Today it begins a serial about "Ladies from Firenze in the Renaissance"

La Bella Principessa by Leonardo da Vinci By Shelley Esaak, About.com Guide http://arthistory.about.com/od/leonardo/ss/leonardo_la_bella_principessa.htm (brief summary)

"This little portrait made big news on October 13, 2009 when Leonardo experts attributed it to the Florentine Master based on forensic evidence.

Previously known as either Young Girl in Profile in Renaissance Dress or Profile of a Young Fiancée, and catalogued as "German School, early 19th Century," the mixed media on vellum drawing, backed with an oak panel, was sold at auction for $22K (US) in 1998, and resold for approximately the same amount in 2007. The buyer was Canadian collector Peter Silverman, who was himself acting on behalf of an anonymous Swiss collector. And then the real fun started because Silverman had bid on this drawing at the 1998 auction suspecting, even then, that it had been misattributed.

Technique

The original drawing was executed on vellum using pen and ink, and a combination of black, red and white chalks. The yellow color of the vellum lent itself well to creating skin tones, and combining with carefully applied black and red chalk for green and brown tones, respectively.

Why Is It Now Attributed to Leonardo?

Dr. Nicholas Turner, former Keeper of Prints & Drawings at the British Museum and an acquaintance of Silverman's, brought the drawing to the attention of leading Leonardo experts Drs. Martin Kemp and Carlo Pedretti, among others. The professors felt there was evidence that this was an uncatalogued Leonardo for the following reasons:

• The age of the vellum.
Vellum, a type of parchment made from animal skin, can be carbon-dated. And dating the physical materials in a previously-unknown-but-maybe-it's-a-masterpiece work is the first step taken in an authentication. (It has to be; there is no point in continuing if "Renaissance" materials date to a later period.) In the case of La Bella Principessa, carbon-14 dating placed its vellum between 1450 and 1650. Leonardo lived from 1452 to 1519.
• The artist was left-handed.
If you look at the larger view of the image above (click, and it will open in a new window), you'll see a series of light ink parallel hatching lines from the nose to the top of the forehead. Note the negative slope: \\\\. This is how a left-handed person draws. A right-handed person would have inked the lines thus: ////.
Now, which other artist, during the Italian Renaissance, drew in the style of Leonardo and was left-handed? None are known.
• The perspective is flawless.
Perspective being a forte of Leonardo's. He had been studying mathematics all of his life, after all.
• The knots on the shoulder of the sitter's dress and the braiding in her headdress are executed with Leonardesque precision.
See above. Leonardo's particular mathematical passion was geometry. In fact, he would go on to become fast friends with Fra. Luca Pacioli (Italian, 1445-1517) and create drawings of Platonic Solids for the latter's De Divina Proportione (written in Milan; 1496-98, published in Venice, 1509). Just for curiosity's sake, feel free to compare the knots in La Bella Principessa tothis etching.
• It is Tuscan in overall style, though finishing details are Milanese.
One of those finishing details is the sitter's hairstyle. Take a careful look at the pony tail (which actually rather resembles a polo pony's, after it has been gathered and taped in preparation for a match). This style was introduced to Milan by Beatrice d’Este (1475-1497), Ludovico Sforza's bride. Called a coazzone, it featured a bound braid (either real or false, as in a 15th-century hair extension) that ran down the center of the back. The coazzone was in fashion only a few years, and only at court. Whatever the Principessa's identity, she moved in the upper echelon of Milanese society.
• Leonardo had been quizzing a traveling French artist about the use of colored chalk on vellum at the time.
It is important to point out here that no one used colored chalk on vellum during the early Renaissance, so this is a sticking point. Whoever created this drawing was conducting an experiment. Perhaps not on the scale of, say, painting a huge mural in tempera on a wall covered with pitch, mastic and gesso -- incidentally, also in Milan -- but, well. You can doubtless guess where this train of thought is going.

However, "new" Leonardos demand conclusive proof. To this end, the drawing was sent to the Lumiere Technology lab for advanced multispectral scanning. Lo, a fingerprint emerged that was "highly comparable" to a fingerprint on Leonardo's St Jerome (ca. 1481-82), notably executed at a time that the artist worked alone. A further partial palm print was later detected.

Neither of these prints were proof, though. Additionally, nearly everything listed above, save for the date of the vellum, is circumstantial evidence. The identity of the model remained unknown and, furthermore, this drawing was never listed in any inventory: not Milanese, not of Ludovico Sforza's, and not of Leonardo's."

"The young sitter is presently presumed by experts to be a member of the Sforza family, although neither the Sforza colors nor symbols are evident. Knowing this, and using the process of elimination, she is most likely Bianca Sforza (1482-1496; daughter of Ludovico Sforza, Duke of Milan [1452-1508], and his mistress Bernardina de Corradis). Bianca had been married by proxy in 1489 to a distant relative of her father's but, because she was seven years old at the time, remained in Milan until 1496.

Even if one were to assume that this portrait depicts Bianca at age seven -- which is doubtful -- the headdress and bound hair would be appropriate for a married female.

Her cousin Bianca Maria Sforza (1472-1510; daughter of Galeazzo Maria Sforza, Duke of Milan [1444-1476], and his second wife, Bona of Savoy) was previously considered as a possibility. Bianca Maria was older, legitimate and became Holy Roman Empress in 1494 as the second wife of Maximilian I. Be that as it may, a portrait of her by Ambrogio de Predis (Italian, Milanese, ca. 1455-1508) done in 1493 does not resemble the model for La Bella Principessa." 

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