You can turn a film negative into an old-timey cyanotone print with a few chemicals and some printing media. Vinegar or other acidic substances can then be used to intensify the effect.
Cyantotone printing was discovered in 1842. It creates a mildly photosensitive paper that can be exposed to light to create silhouettes and other effects. It's the origin of the term "blueprint." This is a follow-up on a few examples Mathieu Stern did earlier this year.
Image: YouTube / Mathieu Stern