of the mold; the materials were then placed in an air conditioned environment rather than bear the cost of sterilization (39).
16.1.24 Some institutions hare photographic materials in their holdings. can they be frozen and freeze-dried?
Generally speaking, yes. However, the recommended procedure for recovery of water-damaged photographic material is to leave their treatment to a professional laboratory (some, for example, Eastman Kodak hare emergency services for that purpose). There are several sources that describe the methods used to stabilize the materials temporarily for transport to the nearest laboratory (66,67,68).
Briefly, the materials are placed in ridded plastic pails which are filled with clean, cold water, The materials (black and white photographs and film) last in water about 72 hours before the emulsion separates from its support. Color materials are handled the same way except that the color layers will separate in some 48 hours.
Leaving the recommended procedure aside for the moment, there are cases on record where photographic materials were frozen then freeze-dried with satisfactory results. For example, after the Corning Museum Library flood, several thousand books were prepared for freezing and subsequent freeze-drying. During this operation many films, negatives, photographs, and slides were inadvertently packed along with the books. After the drying process, the photographic materials were discovered and found to be in better shape than the materials that volunteers tried to recover locally (30).
Another case. During the recovery operations incident to the Stanford Meyer Library flood, samples of microcards along with silver halide photographs, diazo and vesicular microfilm, and some computer tapes, were immersed in water for two hours then frozen at -15 °F (-26 °C) for 48 hours. The samples were then freeze-dried. The microcard disintegrate hated but the film itself was undamaged; the computer tape recovered well; the microfilm had minor spotting but no loss in text; the photographs were slightly cockled but easily pressed; cinema film in a roll in its cardboard container recovered well (4).
Hendricks and Lesser (66) did some tests on still photographic negatives and prints in black and white (and color), which along with other types were immersed in water then dried by various methods. Their preferred method of drying, if personnel and time are available, is first, drying by air, then freeze-thaw-dry by air, lastly freeze-drying in a vacuum chamber. They stated that wet collodion glass plate negatives must never be freeze-dried; none will survive.
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