Portrait of Lucrezia Panciatichi
Agnolo Bronzino -- Italian Mannerist painter (1503-1572)
c. 1540
Uffizi Gallery, Florence
Tempera on wood
102 x 85 cm
Inv. 1890, n. 736
Jpg: Virtual Uffizi
 
Lucrezia Panciatichi was the wife of Bartolomeo Panciatichi whom was the official portraitist at the Medici court of Cosimo I. They were married in 1528, or roughly 12 years before this painting. 

Bronzion intentionally features her wonderfull  red dress. Adam Sutcliffe, whom has seen this in person, calls it a tour de force for the depiction of scarlet fabric.  According to the Uffizi, there is a small plate engraved on the gold necklace with the words "Sans fin amour dure", alluding to love and faithfulness; and the painting over all went with a pendent portrait of her husband Bartolomeo -- also at the Uffizi.
(The Uffizi Gallery)
 

 Notes: 

John Singer Sargent 
 
Dr. Samuel Jean Pozzi at Home  
1881 

In the forum on Dr Pozzi, Adam Sutcliffe submits Bronzino's "Portrait of Lucrezia Panciatichi". Could this be another source that inspired Sargent's own painting in red?
 

 
   
Created 1/10/2003