Browning of paper in old book is caused by?

Browning of paper in old books is primarily caused by a process known as acid hydrolysis, which is a chemical reaction that occurs when paper is exposed to acidic substances. Paper typically contains cellulose, a polymer of glucose, which is susceptible to degradation when exposed to acids. The acidic environment breaks down the cellulose chains, leading to the formation of smaller molecules, such as glucose, that can be oxidized to produce brown pigments. These pigments, known as oxycelluloses, are responsible for the characteristic yellowing or browning of aged paper.

Several factors can contribute to the presence of acids in paper, including:

1. Lignin: Paper often contains residual lignin, a natural component of wood that is not completely removed during the papermaking process. Lignin can undergo oxidation in the presence of oxygen and light, producing acids that can accelerate paper degradation.

2. Alum-rosin sizing: A common technique used in papermaking to enhance its resistance to water damage involves the use of alum (aluminum sulfate) and rosin. Over time, the alum-rosin sizing can break down and produce acidic compounds that contribute to paper deterioration.

3. Atmospheric pollutants: Paper can be exposed to various pollutants in the surrounding environment, such as sulfur dioxide and nitrogen oxides, which can react with moisture to form acidic substances.

4. Improper storage conditions: Storing paper in humid or poorly ventilated environments can accelerate acid hydrolysis. High humidity promotes the growth of microorganisms that can produce acids, while inadequate air circulation hinders the dissipation of acidic vapors.

To mitigate the browning of paper in old books, several preservation strategies can be employed, such as:

- Deacidification: A common method involves treating paper with alkaline solutions to neutralize existing acids and prevent further degradation.

- Proper storage conditions: Books should be stored in cool, dry, and well-ventilated environments to minimize exposure to humidity, heat, and pollutants.

- Encapsulation: Enclosing rare or valuable books in protective enclosures can help shield them from environmental factors and handling damage.

- Digital preservation: Converting books into digital formats can provide a means of preserving their content and reducing the handling of fragile originals.

By employing appropriate preservation measures, it is possible to slow down the deterioration process and extend the lifespan of valuable old books.