Why art now isn't just for art's sake

Prices for blue-chip artworks have soared. A Giacometti sculpture valued at £12-18 million sold for £65m in February, the highest auction price for an artwork.

• Proven history. The painting was bought directly from Scottish landowner and collector Hugh Munro of Novar, Turner's close friend, in 1839.

• Novelty value. The work has not been on the market since 1878, when it was bought from the Munro family by the 5th Earl of Rosebery and his wife Hannah Rothschild. The couple had honeymooned in a British stately home, Petworth, famous for its Turner collection.

• Condition. The Tate has a huge Turner collection but some paintings are in a poor state.

• Technique. The painting shows Turner's cross-hatching of dry paint, his working with bristles, and subtle nuances of colour.

• Setting. Turner's final painting of Rome, a city that fascinated him, after two visits there. Familiar landmarks on skyline.

• Light. Turner captures city in an atmospheric moment between day and night, but also fuses modern life and past.