I used to do black & white photography and develop my own film/prints. The silver would start to build up on the trays, just like in my colloidal silver jars. My photography teacher told me to soak the trays in bleach and water and that cleans it right up. That's what I still do with my colloidal silver jars, too, then I run them through the dishwasher. Be sure to rinse really well.
Teri Confessions of a Fiber Drunk (updated 1/3): http://fiberdrunk.blogspot.com/
