We all want the best for our manga collection. Keeping your books in good condition is crucial to preserving their value and longevity.
Should I bag my manga?
The purpose of bagging manga is primarily to slow the oxidation process (yellowing), and to prevent from dust, dirt, and staining. All books will yellow with time. It's impossible to prevent this completely - we can only slow down the process. Keeping the pages and covers tightly sleeved in a bag or sleeve also protects against shelf wear, lamination peeling, corner bending, etc.
If bagging your books is going to be helpful or harmful to your collection is dependent on your environment. The two primary aspects that affect book preservation are temperature and light. Where you're located geographically plays a large role in this.
Temperature and climate
Most libraries and archives are kept around 68 degrees or cooler because this is the ideal temperature for paper. Personally I prefer a warmer temperature in my home, so my library is consistently at 72° or above. Somewhere in the range of 68-74° is perfectly fine for your collection. You'll also want to avoid drastic fluctuations in temperature if possible.
Humidity plays the biggest role in book preservation. Paper has moisture, so it's important that your books aren't completely deprived of humidity because the material will become brittle and dry. On the other hand, you're more likely to come across mold and mildew at high humidity (65%+), and even insects that destroy books like silverfish. Even if you don't see any of these, a musty odor can be an indication that your environment is too humid. The recommended range of relative humidity is 40-50%.
Foxing is another kind of yellowing that appears in yellow or brown spots on the edges or flat surface of your books. "There are two major causes of foxing: contamination from metals and from growth of microorganisms such as fungi" (Boothe). You may notice foxing both inside your volumes and along the edges of the pages as well.
In some environments, bagging manga can actually seal in humidity and expedite yellowing. If you live in a very humid environment, a dehumidifier will help tremendously. Silica gel packets can also help to absorb excess moisture (but remember to keep these away from little ones and pets!). Plants release moisture as well, so avoid placing these too close to your collection. If you notice any mold or mildew on your books, don't panic - early action can still help preserve your books. Use a respirator and take a look at some mold and mildew-removing techniques.
Light
Light speeds up the oxidation process of paper, which is what causes it to yellow. Manga is printed on acidic paper, so the most damaging forms of light are those that contain UV rays (such as sunlight or fluorescent lighting). Sun damage and bleaching can occur if books are exposed to direct light for a long period of time, the length of which again depends on your environment.
Storage and bags
The best method for storing your books is upright on a shelf, either packed tightly (not crammed) or standing up with bookends. Avoid leaning your books drastically as this will weaken the spine. Don't place your shelves against a window, and try to avoid putting them against outer walls if at all possible. Try not to place your shelf directly opposite a window either, as the light can cause damage to the spines too.
Metal or sealed wood are the ideal choices for shelving units. "Unsealed wood releases damaging acidic vapors into the environment and can accelerate the deterioration of books" (“Appraisal and Preservation Resources for Books"). If you want to use natural wood shelving, consider a sealant such as linseed oil or a water-based aliphatic urethane topcoat like this one.
As long as your storage environment isn't overly humid, bagging your manga should not cause any damage or accelerate yellowing on your books.
Polypropylene plastic bags are the most commonly used for manga and other comics, and are the most suited for long-term storage (polyethylene and polyester plastics are fine too). These bags offer protection against light and other environmental factors such as fluctuations in temperature. Whatever brand you use, make sure it's acid-free. Keep in mind the effectiveness of these bags is dependent on your environment.
The ideal storage
The best solution for manga preservation and longevity is a dark, cool room with below average relative humidity. This does not mean you should store books in your attic or basement, because "these areas have the greatest fluctuations in temperature and humidity levels and can be very damaging to books," according to “Appraisal and Preservation Resources for Books” (The Art Institute of Chicago). A window-adjacent inner wall is the best choice for your metal or sealed wooden shelves.
I know - that's a lot to ask! This kind of storage just isn't feasible for most of us who just have a couple IKEA bookshelves in our dining room, and that's okay. Being vigilant about the environment and its affect on your books (take pictures to assess changes over time!) will help you figure out what works and what doesn't for your collection. Worst case, you notice your books yellowing and you move your shelf to the other side of the room further from the window.
Unfortunately, there's not much that can be done for your precious volumes that are already showing signs of yellowing. You can sand the edges of the pages with a fine grit sandpaper (400+), but it won't make the book look like it's brand new - it'll only mask the yellowing a bit. The only thing you can do at this point is slow down the oxidation process moving forward.
Yellowing is a completely natural part of owning a collection of books. Don't throw out a volume because of its cosmetic blemish - just give it a read and remember where you bought it and how much you treasure it.
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References
“Appraisal and Preservation Resources for Books.” The Art Institute of Chicago, Accessed 26 July 2023.
“National Preservation Research Agenda for the Human Record.” National Preservation Research Agenda for the Human Record: Scientific Research Focuses of the Library of Congress Preservation Directorate - Preservation Science(Preservation, Library of Congress), www.loc.gov/preservation/scientists/projects/agenda.html. Accessed 26 July 2023.
Boothe, Rory. “Foxing.” A Moment of Science - Indiana Public Media. Accessed 26 July 2023.