[3] According to Kemp and Cotte, the sheet was cut from a Milanese vellum book, La Sforziada, in Warsaw, which celebrates the marriage in 1496 of Galeazzo Sanseverino with Bianca, the illegitimate daughter of Ludovico Sforza, Leonardo's employer.

[5] The portrait is a mixed media drawing in pen and brown ink with red, black and white chalk, on vellum, 33 by 23.9 centimetres (10 by 9 in)[6] which has been laid down on an oak board.

[7] It represents a girl in her early teens, depicted in profile, the usual way in which Italian artists of the 15th century portraited women.

Reflecting the subject of an Italian woman of high nobility, Kemp named the portrait La Bella Principessa, although acknowledging that Sforza ladies were not princesses.

[12] The work was included in a sale at Christie's in New York on January 30, 1998, catalogued as Young Girl in Profile in Renaissance Dress, and described as "German School, early 19th Century".

[2] In 2007, art dealer Peter Silverman, purchased the portrait from a gallery on East 73rd Street, owned by Kate Ganz.

In 2010 one of those experts, Martin Kemp, made it the subject of his book co-authored with Pascal Cotte, La Bella Principessa: The Story of the New Masterpiece by Leonardo da Vinci.

The drawing was shown as a Leonardo in a 2010 exhibition called And there was Light in Eriksberg, Gothenburg, in Sweden,[13] and was estimated by various newspaper reports to be worth more than $160 million.

[citation needed] Pascal Cotte of Lumière Technology in Paris performed a multi-spectral digital scan of the work.

[21][22] In 2010 David Grann published an article about the drawing in The New Yorker, which implied that Biro had been involved with forged paintings attributed to Jackson Pollock.

[26] Geddo attributes this work to Leonardo based not only on stylistic considerations, extremely high quality and left-handed hatching, but also on the evidence of the combination of black, white and red chalks (the trois crayons technique).

[1] The physical and scientific evidence from multispectral analysis and study of the painting, as described by Kemp in the first edition of his book with Cotte,[1] may be summarized as follows: In 2011, after the publication of the first edition of their book, Kemp and Pascal Cotte reported that there was evidence that the drawing had once been part of a copy in the National Library of Poland in Warsaw of the Sforziada.

[31] This is a printed book with hand-illuminated additions containing a long propagandistic poem in praise of the father of Ludovico Sforza, who was Leonardo's patron, recounting the career.

The reproduction of the unpublished heraldic figure attributed to Antonio Grifo, illuminated in the incunabulum "Comedia" by Dante (Cremonese, Venice 1491), now at Casa di Dante in Rome, shows the original coat of arms and insignia of the family of Galeazzo Sanseverino, and the comparison with those illuminated by Birago is not corresponding.

[2][30][35] Among the reasons for doubting its authorship are the lack of provenance prior to the 20th century – unusual given Leonardo's renown dating from his own lifetime, as well as the fame of the purported subject's family[35] – and the fact that it was on vellum.

[35] Marani is also troubled by use of vellum, "monotonous" detail, use of colored pigments in specific areas, firmness of touch and lack of craquelure.

[36] Klaus Albrecht Schröder, director of the Albertina, Vienna, said "No one is convinced it is a Leonardo," and David Ekserdjian, a scholar of 16th-century Italian drawings, wrote that he suspects the work is a "counterfeit".

Noting the lack of mention of dissenting opinion in Kemp's publication, Richard Dorment, the former husband of Ganz, wrote in the Telegraph: "Although purporting to be a work of scholarship, his book has none of the balanced analysis you would expect from such an acclaimed historian.

[38] In evidence, Kline points to a drawing on vellum by Schnorr, Half-nude Female, in the collection of the Kunsthalle Mannheim in Germany,[39] which he suggests depicts the same model, although older, as portrayed in La Bella Principessa.

Comparative material-testing of the vellum supports of the Mannheim Schnorr and La Bella Principessa were anticipated to occur in the New York federal court lawsuit Marchig v. Christie's, brought in May 2010 by the original owner of La Bella Principessa, who accused Christie's of breach of fiduciary duty, negligent misrepresentation and other claims.

[41][42] In his memoir A Forger's Tale, written in prison, Greenhalgh claims as a 17 year old to have forged the drawing by obtaining an old piece of vellum from a reused 1587 land deed.