Mutant Maps

Borrowing an idea from biology to enrich history

In London last month, 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 highly prized volume—which fetched $5.2 million at a Sotheby’s auction on July 13. Were publication dates so easy to come by for some other treasures by the Bard and for thousands of other undated books, maps, etchings, and other printed works, many uncertainties in history and other fields might quickly clear up, scholars say.

AGE LINES. Breaks such as those marked by arrowheads (red) in a 16th-century map of Jamaica (top) mar woodblock prints more often in later editions than in earlier ones. However, prints made from copper plates (bottom) fade from early to late editions, as seen in these ship details from the 1576 edition (left) and 1604 edition (right) of the same Italian map book. A new method for determining the publication dates of undated editions tracks such changes. Hedges and B. Bordone; Hedges and Porcacchi
TIME’S IMPRINT. A carved woodblock (top) dries out and cracks between print runs, resulting in broken lines in subsequent prints. Scouring away corrosion on a copper plate (bottom) wears down the metal and may narrow engraved grooves, causing thinner lines in later prints. Hedges

Hungry for missing publication dates, rare-document specialists scrutinize watermarks, book bindings, and even tiny imperfections in individual letters of old tracts.

“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 in University Park.

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.

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.” His method estimates publication dates with extraordinary precision, he reports, as long as other editions or related documents with known dates are available as guideposts.

There were more than 3 million books printed on hand-operated presses using woodblocks and copper plates from the 1400s to the mid-1800s, and many of those books weren’t dated, Hedges notes. “This method could maybe put a date on many of those books,” he says.

If verified, “his process would be to bibliography as carbon dating is to archaeology,” comments David L. Gants, an old-book scholar at the University of New Brunswick in Fredericton.

Adds R. Carter Hailey, another specialist in old books, at the College of William and Mary in Williamsburg, Va., “Dr. Hedges’ approach, if it can be substantiated, is of immense potential value.”

Wear and tear

Hedges’ work began as a hunch about books that’s based on biology. About a year and a half ago, he 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 lines on the maps. “In the first edition, there were very few. The later-dated editions had progressively more breaks. That’s when I got the hunch,” 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. “It’s the same principle—counting the number of differences,” he notes.

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 copper plates, Hedges observed a different pattern of change. Printmakers created designs on copper plates by etching the metal with acid or engraving it, that is, gouging it with a sharp tool. 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.

Researchers had long thought that it was the printing press that caused the damage to the copper plate. “I realized that was not correct,” Hedges recalls. The pressure of the printing press should make the lines thicker, not thinner.

Instead of the squeezing by the press, Hedges claims, a steady process of corrosion as copper plates 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 etched and 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. While scourings would have occurred episodically, the corrosion would have proceeded at a regular pace.

Indeed, the rate at which corrosion penetrates the copper surface is 1 to 2 micrometers per year, Hedges says. This matches the amount of metal stripped off over time by printers, as indicated by line thinnings, he calculates.

Hedges backs up his observations by pointing out that 17th-century instruction manuals for printers indicate that scouring plates between print runs was common practice.

Not everyone is convinced by Hedges’ proposal. For example, John A. Buchtel, the curator of rare books for libraries of Johns Hopkins University in Baltimore, says that he’s seen prints made from copper plates that visibly changed within the same print run.

Tick tock

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.

Because the numbers of line breaks per print were small, Hedges totaled all the breaks per book—which ranged from about 100 apiece for the 1528 edition to about 300 apiece for the 1547 edition. 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.

The dates estimated by line breaks and by the printer’s mark are similar, and both fall between 1537 and 1570, the period in which previous evaluations by rare-book specialists had placed the undated edition, Hedges notes.

Gants says, “For the first time, we can pinpoint to [a few] months when a wood block has been used. That’s much, much more precise than we’ve been able to do before.”

Buchtel adds that on the basis of historical information about Isolario‘s printer, catalogers at Johns Hopkins had estimated that the undated edition was published between 1561 and 1567. “To have [Hedges’] technique plunk it right down in the middle of that date range … does make his findings seem plausible,” he says.

As one way to confirm his approach to dating copper-plate prints, Hedges used the computerized-image analyses to determine the paleness of the prints from editions of two old map books, L’isole più famose del mondo by Thomas Porcacchi and Geographiae by Giovanni Magini, all of which contained publication dates.

For both books, the fading was proportional to the amount of time between publication dates of six editions of Porcacchi’s book, from 1572 to 1620, and of three of Magini’s, from 1596 to 1621. “That demonstrates that the wear happened with time and not with printing events,” Hedges says.

The Magini editions provided a particularly stringent test of the print-clock hypothesis, Hedges notes. Only 2 years separated the first two editions, whereas 23 years elapsed between the second and third.

In another type of test, Hedges used a computer to compare line widths in high-resolution digital images of Porcacchi’s maps in two editions that were 28 years apart. Measurements at 1,200 specific locations per print in two pairs of early and late prints indicated that line thinning was, in fact, responsible for the paling of the prints.

Hedges describes the new methods in a report, already available online (, that’s slated to appear in the Dec. 8 Proceedings of the Royal Society A.

Hailey says, “The approach does seem to me plausible, though substantial additional data will be required to validate it.”

Terra incognita

The new method is about to foray into the world. Gants say that he’s planning to test its validity. Next year, he’ll investigate whether the method can replicate the known dates of 11 editions of a theological tome printed on copper plates between 1611 and 1723. The analysis will focus on an ornamental border on the book’s title page.

The “next challenge then is to do the same thing on different data sets,” Gants says.

Hedges welcomes independent tests of the approach and is interested in seeing how widely it can be applied. He intends eventually to try it on some undated Rembrandt prints. The artist used etching techniques similar to those employed for book plates. To help others use his approach, Hedges offers a Website full of technical pointers (

Hedges’ next project will focus on Shakespearean texts. These plays appeared in many printed versions, including the 1623 “First Folio,” which sold for a huge sum last month at auction.

Hedges says that he’s made arrangements with the Folger Shakespeare Library in Washington, D.C., to apply his computer analysis of printer’s marks this fall to the only two early books of Shakespeare plays—one of “Romeo and Juliet” and the other of “Hamlet”—that lack publication dates. Those books figure prominently in heated scholarly debates about the development of Shakespeare’s work. Fortunately, the volumes share a printer’s mark with other dated, non-Shakespeare volumes.

“People want to know how [the plays] evolved and why things were added and when it happened,” Hedges notes. His method may soon shed some welcome new light onto the world stage.

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