"The Funeral of Shelley" (Fournier, 1889)

An 1889 oil painting by Louis Édouard Fournier, The Funeral of Shelley, depicts a somber ceremony as the body of Percy Bysshe Shelley is cremated on a Tuscany beach, July 18, 1822.

The scene is wildly inaccurate in all its details.

Death shadowed Mary Shelley’s life. Her very birth came at a terrible cost as her mother died within ten days. As a young woman, Mary would lose three of her four children to childhood illness, and she was still recuperating from a miscarriage that almost killed her when her beloved Percy drowned at sea...

On July 12, Percy’s body washed ashore on a beach near Viareggio and was promptly buried there by locals. Italian health laws prescribed cremation and on July 18, Shelley’s friends and fellow authors carried out the grim ceremony.

Fournier’s 1889 painting depicts a bleak, windswept beach, the witnesses swaddled in heavy coats against the cold. At the back, Mary Shelley kneels in prayer. In the foreground, friends and fellow authors Edward John Trelawny, Hunt and Byron strike dramatic, grieving poses. A peaceful Shelley, as if asleep, is stretched out on his smoking pyre. But it’s all wrong.

July 18 was actually a hot, sunny day. Mary Shelley, as was the custom of the times, did not attend. Leigh Hunt sat out the event in a nearby carriage. Byron, upset at the proceedings and suffering from the heat, cooled off in the surf, eventually to swim out to his own boat, leaving Trelawny alone on the beach. Shelley’s body, badly decomposed, the face and hands gone, was burned in a metal furnace lugged out to the shore by hired help. ..

In the end, Trelawny plucked Shelley’s carbonized heart from the ashes as a gruesome souvenir for himself, but he was eventually persuaded to give it to Mary, who preserved the relic for the rest of her life. Contrary to various reports, the heart was not returned to Shelley’s grave or buried with Mary, in 1851. It was interred with their son, Percy Florence Shelley, in 1889, the very year that Fournier painted The Funeral of Shelley. 
Excerpts from Frankensteinia: The Frankenstein Blog.  My understanding, FWIW, is that the organ plucked from the cremation ashes was likely not Shelley's heart, but more likely his liver.  Perhaps I can look that up sometime.