Okay, so I was trying to clean an old silver coin with a bit of aluminum foil and hot water, and I swear I saw a faint rotten egg smell for a second. I thought the reaction was supposed to just remove tarnish, but that smell threw me. Did I accidentally make something nasty? I’m a bit worried I created hydrogen sulfide without meaning to.
That rotten egg smell worries you a bit. The aluminum foil method with hot water cleans tarnish by a redox exchange where aluminum donates electrons and silver sulfide is turned back toward silver. In a basic hot bath the reaction is usually gentle and does not make hydrogen sulfide in noticeable amounts. Hydrogen sulfide would need sulfur containing material and conditions that release gas. A faint whiff could come from the tarnish itself or from something in the water or air that day. If you are worried about exposure keep the room ventilated and wash your hands after. Did you notice the smell fade when you moved away from the sink?
Honestly I would not panic. The amount of gas you could get from that neat foil trick is tiny and the rotten egg note can easily come from something else in the air. Many people have smelled sulfur compounds in the kitchen or in water and they assume it is the metal cleaning. The method itself is common and not known to produce dangerous fumes in normal use. If the coin has value you might want to rinse and dry it and do a gentler polish with a dedicated silver cleaner. Do you have another coin you could test with to compare experiences?
From a chemistry view the tarnish on silver is silver sulfide and the foil bath uses aluminum to drive the reduction. The key is that aluminum oxidizes and the sulfide moves away from the silver as aluminum sulfide forms. The overall effect is the silver surface freed from tarnish. The smell would only come if sulfur is converted to a volatile form in enough quantity. In a basic hot bath there is little chance of that gas. Still a trace smell does not mean you created a dangerous gas and it will fade after the bath. Do you want a quick check you can do with no risk?
Totally not sure what happened but I would not overthink it. It is easy to misread a smell. The water was hot and you might have stirred up something in the room. I once smelled something weird cleaning coins and it vanished as soon as the air moved. It might just be a temporary sensory glitch rather than a chemical accident.
I wonder if we are focusing on a gas when the real point is the process of removing tarnish. The foil method relies on a surface exchange not a big chemical event on the coin atomic level. If you want to test it think about using a non precious metal piece and see if you get the same aroma. That reframes the issue away from danger and toward how the cleaning feels to the senses. Could be useful to note how durable the coin should be after a light polish.
Another angle is the idea of galvanic reactions and redox potential which means different metals push electrons differently. That is a general idea not a recipe. The key is that the reaction uses electricity like transfer through the water so you see the tarnish go away while the metal itself stays safe if kept away from harsh acids. hydrogen sulfide is something else. Have you seen the same effect with other tarnished pieces or did this one give you a clear sulfur smell only once?