The Vinland Map -- a drawing that suggests Norse explorers charted North America long before Columbus -- has given scientists and historians a fertile platform for debate throughout its contentious history. Several studies have questioned its authenticity, but disagreement about techniques and interpretations has left some adherents to the map's 15th-century origins unconvinced.
While other evidence has already established the pre-Columbian presence of the Vikings in North America, the map still serves as an important piece of history and has been valued by some at more than $20 million. It resides at the Beinecke Rare Book and Manuscript Library of Yale University.
"The Vinland Map is arguably one of the most important maps in the world," said Robin Clark, D.Sc., Sir William Ramsay Professor of Chemistry at University College London. Clark and Katherine Brown, a doctoral candidate, used Raman microprobe spectroscopy to identify the chemical components in the inks on the Vinland Map.
In this technique, a laser beam is directed at an object; a small portion of the light scatters off the molecules as radiation with different colors. Every material has a unique scattering spectrum that acts as a fingerprint, allowing scientists to identify it.
The ink is made up of two parts: a yellowish line that adheres strongly to the parchment overlaid with a black line that appears to have flaked off.
The yellow line contains anatase -- the least common form of titanium dioxide found in nature. Some scientists have concluded that the map must be of 20th-century origin because anatase could not be synthesized until around 1923. Others have suggested that anatase could have been formed during the medieval production of iron-based inks.
The current study is the first to establish precisely where the anatase is located on the map. The Raman technique allowed the researchers to examine the entire map in place, as opposed to other methods that drew individual samples from the map. "Anatase was detected solely in the ink lines and not elsewhere on the parchment, so [it] must be an integral part of the yellow line," the authors assert in their paper.
Prior to the development of the printing press, manuscripts were generally written in either carbon-based inks or iron gallotannate inks. Erosion of the latter makes the parchment brittle and often leads to brown or yellow staining. "Knowing that such yellowing is a common feature of medieval manuscripts, a clever forger may seek to simulate this degradation by the inclusion of a yellow line in his rendering of the map," the researchers suggested.
The study shows, however, that the black ink is made from carbon, not iron gallotannate, which makes the natural occurrence of yellowing impossible. Also, the map has not grown brittle over the years, as would be expected with an iron gallotannate ink.
"The Raman results provide the first definitive proof that the map itself was drawn after 1923," Clark said. "The results demonstrate the great importance of modern analytical techniques in the study of items in our cultural heritage."