Wetple collodion is one of the oldest photographic techniques, invented in 1851 for the purpose of making negatives. Its variation, the ambrotype was discovered a short while later and enabled photographers to create amazingly beautiful, thoroughly unique images on black glass, blackened metal or even composite plates.

More about the technique: https://youtu.be/332UUzjP204

The process itself is best illustrated by a short backstage movie from one of my shoots.