After 18 months of meticulous restoration, Leonardo da Vinci’s Virgin of the Rocks went back on display in London’s National Gallery Wednesday. The painting has been coated in a layer of varnish in 1948 (yeah, go figure) which had become badly discolored, tinting the masterpiece with a yellowish wash. The varnish layer was also cracked and had absorbed dust and dirt, obscuring the subtlety and depth of the design.

Conservators removed the cracked and yellowed varnish, but left a very thin layer so as to protect the top surface of the paint. The change in color is noticeable but not a huge night-and-day alteration. The colors are more saturated, and you can see a lot more detail in the dark areas.

By removing the varnish, restorers revealed not only fresh details but also were able to identify more areas that were likely painted by Leonardo’s hand than they expected. There’s another Virgin of the Rocks in the Louvre that was made earlier, you see, between 1483-1486. That one was thought to be mostly the work of Leonardo himself, whereas the National Gallery version was painted considerably later (some time before 1508) and although it was attributed to the master, because of this dating it was considered to have been primarily painted by his assistants.

The conservation work and study of materials and techniques uncovered different parts of the painting reached different stages of completion – the angel’s hand was barely sketched while the heads of the main figures appear completely finished, the gallery says.

“In the past, gallery curators, like many scholars of Renaissance painting elsewhere, have explained the different levels of finish and resolution in the picture by arguing that Leonardo was helped by assistants,” the gallery said.

“It now seems possible that Leonardo painted all the picture himself, leaving some parts just sketched or yet to be completely resolved and others fully worked up.”

The original commission for the painting was made in 1483 (the commission papers are still extant), and he finished the first version fairly quickly, but he never actually sold it to the people who commissioned it. It was supposed to be central panel of a carved altarpiece at the oratory of the Confraternity of the Immaculate Conception abutting the Church of San Francesco Grande in Milan. But when the confraternity didn’t pay him what he expected (there was some dispute about how dark and unsettling the imagery in the final work was) , Leonardo sold it privately for more money, probably to Ludovico Sforza, the ruler of Milan.

The National Gallery’s version was begun 10 years later and is believed to have been made once the artist and the confraternity paid up and Leonardo made some changes to the composition. He never quite finished the second iteration, but it was close enough for the confraternity. The Virgin of the Rocks was installed in the oratory in 1508. It remained there until 1785 when the confraternity was suppressed by order of Tuscany’s Grand Duke Leopold I of Habsburg-Lorraine. Its assets, including the Leonardo da Vinci masterpiece, were sold off and the church was demolished in 1806. The Virgin was sold to Scots artist and dealer Gavin Hamilton. After passing through assorted titled hands, was bought by the National Gallery in 1880.