The art and science of dating

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

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.

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.

"The assumption in published papers in this field was that prints from later editions of the same copperplate are paler because the enormous pressures in the printing process were flattening the copperplate, but my laboratory studies proved that this is not the case," Hedges says.

"My measurements of copperplate prints from different editions showed that the plates used to make the prints did not become flatter with time." Hedges found clues to the cause in printer's manuals from the 1600s, which instruct printers to scour and polish the copperplate to remove corrosion before each print run.

Over time, cracks develop in the woodblock resulting in line breaks on prints.

With copperplates, erosion of the surface produces thinner groves and thinner lines in prints. A new and relatively simple method for discovering the date when centuries-old art prints and books were produced has been developed at Penn State.

"This study used two independent print clocks and analyses of 23 copies of the book, each containing 112 different prints, for a total of 2,576 woodblock prints," Hedges explains.

"The resulting estimate for the print date of the undated Isolario is mid-February 1565, plus or minus 1.3 years, making it clearly the fourth or last edition." Because the dated editions were unevenly spaced in time, they helped to determine that the deterioration in print quality is related to the relatively constant age-related deterioration of the woodblocks rather than to events during each press run.

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.

Tags: , ,