It was May 1826 and France was celebrating the first anniversary of the coronation of Charles X. French troops had occupied Spain; Mexico had gained its independence and Latin America was in turmoil. But, sitting in his office in the library of the National Assembly, deputy-curator Pierre-Paul Druon was feeling pleased. For the past 30 years this former Benedictine monk had laboured to track down rare works and add them to the 12,000 items inherited from the French Revolution and now entrusted to parliament. Never before had he had the opportunity to acquire such a treasure, even if the source of the Nahuatl manuscript he purchased for 1,300 gold francs at auction was unknown and two of its pages were missing. He was, nevertheless, convinced of its worth.

The document, in its present state, is 14 metres long, comprising 36 fan-folded sheets, each 39 sq cm. It details the cycles of two calendars, one divinatory, the other solar, used by the Aztecs before the Spanish conquest led by Hernán Cortés in 1519. It represents several hundred brightly coloured figures and creatures, each of particular significance.

Here “lords of the night” and “numbers”, accompanied by glyphs, “day signs” and ritual birds, surround the deities who preside over the 20 13-day weeks in the “book of destinies”. There Cipactonal and his wife, Oxomoco, the first couple on Earth, celebrate the festival of the “new fire”, which marks the moment when the two systems coincide, every 52 years. Then come the religious ceremonies associated with the 18 20-day months that make up a year.

Here again, priests wearing flayed human skins, desiccated hands falling loose at their wrists, present sceptres and shields to sacrificial victims who will be skinned to hail new growth. Elsewhere they dance round the ritual xocotl tree, in an end-of-year offering to the flowers.

This extraordinary document, referred to as the Codex Borbonicus in reference to the Palais Bourbon, seat of the lower house of the French parliament, is one of France’s national treasures. It is one of six documents – an original parchment dating from the trial of Joan of Arc, a ninth-century Bible, two Rousseau manuscripts and the Serment du Jeu de Paume (Tennis Court oath) – that have not been allowed out of the country since the 1960s. Does it predate Cortés? Or, as suggested by the catalogue of a 2008 exhibition at Quai Branly, is it a colonial-era manuscript, resulting from the clash between Meso-American and western cultures?

Now, 188 years after the manuscript’s first public appearance, specialists from the Natural History Museum in Paris, the National Centre for Scientific Research (CNRS) and the arts ministry are trying to answer that question.

In May 2013 an “exceptional” decision by MPs authorised the experts to analyse the materials used in the Codex in an attempt to date it. This is a technical and scientific challenge in itself, because the research must be carried out in a strong room under the parliament, at a constant temperature of 18C.

Facebook Twitter Pinterest Detail from the Codex Borbonicus, showing Quetzalcoatl, the mythical Aztec feather serpent. Photograph: Mireille Vautier/Alamy

Interest in the Codex goes beyond conservation. Pre-Columbian documents describing the beliefs and rites of Mesoamerican civilisations, between central Mexico and Costa Rica, are extremely rare. Very few survived the Spanish inquisition. From 1525, in order to speed up conversion of the “Indians”, their temples were demolished and their “idolatrous” books banned. In 1562, for example, 27 “demoniac” documents were burned at Mani, Yucatan, and their owners put to death.

According to historical accounts, when Cortés and his companions entered the Aztec capital Tenochtitlan on 8 November 1519, they found libraries containing thousands of works on many subjects. Now only about 20 pre-Columbian Mesoamerican documents remain. Only five of these, belonging to the Borgia group, are categorically genuine. The majority of them have been shown to be of Mayan or Mixtecan origin. None were produced by the Aztec empire or in its main language, Nahuatl.

What does remain are roughly 500 “colonial” manuscripts, some drafted by tlacuilos, or indigenous scribes, between the 16th and 18th century on the instructions of the authorities in New Spain. The aim was to gain a better understanding of the history and customs of the native peoples the Spanish sought to govern and convert. Some of these documents – such as the Florentine Codex begun in 1547 under the supervision of the Franciscan missionary Bernardino de Sahagún – were designed with the help of scholars familiar with the Nahuatl language and were real encyclopedias. They were packed with information on the Aztecs, the “people of the Sun” who came from the north and for 200 years leading up to 1521 took possession of the Mexican plateau. They took their spiritual lead from Huitzilopochtli (held by some to mean “left-handed hummingbird”), a deity who demanded that his people wage war to provide human sacrifices, thus feeding the sun with blood to sustain its daily journey.

Where does the Codex Borbonicus fit in? The commonly accepted explanation is that the French stole it from the library at the Escorial palace, outside Madrid, when Napoleon’s army invaded Spain in 1808. But there is no proof of this and the year of its purchase by the French parliament, 1826, coincides with unrest in Latin America, which may explain why the manuscript came up for sale in Europe.

Analysis of the Codex’s contents is more helpful. According to several specialists, such as anthropologist Ernest Théodore Hamy who made a facsimile copy in 1899, it is pre-Columbian and may date back to 1507. But other experts maintain that it is a copy of a pre-colonial original. They cite its size – larger than other known codices, which were designed to be portable – the grid pattern on some pages and empty spaces set aside for comments in Spanish. Some go so far as to claim that a particular scene represents the crucifixion. If it is a copy, it would have been produced just after the Spanish conquest, but using traditional techniques. Pointing out the differences in style and colour between the first and second part of the codex, some commentators have suggested that it is a composite work, either finished after the conquest or a marriage of two distinct works.

However, on two points there is consensus: the Codex Borbonicus was the work of Aztecs, perhaps even based in their capital; and it is essential to an understanding of how all the Mesoamerican civilisations of that period represented time.

Jointly funded by the Foundation for Cultural Heritage Sciences (FSP) and France’s National Assembly, the new research programme hopes to find out whether the whole manuscript was produced using traditional methods. If not, it may contain traces of materials imported from Europe by the Spanish. “It is impossible to date the Codex directly,” says Fabien Pottier, a PhD student working on the subject. “We’re interested in knowing whether it was made just before or just after Cortés arrived in 1519, which means a window of between 20 and 40 years at the most. None of the known dating techniques, even using carbon-14, is accurate enough for that.”

Instead, the Patrimex system is being used. There is only one in France and it all fits into a van and is mobile for “analysing monuments or objects which cannot be moved”, says FSP general-secretary Emmanuel Poirault.

Work started on the manuscript at the beginning of September. The Patrimex hyperspectral imaging system operates in a series of spectral bands ranging from the visible to infra-red and will photograph the Codex in 900 bands. With digital processing, researchers should be able to discover the characteristic spectrum of each organic dye and the paper itself, probably amate. This was made with fig-tree bark coated with gypsum. By comparing individual pages, and others made using traditional techniques but artificially aged in a laboratory, the scientists hope to settle the question of European input.

“Of course people could always say that even if they had no part in making the document itself, the Spanish influenced the process simply by being present,” says José Contel, from Toulouse University. Would this be a recognition of failure? “Not at all,” says Elodie Dupey Garcia, a CNRS researcher currently in Mexico City. “In the last few years we have begun to realise that in Mesoamerican civilisations there is a symbolic side to every technique, over and above its practical value,” she explains. “They deliberately used pigments obtained from hard-to-find plants or animals (such as cochineal for scarlet, or Mayan indigo) despite having ready access to a whole range of mineral dyes. Organic pigments have a special texture and brilliance that suited the aesthetic standards of the Aztecs – a beautiful colour should be bright. So confirming that the Codex Borbonicus, the only Aztec manuscript we have that might be pre-Columbian, was really made in the traditional way would be really great news. Much more important than finding out it was produced just before or just after Cortés reached Mexico.”

This article appeared in Guardian Weekly, which incorporates material from Le Monde