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Silver tarnish


#1

G’day; here we go again. Tarnishing of silver is not oxidation.
Silver oxide is quite difficult to make in the lab. What you see is
silver sulphiding, and the sulphur comes from the environment. I
live in a little village near the sea, but here silver takes a long
time to tarnish. But a piece I sent to my brother’s wife in Glasgow
was black in a week, according to my brother! Exhaust from vehicles,
chimneys, furnaces, rubbish tips, cooking (eggs, onions) and yes -
even people (blush) - all provide the sulphur gases necessary for
tarnishing silver. Chalk- with some imagination - MIGHT help, but if
you want to go that route, a small dish of freshly slaked lime
(calcium hydroxide) would absorb sulphur gases better. In a town’s
atmosphere, a strip of special inhibiting paper (think 3M sells it) in
the drawer or cupboard would help, and the treatment for tarnish is to
dip the items in acidified thiourea - which is probably what
Goddard’s Dip is, or an alternative is to put some warm water in a
vessel, add a tablespoon of washing soda (sodium carbonate) and a
strip of aluminium foil. Let the silver touch the aluminium. Your
silver, unless very badly tarnished, will come out sparkling. As said
on TV “It reallyworks” Cheers, – John Burgess; @John_Burgess2
of Mapua Nelson NZ


#2

John Burgess, How do you acidify thiourea? I tried a solution of 60
grams thiourea in 600 grams of water and it turns silver brown.


#3
    John Burgess, How do you acidify thiourea? I tried a solution
of 60 grams thiourea in 600 grams of water and it turns silver
brown. 

G’day. I hang my head and have to tell you I have never dissolved
the stuff myself - assistants at the University did it! However, I
would take a little of the plain solution in a small transparent
vessel like a plastic shot glass, and add a LITTLE DILUTE sulphuric
acid (battery acid) and see if it stays clear, and then removes mild
tarnish on silver. Then you can add an appropriate amount of the acid
to the larger amount you have. Add too much acid and you will
doubtless find some sulphur precipitating and giving you that brown
colour. The makers of Goddard’s Silver Dip seem to do it somehow. If I
have given you a bum steer, then please accept my most humble
apologies; I should have tried it myself before rushing into print,
only I don’t have any access to thiourea these days. Local
pharmacies don’t have any call for it, and the lab suppliers won’t
deal with individuals! So I use Goddards. (humble) Cheers

	John Burgess;   @John_Burgess2 of Mapua Nelson NZ

#4

Hello: I make this solution: 900 grams of water, 50 grams of
thiourea, 10-12 grams of sulphuric acid.

Regards from Daniel Mischelejis
Buenos Aires, Argentina
email: mischelejis@fibertel.com.ar
web page: http://www.mischelejis.com


#5

Hi there, I missed the beginning of this thread so I apologise if I’m
way off base with this response. Thiourea is not the active
ingredient in silver dip. It’s an inhibitor. It stops the acid
(active ingredient) from attacking the freshly cleaned metal. In
conservation, we use a silver dip made of 1% sulphuric acid,
0.5%thiourea, 1% detergent (Terric GN8) in water. To clean silver,
just dip, scrub (gently) and wash until you’re happy with the result.
Soaking is not recommended as silver left in this solution too long
will be attacked by the sulphur in thiourea. A couple of points:
don’t mix your metals with this recipe - you’ll get one metal plating
out on another (copper on silver or gold, silver on gold…). Make a
batch for each metal type and lable. Secondly, there will be a time
when the acid is exhausted and the dip will no longer work. It won’t
have that blue colour from copper to tell you when. Dispose of the
solution by evaporation - don’t tip it down the sink. Thiourea is a
possible carcinogen so take care.

Eileen