Sotheby’s online magazine describes the roundel featured in Botticelli’s Portrait of a Young Man with a Roundel as follows:
The [….] painting differs from any other portrait of the time in the fascinating way in which Botticelli has shown his sitter holding a small roundel in his hand depicting a saint. This roundel is an original 14th-century work attributed to the Sienese painter Bartolommeo Bulgarini, which was inserted into the panel on which Botticelli painted his portrait. The significance of this striking visual device remains to be decoded, but must relate in some way to the identity of the handsome young nobleman who shows it off so proudly.
Further information is included in Sotheby’s auction catalogue:
The grain of the wood and the truncated punchwork of the background confirm it as a fragment—one not always round in shape, but rather cut out of a larger, vertical panel. While some of the gilding around the curved edges has been repaired, the figure of the saint has survived, like the rest of this painting, in very good condition. He is depicted half-length with a long grey beard, balding head and wearing a grey mantle atop an orange robe. Set against a gilded background, he is surrounded by a network of geometric punchwork that serves to frame his figure in a manner not unlike the painted architectural setting behind the young man. The saint lacks any identiable iconographic attributes, and only his right hand is visible, raised in an apparent gesture of blessing.
Botticelli’s portrait of the Young Man Holding a Roundel is referred to by the Italian painter Andrea Mantegna in his painting known as Parnassus, and identifies the sitter as Piero di Lorenzo de’ Medici. Mantegna also makes reference to the roundel and links its inclusion to Siena, as Bottcelli intended.
• More on this in my next post