Willem Bartsius (1612, Enkhuizen – 1657, Enkhuizen), was a Dutch Golden Age painter. Biography According to Houbraken, who mentioned his sister as Paulus Potter's mother, his father was Paulus Bertius, the city secretary of Enkhuizen, and his mother was descended from the House of Egmont. Paulus Potter Biography in De groote schouburgh der Nederlantsche konstschilders en schilderessen (1718) by Arnold Houbraken, courtesy of the Digital library for Dutch literature
According to the RKD he became a member of the Alkmaar Guild of St. Luke in 1634 where he took on the pupil Abraham Meyndertsz, but in 1636 he moved to Amsterdam and little is known of him after 1639.Willem Bartsius in the RKD He is known for both landscapes and portraits, including a schutterstuk in Alkmaar.
References
SK-A-2214 A captain with ceremonial commander's staff at the Rijksmuseum Image database (search term: Bartsius) of Stedelijk Museum Alkmaar shows schutterstuk by Bartsius, completed in 1634 Willem Bartsius on Artnet