MASTERPIECE #1447 Arnold Böcklin, Swiss Medusa on Instagram #NoMakeup, 1878 Oil on wood Follow That Is Priceless on Social Media and GoComics (the Link button): Click to Follow This Blog or Share This Masterpiece: If You Liked That, Check These Out:
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Museum online page (Google translated, Bing translated) says it's oil on wood/panel. I've updated what I think was the source for this, which had previously said it's oil on canvas.