The graph shows mean gray-level data for two prints, Cuba and Hispaniola.
In addition to image analysis and conventional statistics, the method also requires at least two images with known print dates made with the same woodblock or copperplate that can be used to calibrate the print clock.
His methods include taking digital photographs of the prints, which he analyzes with standard statistical methods and with widely used image-analysis software.
Working with black-and-white pixels, the software can detect and count breaks in the lines of woodblock prints and can measure fading of the etched and engraved lines of copperplate prints.
"Genetic mutations--like the deterioration of a printer's wood blocks and copperplates--occur sporadically at random intervals, but if you have enough of them, you can get an average rate that is quite useful for dating these very different kinds of materials," Hedges explains.