It’s probably not the best day to be a senior partner at Gibson, Dunn & Crutcher. The Montana Supreme Court has upheld a $9.9 million punitive damage award against the firm, finding that Gibson Dunn acted with “actual malice” in suing an art expert who declared a painting with the signature of C.M. Russell was done by Olaf C. Seltzer, thus greatly reducing its value. The firm’s use of the judicial system amounted to “thuggery” said the Court. Wow. Strong language.

Every week or so, somebody asks Ginger K. Renner to verify that a painting is an original by C.M. Russell, an artist known for his early 20th-century depictions of cowboys and Indians.

More often than not, Mrs. Renner declares the artwork a fake, and most people accept that as the last word since she is a leading authority on the famed artist. But in 2003, a collector named Steve Morton sued her and another Western art expert, insisting they were wrong about a painting he had purchased that has “C.M. Russell 1913” scrawled on the lower left corner, along with a sketch of a steer’s skull, the artist’s trademark.

The suit backfired. The unusual result: a multimillion-dollar judgment against Mr. Morton and one of the nation’s top law firms.

Charles M. Russell (1864-1926) lived most of his life in Montana, after moving to the state as a teenager to become a cowboy. He turned out 4,000 oils, watercolors, sketches and sculptures. His 12-by-25-foot “Lewis and Clark Meeting Indians at Ross’ Hole” hangs in Montana’s Capitol. Mr. Russell’s work can fetch more than $1 million. At the Coeur d’Alene Art Auction in Reno, Nev., last year, LaJolla, Calif., collector William Foxley sold “Piegans,” a 1918 oil of Indians on horseback, to an anonymous buyer for $5.6 million.

Unlike his contemporary Frederic Remington, who strove to please art critics, Mr. Russell had mass appeal that spurred an industry of imitators.

“I don’t think there has been an American artist who has been faked as often as Charlie Russell,” says the 84-year-old Mrs. Renner. “In the three weeks before Christmas, I had seven fakes come through the door.”

The C.M. Russell Museum in the painter’s hometown of Great Falls, Mont., hears regularly from people who wrongly think they own a Russell, says its director, Anne Morand. (The 38th-annual C.M. Russell Art Auction, a five-day event that benefits the museum, began yesterday in Great Falls.)

Mrs. Renner became a Russell expert through her late husband, Fred, who grew up near Mr. Russell in Great Falls and later amassed more than 100 works by the artist. In 1966, Mr. Renner published a catalog of Russells housed in the Amon Carter Museum in Fort Worth, Texas. The book’s first edition included “Lassoing a Longhorn,” a watercolor depicting a cowboy on horseback roping a steer.