Elmer Eusman, of the Library of Congress, has shown that tidelines can be produced in pure
cellulose paper (Whatman's filter paper) using
distilled water. His inference was that since paper begins to oxidize as soon as it is made, oxidized parts of the paper (hornification
products)are moved to the edge of the wetted area
of the paper,where they concentrate. He demonstrated that the tidelines that they caused
could be seen, first in UV light and later, as they aged, they darkened and became visible in
ordinary light. Any soluable dirt in the paper or
dirt in the water could, as Rebecca noted, also
contribute to the creation of such lines. Since
any paper can form tidelines, if it is wetted, locally, great care must be taken to keep liquid water away from paper. Solvents applied to some papers can move additives, such as optical brighteners, also causing tidelines, over time.
Hugh