The study included historical materials as well as sized and unsized modern papers similar to historical materials. We characterized changes across the wet-dry boundary of each, recorded the migration and deposition of inorganic materials associated with tidelines, and investigated the effects of artificial aging using scanning electron microscopy, scanning and mapping micro X-ray fluorescence (XRF), synchrotron XRF, and high resolution X-ray microcomputed tomography.
Modern model papers were held with their lower edge in water for 15 hours, then lifted and allowed to dry. In the model samples that had tidelines created on them in the laboratory, these tidelines ran across the entire width of the sample and had an approximate width of 0.5 mm. Modern gelatin-sized papers immersed for 15 hours did not form tidelines visible to the naked eye. However, UV light revealed a faint fluorescence at the water immersion line in these samples. This suggests that un-degraded sizing effectively prevents vertical transport of water and thus provides protection against tideline formation. A sample immersed for over 53 hours began showing fluorescence at day 20 of aging, suggesting that longer immersion or aging permits tidelines to form. Aging of the tideline formed on the Whatman No.1 paper produced gradual diffusion of some elements and showed a distinctive multi-zone appearance in UV light after 21 days. Artificial aging of tidelines generally produced darkening from yellow to brown.
Although the historical samples were all sized papers, they had tidelines and new tidelines could be produced on them. This suggests that sizing will naturally break down over time and allow tideline formation.
This project produced visually arresting images of the chemistry and microstructure of tideline regions and new information about the inorganic chemistry associated with tidelines (the distribution of various elements and compounds in the original paper which help explain the reactions taking place). The insights we gained will be useful to the scientific community and conservators, but raise further questions that we hope to answer with future work.
This project was funded through a grant from NPS National Center for Preservation Technology and Training (NCPTT)