Gary,
Whilst I don’t want to comment on polishing new silver chain, I
would like to suggest that you take care using ammonia to clean any
valuable coins, it is an old fashioned method that we now know is not
appropriate. The problem with using ammonia is that it can cause
embrittlement of the metal and lead, over a period of time, to the
metal literally turning to dust. I frequently come across this
problem in old clocks where, for many years, the standard
professional method of cleaning them was to scrub the parts in a
solution of soft soap and ammonia, rinse in clean water and then
allow them to dry in a warm place. When I see them now, I find that
individual teeth or whole sections of wheels have fallen away and
turned to dust. The problem is twofold - partly mechanical and partly
chemical. When an ammonia solution is used it penetrates into any
cracks or gaps in the metal structure. These micro cracks are most
often found in metals which have been forged or coined where the
hammer blows have flattened the individual grains of metal into hard
plates - ordinary cast brass is less susceptible as the grains are
rounded and softer and so can absorb the stresses better. The rinsing
of the metal does not remove this ammonia from deep within the crack.
As the metal dries, the ammonia is concentrated and drawn into the
furthest recesses of the cracks by capillary action until it forms a
compact anhydrous plug at the bottom of the crack. Now, the next time
the metal gets wet or the atmospheric humidity changes, this
anhydrous material swells as it takes in the water and exerts a very
strong force on the metal grains. Over a period of time this
continual prising action loosens the grains and they simply fall
away. This is the primarily physical action. The chemical action is
known as Hydrogen embrittlement where the atoms of the Hydrogen in
the Ammonia (NH_3 or, in its Hydrate form HOH-NH_3 ) diffuse into the
brass and chemically change the atoms of the copper and the Zinc to
form a copper-Hydrogen complex (either CuH_1 or CuH_2 ) which in turn
releases electrons which attach to the Zinc and effectively corrode
it. I’m sure John Burgess can correct any of the chemistry here and
improve on my description - its a long time since I did high school
chemistry!! Anyway, the point is that, whilst using Ammonia solution
to clean copper or silver (a silver / copper alloy) coins may produce
a good finish in the short term, it could easily cause damage in the
long term which would destroy the coin. Remember, these items are
only on loan to us to hold in trust for the next and future
generations. A better system of cleaning the coins would be to set up
a simple ‘reverse plating’ bath - also known as an ‘ionic cleaner’
which have been described on this list before. I made the one I use
very simply from a small glass jar with straight sides, a piece of
stainless steel shim salvaged out of an old computer disk drive and a
’wall wart’ low voltage mains adaptor which supplies 6 or 9 volts DC.
This is the system widely employed by museums for cleaning off metal
articles salvaged from the seabed or for other archaeological metal
artefacts which are too delicate to clean by physical abrasion.
Best wishes, Ian
Ian W. Wright
SHEFFIELD UK