Molds are fungi and obtain nutrients from organic compounds such as cellulose. Library materials are composed largely of cellulose (paper and cloth) and will support mold growth if the right conditions are present. Moisture provides the necessary conditions for mold germination. It can sometimes be difficult to distinguish mold from dust and dirt. Mold can be almost any color, and often has a musty odor, whether active or dormant. The pictures on this page show some examples of mold from materials sent to the Preservation Unit.

Any book with evidence of mold should be put in a resealable plastic bag and brought to the attention of library staff. Contact Circulation (314-935-5420) or Preservation (314-935-4287) if you suspect a library book has mold.

Foxing is a condition found in some older books. These brown-red stains are circular or irregular in shape. The origins of the marks are not yet fully understood (e.g. fungi? oxidation?) but they do sometimes occur. They tend to be distributed more evenly over the page, whereas mold tends to form on either the outer edges of the page or near the spine. We do not consider them as "mold" in the strict sense. Books with foxing ordinarily do not need to be sent to Preservation.

Mold can form in 48 hours. Active mold is fuzzy, damp, slimy, and smears. Inactive mold is dry and powdery. Mold can begin to form if a water or liquid is spilled onto a book and the book remains wet.