The following article chronicles the debate between microscopist Walter McCrone and blood chemists, Dr. Alan Adler and Dr. John Heller. McCrone claims that the image is the result of iron oxide suspended in a light protein binder and the bloodstains are actually vermilion paint. Other Shroud scientists dispute his claim and this article tell why. A very entertaining read.
http://www.shroud.com/pdfs/ford1.pdf