A secret lifted at the Prado Museum:

An obscure painting, after restoration in Spain, turns out to be a masterpiece of French primitive art, and the only painting showing Louis I d'Orléans.

"La Oracion en el huerto con el donante Luis Ier de Orleans ["Prayer in the Olive Garden with the Donor Louis Ier of Orleans"] after restoration. The figure of Louis I d'Orléans (bottom left) was covered by a layer of paint on the original painting, as was Saint Agnes, before whom he is kneeling.

Flair, modern technical means, long hours spent in libraries, a passion for enigmas and a sharp mind for synthesis. It took all this, and a little more, to affirm that a painting from a private collection, undated, without an author, and with an unbalanced composition, is in fact a "jewel" of French primitive art and the only painting on wood representing Louis I d'Orléans (1372-1407).

The Prado Museum proudly unveiled La Oracion en el huerto con el donante Luis I de Orleans ["Prayer in the Olive Garden with the Donor Louis I of Orleans"], "one of the most important discoveries of French primitive painting," announced the Madrid institution. The number of French paintings from this period is very small. And very few, if any, reach the quality and importance of this one. Hence its exceptional nature."

Three days earlier, in the Prado's restoration room, amidst the large master paintings awaiting treatment, Pilar Silva was jubilant as she explained her discovery. The head of the Spanish painting department (1100-1500) and a specialist in Flemish painting and the Northern schools, had made a point of previewing her "treasure" to the French press.

Until last year, this painting, which has never been exhibited in any museum, sat in the living room of a modest Madrid family. The painting, which some have been too quick to attribute to the Prague school, is thought to have been produced just a few months before or after the death of the Duke of Orléans, in 1407, assassinated in the streets of Paris on the orders of his cousin and political rival, the Duke of Burgundy, John the Fearless.

There was no reason to believe that the painting, consigned to the Prado in February 2011 for expert appraisal prior to a possible auction, would hold so many surprises. Its owner, "a very elderly lady" who wishes to remain anonymous - we won't know more - has always seen the work in her home. She doesn't know when or how it came into her family, descendants of French settlers in Spain during the Napoleonic era. Was it the crisis that prompted this lady to sell it? Was it to facilitate the division of an inheritance? Or did she want to have the painting appraised to find out more about its artistic and financial value?

Whatever the case, the work immediately caught the attention of Pilar Silva and restorer Maria Antonia Lopez de Asiain. In the upper half, Christ kneels on a hill and prays to the Lord. In the lower half, on the right-hand quarter, are the apostles James, Peter and John. On the left, however, nothing. Or, rather, a mass of earth. A thick layer of brown paint that contrasts with the delicacy of the rest of the work. "I immediately thought there was something under the paint, so we put it under infrared," says Pilar Silva.

To hear her explain her research with an eye for detail, sparkling eyes and rapid speech, you'd think you were in a detective story or a Sir Arthur Conan Doyle novel - the curator resembles actress Angela Lansbury, the amateur detective in the TV series "Arabesque". And her intuition is right.

Thanks to an X-ray of the work, the team from the Prado's technical laboratory discovered that beneath the thick layer of brown paint were two figures: a man kneeling on the ground and a woman protecting him. From there, two studies are launched: the restorer must ensure that it will be possible to "free" this drawing, and the curator must find out who the figures are, hidden from view for centuries.

The female figure is quickly identified as Saint Agnes, recognizable by the sheep accompanying her. The man must be the commissioner of the painting, the one to whom it is dedicated. But who is he? Pilar Silva immerses herself in a documentary and historiographical study. The frame is made of Baltic oak, a noble wood that suggests the donor was a very important person. The dense blue sky, made of lapis lazuli pigment, is another sign of the patron's wealth. His clothes, with wide sleeves covered in gold leaf, correspond to the fashion of the French royal court in the years 1400-1415, when, surrounded by his brother the Duke of Orleans, the mad King Charles VI reigned.

Meanwhile, scientific analysis of the painting yielded some very good news for Maria Antonia Lopez de Asiain. A layer of varnish separated the original painting from the brown overpainting, added probably two centuries later," explains the restorer who worked on the work for a year. This varnish was essential for restoring the original painting without damaging it. It also guaranteed that this part of the painting was in good condition, even better than the rest."

Convinced that it had found a treasure, the Prado Museum decided to purchase the work in May 2012 for 850,000 euros. A good deal, as Gabriele Finaldi, deputy director of the Prado, implied at the official presentation of the painting, recalling that for a larger work from the same period, attributed to Jean Malouel (1370-1415), but less well preserved, the Louvre had paid 7.8 million euros in January 2012.

To discover the identity of the donor, Pilar Silva turns her attention to Saint Agnes. I had an intuition: the donor must have been Louis I d'Orléans, son of Charles V and married to Valentine Visconti, daughter of the Duke of Milan. Through his father and his wife, he had a double reason to be under the protection of Saint Agnes."

Intuition is not enough. No painting of Louis I d'Orléans exists. His face appears only in a few illuminations, which may have been produced by painters who never saw him. After months of research, Pilar Silva finally found proof during a trip to Paris, when she scoured the period books at the Bibliothèque nationale de France. In a miniature, she recognized the particular shape of the leaves on the sleeves of the donor's clothes: nettle leaves.

For her, there is no longer any doubt. Everything points to Louis I d'Orléans as the donor. "He had taken as his motto the nettle, heraldic symbol of the thorn of death, as his political ambitions and disagreements with the Dukes of Burgundy, his uncle Philip the Bold and his cousin John the Fearless, increased."

This solves the mystery of the donor featured on the painting. The question now is who did it? Pilar Silva is convinced that it comes from the Parisian school, as the painting's line, style, rhythm and composition are very different from Burgundian or Flemish paintings. It is "more elegant", the figures are "finer", the folds of the clothes more "delicate". One thing is certain: none of the rare paintings of the period is by the hand that painted this work in Madrid.

So, after investigation, Pilar Silva is leaning towards Colart de Laon, "a recognized painter and valet de chambre to Louis I of Orleans from 1391 until his death". But this time, she remains at the stage of intuition, since no painting by this artist has come down to us. His name is therefore accompanied by a question mark on the label that accompanies the work hanging in the museum.

Other mysteries will undoubtedly remain unanswered. Under what circumstances did the painting arrive in Spain? Why was the figure of Louis I d'Orléans hidden? Was it to make him disappear from history, to kill him a second time, because his presence was a nuisance? Or did the buyers want to accentuate the purity of the religious scene?