More of the particles pass through the thinner watermark areas, resulting in greater exposure of the film and a defined image. In 1979, Northwestern University Library's Special Collections and the Newberry Library jointly purchased a carbon-14 radiography source. It has since been used to reveal the secrets of hundreds of maps and other documents from various institutions and private collections. The technology can be used to pinpoint the paper's date and place of manufacture and to compare papers to determine whether they were made sequentially in the same mould or in different moulds with similar watermarks. Digitizing the radiograph and applying various image-enhancement techniques can reveal subtle changes, such as the minute shifting and distortion of a watermark as the craftsman made paper and applied his cleaning brush.
Radiography sources are expensive and require licensing.
While certain safeguards are needed, they are simple to implement. Risk to the user is negligible, and the benefits of efficiency and clarity are significant. The Office of Research Safety oversees the safety and regulatory aspects of this valuable tool and can provide more information about its use.