Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: thedoc on 19/09/2012 15:30:01

Title: Why does silver develop a black tarnish patina?
Post by: thedoc on 19/09/2012 15:30:01

Dorothea asked the Naked Scientists:
   
Dear naked Scientists,

First, I totally love your podcasts (http://www.thenakedscientists.com/HTML/podcasts/) and am simply addicted to them. Over the years I must have learned a lot from you - if it sticks in my brain cell, that is.

Well, here is my question:

I wear a lot of silver jewelry, sterling silver most of it (you can never be sure about old ethnic silver). It strikes me that it gets the typical black patina on me really, really quick and becomes pretty black in a short time, if I wear a necklace daily it will be black within a week. My sister, who makes silver jewelry as a hobby, always jokes, that whenever she needs a patina she only has to lend her treasures to me for a week...

Why is this? As far as I know silver turns black if there is hydrogen sulphide involved. Thus I must be some kind of volcano?

I would be delighted if you could come up with an answer why silver turns black on me so quickly.

Thank you so much for everything!

Love you, folks.

What do you think?

Title: Re: Why does silver develop a black tarnish patina?
Post by: lightarrow on 20/09/2012 12:34:43

Sulfur is normally present in the aminoacids which make up the proteins, so we have a lot of it in our body. Probably different skin's metabolic processes of different people produces different amounts and different kinds of compounds containing sulfur (not hydrogen sulfide, this is not necessary to tarnish silver).

Another possibility is that you stay some time, at work or at home, near a source of gaseous sulfur compounds (you didn't specify if your sister lives and works close to you).

Title: Re: Why does silver develop a black tarnish patina?
Post by: damocles on 23/09/2012 22:30:49

The natural gas (mostly methane) that we use for heating and cooking is nearly odourless. A small amount of mercaptan (containing sulfur) is added so that any gas leaks will be noticed. This, in turn means that silver items will tarnish quickly in kitchens or rooms with open gas fires.

Talking of kitchens, if you regularly eat fried eggs your silver is probably doomed. A hot egg will give off a significant amount of sulfur-containing gas.

Title: Re: Why does silver develop a black tarnish patina?
Post by: evan_au on 24/09/2012 23:45:48

Garlic, Onion and related vegetables have sulfur-containing compounds, which can find their way onto the skin.
It probably depends on how well your body breaks it down in the liver.
http://en.wikipedia.org/wiki/Garlic#Adverse_effects_and_toxicology
http://en.wikipedia.org/wiki/Onion#Eye_irritation