The paint is thickly caked on the canvas, just as the dirt appears to be heavily layered on the arms and the collar of the pilgrim's cloak, yet this impoverished Saint James the Greater is likely to fetch more than £12m when he is sold at auction on Thursday in New York.

The portrait by Rembrandt, dated 1661, comes from the final decade of the painter's life, when he was facing penury in Amsterdam. It is one of a mysterious group of works that has intrigued art historians for many years. They are all sensitive character studies of predominantly New Testament figures, but it has remained unclear why they were painted.

This portrayal of Saint James the Greater is the only one of Rembrandt's late religious pictures to remain in private hands and, during preparation for its sale as the jewel in the crown of Sotheby's Old Masters auction, the close study of its surface has shed new light on both the life of its creator and on the group of paintings to which it belongs.