Van Dyke Brownprints
The van Dyke Brownprint process (there are many variants on this name) is a hybrid iron/silver process. Like the cyanotype, it relies on the light sensitivity of salts of iron. The sensitised iron salts react with salts of silver to produce very fine particles of silver, which have an attractive brown colour. This can be considerably enhanced, with a shift to purple, by gold toning. It is a contact process and relies on a source of UV light for exposure. The sun is a possiblity, but my preference is to use a sun lamp for consistency.
I find this process can be very tricky to control. It requires a fairly contrasty negative and seems to be quite sensitive to the paper used. To date I have used film rather than digital methods for my brownprint negatives.