I stupidly left some very EXHAUSTED film FIXER in a clear PLASTIC measuring beaker for about a week. When I dumped it out, the inside of the beaker is coated/plated with a opaque black silvery material. This coating does not want to wash out. I do not want to scrub it out with a abrasive material as it will scratch to inside of the plastic beaker. How do I clean this material from my beaker please. With thanks, Sam H.