Experts believe they have found Rembrandt’s fingerprints while examining and restoring a rare oil sketch by the Dutch master. The prints, which were pressed into wet paint, are impressions of the artist’s thumb. They were found hidden underneath overpaint and darkened varnish layers. Study of a Head of a Young Man (c.1650), the painting in question, will be the highlight of the upcoming Old Masters Evening Sale held by Sotheby’s on 5 December. The 17th-century...