The Fort Worth Press - Sketch attributed to Michelangelo to be auctioned in New York

The company's experts were examining a drawing by another artist of the same period for a forthcoming sale when they saw, stuck to the back of the frame, "this little piece of paper with a drawing and a letter," Stijn Alsteens, international head of the Old Master Drawings department at Christie's, told AFP.

Cosimo Buonarroti, Michelangelo's last direct descendant, indicates in the November 3, 1836 letter that he is offering "the enclosed essay" by his "illustrious forefather Michelangelo" to Sir John Bowring, the future governor of Hong Kong, whose signature appears at the bottom of the sheet.

The piece under inspection was sold to Christie's in London in 1986, with the catalog indicating the presence of a square "probably in Michelangelo's hand," though the reference did not initially generate any special interest.

Christie's has since estimated the square's worth at nearly $10,000.

Though it is unsigned by the great artist, Christie's said it guarantees it is "100 percent Michelangelo."

While working on his famed Sistine Chapel ceiling, the Italian painter, sculptor and architect created a series of small diagrams indicating the type of marble block he would need, according to Alsteens.

The square on auction, with the label "simile" or similar, is believed to be one such drawing.

Before his death in Rome in 1564, Michelangelo burned most of his drawings and sketches.

Those his family managed to preserve were bequeathed to "Casa Buonarroti," a museum celebrating the artist in a building where he lived in Florence.

But Buonarotti was also in the habit of offering small fragments containing Michelangelo sketches as tokens to friends and acquaintances.

Many of these gifted pieces included block drawings and were sold at auction or entered various foreign collections, art historian Mauro Mussolin wrote on the San Francisco Museum of Fine Arts website.

According to Alsteens, only a handful of the master's drawings and manuscripts are still in private hands.

The work goes on display Friday at Christie's New York headquarters as part of a sale devoted to old masters, which opens next week. The square itself will go under the hammer in April.

H.M.Hernandez--TFWP