freezing books to kill mildew, dust mites, etc.

14 years ago

We have a ~lot of books, and want to treat them this way before shelving them. Has anyone done this? Any suggestions as to time required or anything else? I know I could find this info on a forum for the chemically sensitive, but am asking here first. I really don't want to keep up with one more forum!

Comments (11)

  • housekeeping
    14 years ago
    last modified: 6 years ago

    OK, if there's one thing I've spent a lot of time doing it's treating books for problems, including the dreaded mildew.

    One thing that I know is freezing does nothing for mildew. If it did the books I am currently bringing in from dry, but below 0F storage would be in tip top shape, and it just ain't so, alas.

    There is some research that shows that many of the environmental things that we do to try and treat a mold infestation can actually prod resting spores to come alive and get busy.

    Fungi have been with us forever, and I believe they will outlast us.

    But here's what you can do for books with some mildew issues.

    First, you have to accept that bringing books with mildew into your house will put all your other books at somewhat higher risk. It's a trade-ff, and a difficult one to make. Some people won't have it; some people quarantine the mildewed ones, permanently. My choice is to do what I can to clean them very well and then accept that I will have to monitor not only them but all the others and proactively manage my indoor environment as best I can, forever.

    Here's how to get them cleaned up:

    Mildew suppression and removal can be considered something of an emergency, so if you are going to do something about it, it must be started at once. (When I have had to deal with an outbreak, all other household work stops, and DH resigns himself to sandwiches until it's done. It can be hard, intense work if you have a big problem.)

    First, they must have the humidity lowered before being cleaned in order to avoid rubbing the active mildew colonies into the crevices on the surfaces. This is most easily done using a dehumidifier in a detached, closed room. (The dehu will have to thoroughly cleaned afterward.) Lowering the humidity to 40% would be a good target if you can do it in a day or so. Next best for drying the books is out of doors on a dry, breezy day away from your house and open windows to your house. Fast changes in humidity are hard on books and can create problems of their own, but mildew removal is the first, most urgent, concern.

    After the books are dry, or at least surface dry, take barely damp, cut-up squares of paper towels and brushing in only one direction (away from your face and away from the spine), lightly brush the mildew or efflorences off. The purpose of this step is to do a quick removal of the gross contamination and arrest any further development. Be prepared to use a lot of towels and try not to wipe "cleaner areas" with a soiled section.

    Another way to do this is to vacuum the books with a clean upholstery brush, BUT, this has the potential to seriously contaminate your vacuum and then spew book-mildew spores out every time you use it ever afterward. I have solved this by having a special, dedicated, true-HEPA museum vacuum that I only use on books, and can disassemble completely for cleaning. I live with nearly 7,000 books so it's an important cleaning tool for me, but probably not so important for others. Simple hand wiping, however, will do as good a job, and leave your vacuum safe. If you do use a vac, you will need to use it carefully to avoid damaging the books or sucking loose pieces of pages or the dust jackets in. (BTW, remove any dust jackets and treat them separately.) It's also a good idea to wash any brushes and dry them completely between days of work, if it extends over more than one day.

    But you're not done now. You still need to expose the books to sunlight for a few hours. Of course sunlight can fade bindings, so this has to be controlled carefully. But a few hours of good strong sun will do wonders to kill many of the remaining spores that you can't see. After the sunbath, and still working outside on trestle tables covered with clean sheets, I go on to the next phase of the cleaning.

    What I do next is to wipe the covers down with grain alcohol, which will kill even more of the remaining spores, without in my experience, doing much or any damage. Obviously, each book has to be carefully tested to make sure the alcohol won't damage the covers, but I rarely find it does. I know some people have used isopropyl (rubbing alcohol), but I've had many more bleeding and damage problems with that than with grain alcohol. I get grain alcohol (which is not for consumer sale in NY), by going across state lines and purchasing it from Vermont State Liquor Stores. (I have been told that in some states in can be bought in ordinary liquor stores or in pharmacies.) Even there it has become more difficult to get because it is so damaging to drink. I have to sign a statement that I am using it for scientific or experimental purposes; I just state I am using it to clean books. I got the idea of using the grain alcohol from studying both the solinet and palimpsest preservation boards. They suggested using cut-up squares of paper towels saturated in grain alcohol and kept in zip-lock bags. My process is to slice a roll of Select-a-size Bounty in half or thirds and then use the pieces. I have a small container with a siphoning laboratory top that allows me to just tap and draw up the alcohol. I wipe the entire cover surface (from spine to edge) discarding the little towels as soon as they look dirty. (I wind up standing in a little snow storm of squares of discarded towel if I'm doing a big project outdoors. The alcohol-saturated towels must be thoroughly air-dried before being discarded in the trash can in order to avoid spontaneous combustion, so I just drop them to the ground and then collect them in batches.) After I've done the spine and covers, I open the book and do the turn-over of the cover, and sometimes the end paper and free fly leaf in the gutter.

    Then I stand the book upright on a clean surface to air and finish drying while I go on to the next one. I work carefully so that I am cleaning and dusting down wind, and have a freshly sheeted table upwind for the ones that have been wiped. It's not the same as a hospital operating field, but the principle is the same: pick-up from table with "dirty" books, clean in the air holding the book , then set it down on the clean surface. After the first table has been done, I pack the books loosely in clean liquor boxes and bring them indoors to a room where I continue to dehu them down to very, very, dry. I keep the clean liquor boxes shut-up away from the cleaning operation until needed so as not to be putting cleaned books in boxes that have been exposed to air-borne spores disturbed in the wiping operations. I make sure to wash my hands and change clothes before handling the cleaned books.

    The purpose of this multi-step process (drying, physical removal, UV exposure, and alcohol wiping) is to use a series of suppressant techniques to increase the effectiveness of the "kill", kind of like using a broad-spectrum antibiotic when you're sick. You want to demoralize as many different resistant beasties as you can. It is a lot of work, but when the other option is to discard the books, I prefer it.

    After your books are clean, and have had additional drying then I would re-inspect and see if they seem OK for being shelved with the others. If in doubt keep them separate for a season and see how they do.

    But having had a mildew problem you are now more or less obliged to keep indoor humidity under rigid control forever more. If you don't, you risk the problem coming back, and worse, further extending itself. Pay attention to places within your house that are relatively more or less affected by increased ambient humidity and make decisions about where to store books based on those factors. I keep my most valuable books stored on the second floor because it stays dryer during the hot summer months. I have a couple of dozen of those small max-min hygrometers and I check on them every day to monitor conditions. I do not have a/c, which would make this problem much less onerous.

    As for the various book-eating and book-burrowing pests, freezing might destroy them and if I found an active infestation, that would be my first choice. Being in a freezer with food would hard on books, however. Perhaps you could ship them up to Gloria and she could expose them Alaska's natural freezing?

    Finally make sure you can tell the difference between mildew and foxing on old books. While there are many theories about the origin of foxing, and it is exacerbated by excess humidity, it is not the same as mildew, nor would it be improved by sunning and alcohol wipes. It would be a shame to go to all this trouble in a vain attempt to eliminate foxing, which is more or less permanent on books.

    Are you dealing with any leather-bound books (partially or fully), the process has some similarities, and they have the same, or increased, risk of mildew, but the cleaning procedure and treatment are different.

    I hope I have given you some useful suggestions. I have never had any "chemical" concerns using grain alcohol, which after all, is considered "potable" in some states. One would of course not want to breathe too much of it in a closed room, nor smoke while using it!

    Long-term control of mildew on books is affected by decreasing humidity, avoiding stagnant air (say shelves behind a sofa), avoiding darkness (though this is a two-edged sword due to the risk of fading) and eliminating mold food. Of course molds adore paper pulp and starches (AKA pages and covers), but mold can be somewhat discouraged by keeping the books well-dusted and free from the organic residues often found in a house (soot, grease, smoke, spider droppings,etc.) I regularly go over my books - well actually more or less constantly since I have so many it takes months to clean each one, once. For some reason well-dusted books seem have fewer problems, or maybe I'm just cleaning it off in an earlier, less visible, stage. Either way, I find scrupulous cleaning keeps me from having to deal with outbreaks. So when you are done putting down the mildew-attack, consider going over all the books, page by page to get them completely in good order. There is nothing more satisfying than a clean, sweet-smelling old book.

    And speaking of sweet-smelling, even books "cured" of mildew may have a lingering, musty odor problem. I can add some additional info if that's an issue for you, though the airing and sunning will make a big difference all by itself.

    I can also make some suggestions for books about cleaning and managing a library, if you'd like.

    As you may have gathered, I have faced this problem more than few times. I have had good success with these techniques and hope you will, too.


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  • jannie
    14 years ago
    last modified: 6 years ago

    A few years ago, I head of a museum book restorer who put old books and manuscripts in a hyperbaric oxygen chamber and got rid of mold and mildew that way. It would be very expensive, and where does one find a hyperbaric chamber?

  • talley_sue_nyc
    14 years ago
    last modified: 6 years ago

    here's one! never used, fully functional, designed for field use and so very portable. Russian markings. Not FDA cleared (not a problem for this use) and not serviced by anyone in the U.S. --$17,000

    That's a steal--the other ones i found were $70,000.

  • housekeeping
    14 years ago
    last modified: 6 years ago

    I have read about experiments (both home-grown and professional) and research on the use of ozone chambers as a remedy for mildew on books. I am not aware of any workable system, as yet. Ozone in the concentrations and quantities needed for the task poses some significant health risks and is said to accelerate the degradation of paper.

    For those who want to delve deeper into this topic I recommend the book Fungal Facts. The author is an authority on dealing with mildew challenges in libraries and collections. I should warn you, though, that it can be discouraging reading if one is hoping for a quick, easy and permanent fix. I'll attach the link below.


    Here is a link that might be useful: Scholarly book on the care of libraries and collections affected by mildew

  • liz_h
    Original Author
    14 years ago
    last modified: 6 years ago

    What a lot of info! Thanks everyone.

    Gloria - thanks for the links. I learned a lot.

    Molly - thank you so much for the detailed explanation of your procedures. Again, I learned a lot. I can't imagine doing all that on an ongoing basis! I hope I don't have any leather bound books with actual mildew.

    I've read the info and am very glad that I'm not dealing with any visible mildew - at least not yet. I have 2 large groups of books - those that were packed from my old house and those that have been in storage. My old house had central air and heat. We often ran the central air in the spring just to deal with the humidity. A few of these books have a sort of fusty smell, but dust mites are probably my main concern. Very few of these books have much monetary value, though some have sentimental value.

    The new house has central heat and A/C, and the humidity level is kept at 55%. In addition, we have some very heavy duty whole house air cleaning equipment. This is primarily for the people, but is nice for the books as well. Nearly all of the books will be behind glass doors. From what I've just read, it sounds like the books should stay fairly stable once shelved here.

    A few years ago, as we were planning this house, I developed the plan of deep freezing all of the books before shelving. I think I thought this would kill molds as well as dust mites. I don't even remember where I got the idea now. I do remember one online acquaintance who bought used paperbacks and baked them before reading. It helped her allergies, though not the books!

    If I read the articles correctly, freezing would be "fungistatic" that is, it would not kill any molds or mildew, but would make them go dormant. Once dormant, they shouldn't spread to other books, or to my room air if stored under the above listed conditions. Any mold or mildew could still be problematic for someone reading the book, if that person has allergies or respiratory problems. Does this sound right?

    Also, does anyone know if freezing the books would kill any resident dust mites?

  • housekeeping
    14 years ago
    last modified: 6 years ago

    No, the fungistatic effect would only last as long as the books remained frozen. Dormancy is not a permanent state; it would be reversed when the temps returned to normal.

    And according to some research I have read, (which I touched on above) fungistatic treatments (including freezing, which would only be mildly fungistatic anyway, and only for some low-temperature susceptible fungi) can result in a paradoxical "awakening" of otherwise resting (dormant) spores. Fungi being being different from vascular plants and other life forms can (and maybe must, I don't know) go into a period of resting in which the spores almost cease metabolic activity, but remain viable for very long periods. This is probably why they are so successful in surviving conditions that would otherwise be lethal to them. But when conditions return to what they normally like, there can be a surge of resting spores which are jumpstarted back to life, whereas if the conditions had stayed consistently favorable, the resting spores might have stayed resting.

    So I doubt freezing would be successful in eliminating, or controling a potential outbreak, or even be advisable under the conditions you describe. (And freezing will have its own cost to the books themselves, including promoting foxing, and possibliy inadvertently raising their humidity, which carries its own risks.)

    Now as to dust mites, I don't think dust mites are much of a problem on books (except perhaps books stored on beds and carpeting). It's my understanding that dust mites live primarily on shed proteinaceous cells that have fallen off of mammals, mostly humans and pets, though to some degree also on particles of wool and down used in bedding and clothing. I don't think dust mites would find books particularly tasty, with the possible exception of leather-bound books. I think dust mites' common name comes from their habitat, not their diet. They live in the dust, they don't eat dusty things; simple vacuuming would remove any that had inadvertently got on your books.

    Liz, I was a little confused about one your comments: I didn't give you instructins for cleaning leather-bound books, my instructions above are for hardcover and paperbacks. Leather books need somewhat different treatments.

    From your description, here's what I would do with the books as they come out of storage: pick up the sequence I described above at the airing point. Give the books a few hours of sun outside on a breezy day, along with an inspection and then a careful vacuuming before bringing them in and shelving them. Set aside any that show any visible signs of mildew for further treatment. An airing and general cleanup will take away the "fusty" smell very effectively.

    Even though your books were living in a safe place before, the conditions in storage may have been more challenging. You won't know until you unbox them, which since this is a new house I would definitely do out of doors to avoid bringing anything inside.

    You are correct that books stored behind glass tend to do better than open-shelved books, but that's only if they go into the glass completely free of any spores. Glass cabinets can pose a problem in that they automatically provide one of the key needs of a fungal outbreak: still, unmoving air. But if the other components aren't there, and if the books aren't seriously contaminated when installed, you should be OK. But glass cabinets won't make a mildew, or incipient mildew, outbreak better, just preserve the status quo.

    When vaccuming books, it's usually advised that you put a layer of thin nylon nesh on the end of the wand before connecting the brush. This will serve to trap any pieces of paper that break off during the process. (It will also clog faster, and have to be cleaned; you might want to start with a small pile of pre-cut pieces of netting and just remove and discard them rather than try to clean them.) After removing the dust jacket, hold the book tightly closed in one hand and vacuum the covers and spine. Then with book still tightly held closed run the vaccumm over the top edge from spine to fore edge, down the foredge and then from spine to front along the bottom edge. If you think it's necessary, you can easily flip the book open and do the end papers, front and back, paying close attention to the gutter. With a little practice you can learn to manipulate the books with one hand while holding the vac wand in the other. But it is important to hold the book tightly closed while cleaning the covers and edges, or you will simply knock dirt down onto the pages inside.

    I'll pass along one final tip on managing the books out of doors. I use temporary tables that I set up in this fashion: For each table I used to two sturdy plastic garbage cans (with tops removed); I lay a lightwieght hollow-core door on top and cover it with as many as eight or ten flat sheets (twin size is best) folded once lengthwise, laid one on top of the other. Normally I have a clean table and a "dirty table". I set the first batch of books on one table on the top of the sheets, and do whatever stage I'm working on. For general airing I might lay them flat first, then turn them over, and finally upright, slightly open. When I'm ready to vacuum or wipe I pick one up and handle it, then put it down on the surface of the clean table, one book after the other until all the books have been transeferred to the second table. Then, going back to the now-empty dirty table, I fold the top surface of the top folded sheet back and tuck it under the door, exposing a new untouched-by-any-book surface and reload it with books just starting the process. While they're airing, I pack up the cleaned books or carry them away, etc. When that's done, I open the sheet that covers the "second" table, gaining another clean surface, and so on. If I have a particulary bad group of books, I can quickly remove a couple of layers of the sheets until I get down to one I feel is not contaminated. The folded sheet technique is one I developed over the last few years. I have used paper, and wipeable surfaces, but find the sheets work best and they have the added advantage that at the end of the day I can just launder them in very hot water and they're ready for the next day's work.

    Since you have described your books as smelling "fusty", I would definitely recommend you do, at least, a sun-and-air step, followed by a thorough dusting. Fusty smells are usually low grade, sort of sub-visible, mildew problems. Unaffected books don't have any bad smell to them; they just smell like dry paper. Chances are that these simple steps will completely reverse, and improve their condition remarkably.

    You said you had a lot of books - a lot like I have, nearly 7K? Airing and vacumming all of them would take a week or more, depending on weather. But for most people a lot is a few to several hundreds, which is something that could be done in a day. Or maybe you're one of the lucky people who have 12-30K volumes - now that would be a big chore!

    Have fun!


  • minnie_tx
    14 years ago
    last modified: 6 years ago

    I don't know about anyone else but I'm delighted to find a book on the internet the whole thing. I'm getting rid of a lot of books. DS is selling his on EBAY he just sold a Dore for $200. Some of course you don't want to part with.

  • minnie_tx
    14 years ago
    last modified: 6 years ago

    I'm thrilled to find a book available to read - in its entirety - on the Internet.

  • gloss
    14 years ago
    last modified: 6 years ago

    I've had good results using chlorine dioxide to kill mold in my apartment. I haven't specifically treated my books with it, but that is how I first learned about it.

    The original article I read seems to have a broken link, but I believe it was this:

    There is also this:

    You can get a form of chlorine dioxide release agent from West Marine. It's called Starbrite Mildew Control Bags...

    The fogging referred to in the links above uses an aqueous solution. While I don't recommend fogging, since adding moisture is never a good idea, using an ageous solution that boils off chlorine dioxide gas has been a good treatment technique for me.

  • SouthernJan
    10 years ago
    last modified: 6 years ago

    This message is for Molly who posted her advice on 3/23 & 25/2007.

    Dear Molly,
    I have a severe problem with mold on several 100 leather books with gold tooling that were packed away for several years after Hurricane Katrina in Louisiana. Thank you for your advice on Village Garden Web. I have a couple questions which I hope you may have investigated or have more knowledge.

    Purdue U. says "Place books in a closed container with moth crystals to stop mold growth." What do you think?

    I have read alot about machines that kill mold that can be used for large areas. I am interested in one on Ebay titled Commercial Ozone Generator kills Mold Ebay Item No.Item number: 370286856402. This Seller has a separate web page,, which I have read extensively. I would greatly appreciate your opinion on this type of mold removal machine using ozone, since I have house mold, as well as book mold. My personal email is

    Hope to hear back from you or anyone who reads this and knows how to get my message to you.

    Jan in Mold Trouble