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He found that the number of breaks in the lines of images printed from woodblock carvings increased over time, while the image intensity became more pale in copperplate prints.

"Because woodblocks and copperplates were expensive to replace, they commonly were reused for decades to produce multiple editions of a book or print," Hedges says.

Image courtesy of Blair Hedges, Penn State Woodblock print of Jamaica showing changes over time.

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"For copperplates, the image fading I measured is the result of the thinning of the etched and engraved lines caused by the erosion of the copper surface, which results from the steady corrosive effects of acid in the atmosphere plus the periodic removal of the accumulated corrosion by scouring and polishing of the plates prior to each print run." His analyses indicate that the rate of this deterioration of copper plates is 1 to 2 micrometers per year, which agrees surprisingly well with known rates for the atmospheric corrosion of copper. Close-ups of a small section (1/2 inch wide) of prints from two editions of an Italian Renaissance book by Porcacchi illustrate time-dependent image fading, useful for dating books and prints.All editions of both books were printed with the original copperplates and were unevenly spaced in time, again providing a means of confirming that changes in print quality resulted from the passage of time rather than from the number of prints made with the plates.Image courtesy of Blair Hedges, Penn State Generalized diagram showing change in width of an engraved triangular-shaped groove in a metal (copper) plate through time (four editions), and the printed lines produced by that groove.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.The method could reveal long-sought information about thousands of undated works printed on hand-operated presses prior to the development of modern printing methods in the mid-19th century, including works by Rembrandt and Shakespeare.

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