Some clarification on cleaning zinc discs with toilet cleaner.  First, I
was able to find a small stock of the product at a local hardware store.
It was discontinued five or six years ago, but there is still some about.

It is labeled as "Harpic Powder Toilet Cleaner with Bleach (cleans
under the rim).  It contains amongst other ingredients less than 5%
oxygen-based bleaching agents.  Do not mix with hypochlorite bleach or
any other toilet cleaner or powder.  Made in France for Reckitt &
Colman Products Ltd., Dansom Lane, Hull, HU8 7DS. Tel: 01482 326151."

This did not tell me very much, so I rang the toll free number (0500
646645) and spoke to a company representative.  After much waiting on
hold, I was informed the following:

  "It contains sodium hydrogen sulphate.  This can give off chlorine
  gas when mixed with water.  Also contact with some metals in the water
  solution will give off hydrogen gas."

I have noticed when cleaning zinc parts that small bubbles were given
off.  I guess this is the hydrogen gas.  I originally used this cleaner
on brass, but not all grades of brass came clean.  It was only later by
accident that I discovered it would clean copper, bronze and zinc.
Ferrous metals will rust very quickly when exposed to the cleaner.

Although I have no wish to poison anybody, I must stress that this
chemical has until recently been around in British homes for donkey's
years, and I've no recollection of reports of fatalities.  Used in well
ventilated workshops, it should pose no risk.  British toilets, however,
are notoriously small compartments, and we are all still here.  I am
prepared to send one or two samples across the pond for analysis if
someone so wishes.

Cheers,
Paul Camps