If you have placed some of your favorite books from a decade ago on the shelf and picked one of them up to reread a favorite passage, you may notice that the pages are no longer white, but have turned an unattractive yellow or even brownish color. Since these are books that were in very good condition when they were placed on the shelf, it is distressing that at best they would be classified as acceptable, now. In the interest of preserving the value of the book, you may wonder if the yellowing can be removed and the book restored to its previous condition.
If you want to restore the pristine condition of a book that has yellowed, you have two options. You could apply an expensive and delicate process of bleaching individual pages by immersing them in a chemical solution. Or, you could take steps to prevent the yellowing in the first place.
Unfortunately, cheap paper cannot be restored. Because of the source of the paper, and the manufacturing process, the paper structure will yellow with time. The best approach to avoid yellowing, or at least to slow down the process is to be vigorous in providing the best storage, lighting, handling, and humidity conditions, as well as good airflow.
How to protect books from yellowing
The best way to protect against yellowing is to prevent it from happening. When the paper is cheap, you probably won’t be able to prevent any yellowing, but you can certainly delay the onset through careful storage measures, including avoidance of temperature fluctuations, high humidity, exterior wall placement, and direct sunlight. It is also helpful to ensure that you have good air circulation. For even more protection, slip sheets of acid-free paper between the pages of valuable volumes.
Avoid high humidity and temperature fluctuations
Controlling the temperature and relative humidity is vital to the preservation of archives and library collections. Levels of these two variables can contribute to the deterioration of materials. Heat speeds up the natural deterioration of paper–in fact chemical reactions such as deterioration approximately double with an increase in temperature of just 18°F (10°C).
High relative humidity supplies the necessary moisture to accelerate harmful chemical reactions within materials. In combination with high temperature, it encourages insect activity and mold growth. Extremely low relative humidity, which can occur in winter in centrally heated buildings, may lead to desiccation and embrittlement of some materials. The range of humidity should be 30 to 50 percent, while a good temperature reaches no higher than 70°F.
Avoid sunlight and other illumination
Light speeds up yellowing of library and archival materials. Light weakens or embrittles the cellulose fibers which form the major component of paper, causing the paper to yellow, bleach, or darken. Any exposure to light, even for a short time, can cause cumulative and irreversible damage.
For many years, the accepted recommendation of visible light levels was set at 55 lux for light sensitive materials, and less sensitive materials to as much as 165 lux. Today, these recommendations are under debate. All wavelengths of light are damaging, but ultraviolet radiation is particularly harmful to archival and library materials. High amounts of UV energy are exuded by the sun, tungsten-halogen, quartz, mercury, metal halide high intensity lamps and fluorescent lights.
Lights should be kept as low as possible and used for the least amount of time. When not in use, archival materials should be stored in a light-tight container or a windowless room, and LED bulbs are preferable to incandescent lights. Covering any windows with blinds or shades will aid in temperature control, and prevent sunlight from falling on the books.
Obviously, meeting all of these standards is difficult or impossible to accomplish in most private homes. These methods are targets to extend the life of your books. For truly valuable books, enhanced storage measures can be utilized. Books that are not exposed to oxygen or light will have an extended life.
Pollutants of two major types have major effects on books. Gaseous contaminants include ozone, peroxide, nitrogen oxides, and sulfur dioxides. These bring on harmful reactions that result in the formation of acid. Paper and leather are especially vulnerable to acidic damage. Paper becomes brittle and discolored (yellowed), and leather becomes weak and powdery. Particulate contaminants such as soot, dust, and spores can abrade and disfigure materials.
Controlling air quality is challenging and depends on various interrelated factors. The most reasonable recommendation given the status of technology, knowledge, and environmental factors is that pollutants in the air be limited as much as is practical.
Gaseous contaminants can be removed by wet scrubbers and/or chemical filters. Particulates can be filtered mechanically. Electrostatic precipitators which produce ozone should not be used. The equipment should be linked to the needs of the building and the level of air pollution in the surroundings. Regular maintenance and filter replacement is vital.
The role of off-gassing
The atmosphere is both good and bad for your books. There is a process called ‘off-gassing’. Off-gassing is in its simplest form the process of giving off harmful chemicals in the form of gas. Books and wooden bookshelves go through the process of ‘off-gassing’ quite a lot. The gasses which are released into the room air are not good for your books because these gasses accelerate the yellowing process. Make sure that there is good air circulation in the room where you keep your bookcases.
Off-gassing from the wood in bookcases can also cause yellowing of pages and books. You can protect books from these harmful gasses by placing protective acid free lining paper on the shelves where the books reside. Lining paper serves as a barrier between the books and the wood, which can help to keep your books safe from turning that yellow tinge. If you can fix a fan in the direction of the bookcase, it will be extremely effective in keeping your books safe from turning yellow.
How to reverse yellowing
Reversing yellowing in your home library is difficult to impossible, largely because of the expensive equipment and supplies which are required to affect the coloration. Bleaching is the least complicated process, but is still a series of steps and special handling techniques that can have detrimental results to the paper, as well as to the ink on it.
Expensive and rare books can be treated with a costly and delicate process of bleaching individual pages by immersing them in a chemical solution. This process only works if the paper and ink are both suitable for such treatment. Due to the expense, difficulty, and uncertainty of these procedures, they are typically unsuitable for casual collections.
Paper deterioration is still a problem, but thanks to decades of research by the library community and beyond, it is no longer a mystery. The preservation strategies for paper materials at locations such as the Library of Congress continue to evolve as the scientific understanding of deterioration mechanisms has progressed.
Why do book pages turn yellow?
Books which are past their prime, old newspaper clippings, and aging paper documents are often characterized by a yellow tinge. This change in coloration is due to the fact that paper is made from components which turn yellow over time when they are exposed to oxygen.
Cellulose and Lignin
Most paper today is manufactured from wood, which is mainly formed by cellulose and a component called lignin. Lignin, a complex polymer, enhances the rigidity of the cells of trees and other plants. Cellulose, the other major component of wood, provides the majority of the strength of plant cells.
Cellulose is colorless and is excellent at reflecting light. To our eyes, it is perceived as being white, while lignin is a dark brown color, with some variation due to pH levels. Wood pulp is naturally a shade of brown, and would create brown paper, but most commercial paper in modern times is bleached to produce a white result–at least temporarily.
Lignin which is exposed to air and light undergoes a change in its molecular structure. Lignins are built from repeating molecular units consisting of alcohols(oxygen, hydrogen, and a few carbon atoms). Lignin (and to some extent cellulose) is susceptible to picking up extra oxygen molecules, which change the structure.
The addition of oxygen molecules breaks up the bonds holding the alcohol units together, creating areas in the molecules called chromophores. These regions reflect wavelengths of light that are perceived as yellow or brown in the case of oxidation of the lignin.
Manufacturers of paper try to remove as much lignin as possible during the manufacturing process. The bleaching process varies in effectiveness. The more lignin that is taken out, the longer the paper will appear white. Newspaper, a cheap form of paper, typically contains more lignin than a typical textbook, so it turns that annoying yellow-brown color more quickly than more expensive paper.
How long does it take for a book to turn yellow?
Books made with high lignin content tend to become yellow very quickly. Newspaper is another paper product which deteriorates and turns yellow within a matter of months or years. Under normal usage, books with average lignin content will begin to change color noticeably within five to six years.
Older books, especially those made with better quality paper, can last much longer, sometimes hundreds or even thousands of years. Some Chinese papers are nearing two thousand years of age. The oldest dated printed book in the world is the Diamond Sutra. It is still readable on a gray paper scroll and is dated from 868 AD.
Enjoying your book collection
Unlike costly archival volumes which can be national treasures, your home library cannot be housed in light-free and oxygen-free environments. Using common sense storage and usage methods will lengthen the lifespan on your favorite volumes, and perhaps allow them to be passed down to future generations.