Hyperspectral Imaging Shines Light on a Mesoamerican Treasure

Detail of a page of the Codex Selden, as revealed using hyperspectral imaging. [Image: Copyright © Journal of Archaeological Sciences: Reports, 2016, Elsevier; courtesy of Bodleian Libraries, University of Oxford, U.K.]

For more than 50 years, the Codex Selden, a rare Mesoamerican manuscript dating from around 1560, has frustrated scientific inquiry. Archaeologists have long known that the accordion-folded, 20-page leather codex likely contains a rich pictographic record of Mexico's culture and history, as seen by its indigenous Mixtec peoples during the upheavals of the earliest colonial period. But that original indigenous record has subsequently been completely covered by a layer of white gesso (gypsum and chalk). And recovering the original images without damaging the precious artifact itself has proved all but impossible.

Now, a research team from the Netherlands and the United Kingdom has used a thoroughly modern optical technique, hyperspectral imaging, to noninvasively burrow through the gesso and reveal the scenes beneath (J. Archeol. Sci.: Reports, doi: 10.1016/j.jasrep.2016.07.019). While much work remains to be done, the effort has already yielded some surprises.

A stubborn subject

Archaeologists first realized that the Codex Selden was a palimpsest—an original manuscript that has been subsequently covered—in the late 1950s, when several researchers attempted physically remove the gesso on one of the codex pages to reveal the writing beneath. The effort proved only marginally successful, however, and since then scientists have searched for ways to get at the original writing noninvasively.

The codex, however, has stubbornly resisted most techniques commonly used to investigate artworks of this kind. X-ray analysis, the most common technique, is useless in this case, because the organic paints used in the original pictographic writing don't absorb X-rays. Infrared imaging has revealed only limited information, as has another technique, photothermal tomography (which measures the thermal energy produced when colors absorb light).

Trying out hyperspectral imaging

Netherlands researchers Ludo Snijders of Leiden University and Tim Zaman of Delft University, along with David Howell of the Bodleian Library, University of Oxford, U.K., decided to take another crack at the Codex Selden. To do so, they zeroed in on hyperspectral imaging, which uses a combination of high-resolution, pixel-by-pixel spectroscopy and high-speed, intense data crunching to pull out detailed information on a given scene. Originally designed for military surveillance, the method has increasingly been applied in recent years in other spheres—from food safety to airport security—as costs, especially for data processing, have dropped (see "Hyperspectral Imaging for Safety and Security," OPN, October 2015).

The researchers began with one of the pages of the codex that had already been partly physically uncovered during the initial work in the 1950s. Scanning that page across the visible spectrum using a hyperspectral-imaging setup that had been recently acquired by the Bodleian Library, the team next applied principal component analysis and independent component analysis to reduce the huge volume of hyperspectral data into an interpretable image. Red pigments showed up particularly strongly; the hyperspectral technique was also able to tease out yellow pigments, which had previously been difficult to discern at all, as they tended to blend in with the gesso color.

Striking new view

The team was able to put the data from the hyperspectral imaging together with the previous information from optical photography, infrared photography and other techniques to form a striking new view of the original codex drawings (see image below). One of the researchers, archaeologist Ludo Snijders, noted in a press release that the text revealed by the new method "doesn't match that of other early Mixtec manuscripts. The genealogy we see appears to be unique.

Pages of the Codex Selden on which the white gesso layer had been partly removed in a previous study (top) were hyperspectrally imaged. The hyperspectral data were combined with information from several other methods to reveal, noninvasively, the original pictographic scenes hidden under the gesso covering (bottom). [Image: Copyright © Journal of Archaeological Sciences: Reports, 2016, Elsevier; courtesy of Bodleian Libraries, University of Oxford, U.K.] [Enlarge image]

The researchers are using the hyperspectral results from the previously partly uncovered page as a sort of "style guide" for interpreting results from the rest of the document, where the gesso covering is more complete and the challenges of uncovering the palimpsest more formidable. Revealing the full text will likely be a long and difficult process, even with the hyperspectral addition to the toolkit.

Nonetheless, the results have been sufficient to excite the research community on the codex's potential to illuminate Mixtec history. And coauthor David Howell of the Bodleian Library is bullish on hyperspectral for other applications as well. "This is very much a new technique," he says, "and we've learned valuable lessons about how to use hyperspectral imaging in the future both for this very fragile manuscript and for countless others like it."

Source: Hyperspectral Imaging Shines Light on a Mesoamerican Treasure