All my rusting is scheduled by the clock and all other work in the shop revolves around rusting when I'm rust bluing.
Knowing the solution, damp box and preparation rusting cycle are very predictable and vary little in timing, mainly with the first cycle taking a bit longer.
Visual inspection and experience are the last word for each cycle.
Learning to professionally rust blue require quite a bit of experience that is what I teach, my decades of experience.
BTW I'd never use a crock pot, I just don't need that much humidity. Too much humidity is more dangerous and potentially damaging than not enough.
