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Name: Gary
Status: teacher          
Grade: 9-12
Country: United Kingdom
Date: Fall 2012


Question:
I am a secondary school teacher of Design Technology in the UK. When at college, many years ago, I am sure that I learnt the term "fire black" for the corrosion (tarnish) that forms on the surface of silver. I have checked the search engines on the Internet and I cannot find a reference to "fire black" or indeed any other term than just "tarnish" for the black oxide that forms on silver. Is my memory at fault?



Replies:
Hi Gary,

I have never heard of the term "black fire". I should point out that the black tarnish that forms on silver, is not silver oxide. It is silver sulfide, caused by sulfur emissions from burning of fossil fuels that contain small amounts of sulfur.

Regards, Bob Wilson


Hi Gary,

Silver doesn't tarnish only with oxygen, but involves sulfur to make silver sulfide which is black. There is a trace amount of sulfide in air, like SO2, and when you burn silver there is an oxidation of silver with sulfur to produce silver sulfide. Even without burning, you can see the color of silver products (silverware, jewelry, etc) turns black when they are old. This is also an oxidation of silver with sulfur in air.

Best, Weonkyu Koh Los Alamos


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