We’re still not there yet, that is, at a full answer to the question about how this fragment of Fanny Hill was used as covering material. However, we now have a better sense of what the fragment looks like overall. Thanks to the work of Ted Stanley, Special Collections Paper Conservator, Princeton University Library, we now have two images of the printed fragments of “Fanny Hill.” These pictures were obtained by a method called “infared reflectography.” [http://www.clemusart.com/exhibcef/battle/gloss/g4411438.html ] In brief, he used a high quality SLR digital camera with a filter than excludes visible light but passes infared. The CMOS array of the camera is sensitive to the IR end of the spectrum, 830-1100 nanometers. The technique is useful in this case because the printer’s ink has different optical properties from the pigments of the marbling. In other words, the ink absorbs / reflects light differently than marbling paints. This differential is then carried over into an image which is visible, with the ink rendered darker than the pigments.
[More is available on this technique in C. M. Falco, “Invited Article: High resolution digital camera for infrared reflectography,” Review of Scientific Instruments 80, 071301 2009 [link]