From the fifteenth to the nineteenth centuries, prints were a common form of visual communication, analogous to
photographs. Copperplate prints have many finely engraved black lines which were used to create the illusion of
continuous tone. Line densities generally are 100-2000 lines per square centimeter and a print can contain more than a
million total engraved lines 20-300 micrometers in width. Because hundreds to thousands of prints were made from a
single copperplate over decades, variation among prints can have historical value. The largest variation is plate-related,
which is the thinning of lines over successive editions as a result of plate polishing to remove time-accumulated
corrosion. Thinning can be quantified with image analysis and used to date undated prints and books containing prints.
Print-related variation, such as over-inking of the print, is a smaller but significant source. Image-related variation can
introduce bias if images were differentially illuminated or not in focus, but improved imaging technology can limit this
variation. The Print Index, the percentage of an area composed of lines, is proposed as a primary measure of variation.
Statistical methods also are proposed for comparing and identifying prints in the context of a print database.