On Fri, 26 Mar 1999, Inka Tertinegg wrote:

> Does anyone know how to get silver stain out of
> a white benchtop?

  Inka:  It depends how tough the bench top is.

    1. Nitric acid will remove silver (but not gold).
       Dilute the conc. acid about 50/50 with water.
       OK on epoxy paint. I don't know about formica,
       but would guess probably safe at this dilution.

    2. (From back in the old days) A paste of mercuric
       chloride and water rubbed into the black silver
       stain. More gentle on the skin than nitric acid,
       I've been told. Never really an acceptable method.

    3. Farmer's reducer, as used in photography; but for
       cleaning purposes it needs to be more concentrated.
       Crunch up about one cubic centimetre each of
       potassium ferricyanide and sodium thiosulphate in
       15-20 ml of water. Apply generously to the blackened
       bits of bench. It will remove the silver but not
       quickly. Allow 30-60 minutes and don't let it dry
       out. The slowness is a penalty for (a) spilling
       your expensive silver solution, and (b) using fairly
       harmless chemicals.
         Be sure to use ferrIcyanide and NOT ferrOcyanide,
       which will not work. Despite the -cyanide in their
       names, these are not considered dangerous poisons.

         (Cyanides, well known for their speed, are much more
          potent and also easily available to murderers in
          detective stories. When I was a lad, one of the
          other students in the lab needed to order some
          KCN for histochemical purposes. He needed about
          50 mg but the stuff was cheap and the smallest
          bottle held 500 g. We looked it up in Polson's
          Toxicology and found that this would be enough to
          kill 1500 people, if they could all be
          persuaded to swallow their 333 mg helpings. The
          smell must surely make cyanide a rather poor
          weapon for the enterprising poisoner: "bitter 
          almonds" to the novelist, but "sickly sweet" to 
          anyone working with amounts well below 10% of what's 
          supposed to be lethal.)

    4. None of these methods will remove dark stains due to
       metallic gold, from spilt "gold chloride."  Cyanide
       and oxygen would, and so would aqua regia, but these
       are not for sploshing on the bench's top. To hide
       carelessness with gold, you'll need a pot of paint.

           John A. Kiernan,
           Department of Anatomy & Cell Biology,
           The University of Western Ontario,
           LONDON,  Canada  N6A 5C1