Today Late Turner: Painting Set Free, a major exhibition exploring the late paintings of JMW Turner, opens at Tate Britain. More than 180 works have been brought together to debunk some of the myths surrounding Turner’s work during his final years, which was dismissed by his contemporaries as the product of someone senile or mentally ill.

“The myth is that Turner’s mind and hand increasingly failed him, especially after 1845, that his work declined and he deliberately withdrew from active engagement with the public and critics,” the exhibition’s co-curator Sam Smiles explained to the Guardian. “This exhibition will demonstrate that this is very far from the truth.”

Among the most controversial works in the exhibition are nine paintings on square supports, gathered for the first time and displayed as a show within a show. One of the most captivating works in the group is Peace—Burial at Sea(1842), which depicts the interment of fellow artist Sir David Wilkie on a ship, with black brooding sails like a modern Charon’s boat.

At the time of their creation, Turner’s square canvases were panned by the press. Even the critic John Ruskin, a devoted fan and champion of Turner’s earlier work, said that the square works shown in 1846 were “indicative of mental disease.”

The curators of the show make a compelling case that these were in fact Turner’s most radical years, in the positive sense. By the end of his career, the British painter exhibited less frequently, received fewer commissions, and retreated into his work, which afforded him the freedom to experiment. With their melancholy themes and somber tones, Turner’s later works are now being presented as the result of an aging artist coming to terms with mortality and doing so through artistic experimentation.

Interestingly, exhibition co-curator David Blayney Brown also challenges some modern readings of Turner’s later production. In a recent interview with the magazine Apollo he argued: “[Turner’s later work] is loved today but not always understood. For example, assumptions tend to be made that Turner was a proto-abstract or Impressionist painter, but these often arise from studio or unfinished work that he never expected people to see.”

This large exhibition might open up more questions than offer answers. Of the 180 exhibited works, many are part of the Turner Bequest—the painter left the entire contents of his studio, containing several thousand works, to the country, a gift which the Tate is custodian of. But there have also been significant loans. One of the square paintings comes from Belfast while another has travelled from Fort Worth in Texas. And the watercolor Bamburgh Castle, Northumberland (1837), which for a long time belonged to the private collection of the Vanderbilt family in the U.S., is being shown in a museum for the first time.

The Tate exhibition starts a great season for Turner fans: it will coincide with the UK release of the Mike Leigh biopic Mr. Turner (2014). The film, which was widely praised during its Cannes premiere, also explores the last years of the artist, played by the actor Timothy Spall.