In my arguments with Charles Freeman, I contended, as I have for a long time, that it is near impossible to paint a negative image. I repeated the claim today. Hugh Farey showed me that I was wrong. It is a short, very disturbing video.
Last November, I had written:
Show me one example of someone painting a negative image in the medieval or anytime in history. Find me an artist anywhere in the world who can do so. I’m sure it is possible. So, too, I imagine is patting your head, rubbing your stomach, jumping rope and singing the Halleluiah Chorus backwards all at the same time. Try it. No, I mean try painting a negative without a negative to copy. Try it.
But Charles, I still would like to see a medieval example and find out why a medieval artist would do this.
Shroud of Turin Blog 