Hedges, who does field work in the Carribbean and happens to collect old maps of the area, conceived of the method after noticing that later editions of the same maps had more line breaks. The flaws exist because printmakers often used the same wood blocks and metal plates for decades and those components deteriorated over time.
He started thinking that the flaws were analogous to the mutations that occur in genetic material. Such mutations do not happen at evenly spaced intervals, but if you can find a lot of them, you can come up with an average rate at which the mutations occur over time. It's called a molecular clock technique. "I'm used to using molecular clocks and counting mutations in genes," Hedges says. "I thought maybe the same principle might apply to this case with prints."
To test his hypothesis, Hedges compiled a database of 2,674 maps from the Renaissance. In some cases, he knew the dates of the pieces; in others, he didn't. Using image analysis software called ImageJ, which is freely available from the National Institutes of Health, Hedges was able to detect and count line breaks and measure faded edges. In prints with known dates, Hedges saw that the number of flaws increased over time.
Knowing this, he was able to develop a simple method for finding the date of mystery print. For example, he said, let's say you have a print with 10 line breaks dated at 1550. Another print from the same printmaker has 20 line breaks and is dated to 1560. The average line break will average to 1 line break per year. If a third, undated print from the same printmaker has 30 line breaks, you can calculate the date to 1570.
Hedges has already used his method to establish a date for a much debated edition of Bordone's Isolario,an atlas containing maps of islands. He also hopes to use the genetics-inspired system on two of Shakespeare's plays as well as several Rembrandt prints.