Researchers from the University of Oxford's Bodleian Libraries and from universities in the Netherlands have used high-tech imaging to uncover the details of a rare Mexican codex dating from before the colonization of the Americas. The newly revealed codex, or book, has been hidden from view for almost 500 years, concealed beneath a layer of plaster and chalk on the back of a later manuscript known as the Codex Selden, which is housed at the Bodleian Libraries. Scientists have used hyperspectral imaging to reveal pictographic scenes from this remarkable document and have published their findings in the Journal of Archaeological Science: Reports.

Ancient Mexican codices are some of the most important artefacts of early Mexican culture and are particularly rare. Codex Selden, also known as Codex Añute, dates from around 1560 and is one of fewer than 20 known Mexican codices to have survived from pre-colonial and early colonial Mexico. Of those, it is one of only five surviving manuscripts from the Mixtec area, now the Oaxaca region of Mexico. These codices use a complex system of pictures, symbols, and bright colors to narrate centuries of conquering dynasties and genealogies as well as wars and the history of ancient cities. In essence, these codices provide the best insight into the history and culture of early Mexico.

Since the 1950s, scholars have suspected that Codex Selden is a palimpsest: an older document that has been covered up and reused to make the manuscript that is currently visible. Codex Selden consists of a five-meter-long strip composed of deer hide that has been covered with gesso, a white plaster made from gypsum and chalk, and folded in a concertina format into a 20-page document. The manuscript underwent a series of invasive tests in the 1950s when one page on the back was scraped, uncovering a vague image that hinted at the possibility that an earlier Mexican codex lay hidden beneath.

Until now, no other technique has been able to unveil the concealed narrative in a non-invasive way. The organic paints that were partly used to create the vibrant images on early Mexican codices do not absorb X-rays, which rules out the X-ray analysis that is commonly used to study later works of art.

“After four or five years of trying different techniques, we've been able to reveal an abundance of images without damaging this extremely vulnerable item. We can confirm that Codex Selden is indeed a palimpsest,” said Ludo Snijders from Leiden University, who conducted the research with David Howell from the Bodleian Libraries and Tim Zaman from the University of Delft. This is the first time an early Mexican codex has been proven to be a palimpsest.