In La Bella Principessa, how is the Support for Leonardo attribution discussed?
The first study of the drawing was published by Cristina Geddo. 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). Leonardo was the first artist in Italy to use pastels, a drawing technique he had learned from the French artist Jean Perréal, whom he met in Milan in 1494 and/ or 1499.  Leonardo acknowledges his debt to Perréal in the Codex Atlanticus. Geddo also points out that the "coazzone" of the sitter's hairstyle was fashionable during the same period. Strong support for the attribution has come from Elizabetta Gnignera, the costume historian, in her book La Bella Svelata, which studies a wide range of comparative costumes and hair styles.
Milan Codex Atlanticus hatching Codex trois crayons Jean Perréal