Lignin is a chemical substance found in many plants that bonds the cellulose fibers, in other words, it's what holds the cells of wood together.  Since lignin is naturally occurring, all paper contains it unless the paper is chemically processed to remove it.  Over time, lignin breaks down into acids that cause the paper to deteriorate and become brittle.  And of course, this would likewise affect any items that come in contact with lignin-bearing paper, like mounted photos or mementos.

What you should do:

• Use only acid and lignin freescrapbook paper.
  
• If you have paper that is not acid or lignin free, treat it withBookkeeper.
  
• Store loose papers inarchival boxesthat are also acid and lignin free.