Passage II

SOCIAL SCIENCE: This passage is adapted from the article “Mutant Maps” by Peter Weiss (©2006 by Science Service).

The numerals 1623 stared out from the title page of a rare Shakespeare book. Printed in a quaint typeface, those digits left no mystery about the publication date of that prized volume.

“People want to know when ideas were developed. In the history of society in general, dates are important,” says S. Blair Hedges of Pennsylvania State University.

An unexpected inspiration from genetics recently led Hedges—a biologist with a penchant for old books and maps—to develop a new way to sleuth missing dates for printed works.

His work indicates that the print quality declines with the steady aging of the blocks and plates used in the printing process, not with how often they are used, as most specialists had suspected. As biologists calculate species ages from the accumulation of genetic mutations, or molecular clocks, Hedges employs a “print clock.”

There were more than 3 million books printed on hand-operated presses using woodblocks and copperplates from the 1400s to the mid-1800s, and many of those books weren’t dated, Hedges notes.

About a year and a half ago, Hedges was examining copies of a 16th-century book of Caribbean-island maps printed from woodblocks. Printers from this era would often use the same woodblock or plate in each subsequent edition of a book.

This book—known as Isolario by Benedetto Bordone—appeared in four editions: three dated 1528, 1534, and 1547, and one without a date. For 200 years, rare-book specialists have been debating when the undated edition was printed.

As Hedges looked at the various editions, patterns of defects in the maps caught his eye. He noticed gaps here and there in the lines on the maps. “In the first edition, there were very few. The later-dated editions had progressively more breaks.” Hedges recalls, “I thought these line breaks are kind of like genetic mutations.”

In organisms, the genetic code changes haphazardly, or mutates, at random intervals as a result of chemical reactions or other insults, and the resulting mutations in DNA accumulate over time. Biologists can calculate the average rate of mutation over millions of years of evolutionary transformation. They can then use that average to estimate when one species diverged from others.

As Hedges pored over the Isolario, it occurred to him that random defects in wood blocks—like genetic mutations—may also have accumulated over the years at a constant average rate. If so, then bibliographers could exploit that rate to date documents relative to those with known publication dates.

Hedges knew that line breaks in old wood-block prints resulted from cracks in the blocks’ raised ridges, which produce the lines. The underlying cause—the drying of the wood—would have taken place steadily over time.

In prints made from copperplates, Hedges observed a different pattern of change. Printmakers created designs on copperplates by engraving them. Hedges noticed that the lines of prints from a specific plate are typically thinner in later editions than in earlier ones, so the overall image in later editions appears faded.

Hedges claims a steady process of corrosion as copperplates sat in storage could account for the observed decline in print quality. When the time came to reuse a plate, the printer would have had to scrub and polish its pitted surface. Because engraved grooves narrow with depth, stripping off the top layers of the plate’s surface would have made such grooves narrower and, therefore, the lines of the prints thinner.

Hedges says that his tests so far support the print-clock hypothesis. He found 23 copies of Isolario, including all the editions, in rare-book libraries. He then counted the number of line breaks in the 112 map prints contained in each book. When he analyzed the data, he found that a steady accumulation rate of line breaks per year matched up with the publication dates of the three successive dated editions.

He next turned to the undated copy of Isolario. That edition had more line breaks than any of the others. Hedges calculated that it was published in February 1565, plus or minus 16 months. Conveniently, another characteristic of the undated Isolario edition—a small, intricate pattern on its title page, called the printer’s mark—gave Hedges an independent way to estimate that date. He took advantage of computerized-image-analysis methods that are used by medical scientists. He applied them to high-resolution digital images of printer’s marks from Isolario and other dated books from the same Renaissance printer. His second estimate for the undated Isolario was April 1565, plus or minus a year.