This is a mild carbon steel that we let rust naturally. There are many factors that determine how quickly steel will rust...it varies depending on things like your location and the weather/time of year. You can definitely spray water on it if you prefer it to speed up the process.
I worked at a blacksmith shop over a summer doing a lot of stuff that used salvaged materials and things that were supposed to look worn. When I asked one of my mentors, a brilliant sculptor, metalworker, and blacksmith what I should do to make something rust quickly and somewhat evenly, he advised that I relieve myself on my work. It was one of few occasions where my process veered from his guidance. I simply used white vinegar to remove any oils or mill-scale from the metal. The vinegar will remove a great deal of the rust at first, but make it more susceptible to rusting in the future.
D-CRAIN Design and Construction
rbcola
Exult Design
Noah Modie