Georgia O'Keeffe's White Calla Lily was the headline lot of a sale of American art at Sotheby's.

The auction took place in New York on May 20, with the painting achieving $8.9m.

O'Keeffe used the calla lily as a motif in much of her work

Sotheby's explained in the run-up to the sale: "Between 1918 and 1932 O'Keeffe executed over 200 flower paintings, but it was arguably in the calla lily that the artist found her ideal motif, one that provided the perfect synthesis of subject and form that now defines her most celebrated work.

"The artist clearly held the present painting in high regard, as she kept it in her personal collection until her death in 1986."

O'Keeffe's price point has advanced significantly in recent years. Her Jimson Weed/ White Flower No 1 sold for $44.4m at Sotheby's last year, setting a new world record for a female artist at auction.

Another artist whose work has grown considerably in value is Norman Rockwell.

His Man with Nose in Book (The Bookworm) was another highlight, pulling in $3.8m against an estimate of $2.5m - an increase of 52%.

In 2013, his Saying Grace set a new artist record when it sold for $46m - precipitating a major upward swing in his estimation among collectors. His work now consistently outperforms valuations at auction.

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