Today, we put up our Christmas tree, which the girls enjoyed immensely. I took on the task of polishing the Wallace silver bells, which have been a family tradition since I was a wee lad. We have the annual bell from 1981 through 1991.
While I was on the subject of polishing silver, my thoughts turned to my penguins, a set that I got from Restoration Hardware about 10 years ago. I have 2 of them that hold a small dish, and another 2 that hold a pair of candles each. They’re cast brass with silver plate, and the silver is badly tarnished. A few years ago, I tried to polish them using silver polish, but that required enough elbow grease to lube a semi, so I gave up on that idea.
So, I googled for a way to do it by soaking them in something. I found a tip that involved lining a container with aluminum foil and making a solution of hot water, salt, and banking soda. After about 5 minutes, I pulled the first penguin out of the soup, and wiped off a swath of tarnish with a light pass of my thumb. “Whoa,” I say, and start attacking the bird with a cloth. I replenish the salt and baking soda and put it in for a while longer. By the time I got done wiping it the second time, it was back to its original shiny glory. I am excited.
Since I’m a big ol’ geek, I start to wonder what the chemistry of this is, and how damaging it is to the silver, as the page where I found it recommended not going overboard with it on silverplated items.
Luckily, my dad shows up on IM and I ask him. I’m not even as big a geek as my parents are, if you can believe that. Both my parents have graduate degrees in scientific fields, and my mom has a PhD as well. My grandfather on my dad’s side was a metallurgist, so my dad more than likely knew about this particular trick.
Sure enough, he tells me that this was a trick his dad used all the time. I figure if a metallurgist used this, it can’t be all bad. As it turns out, unlike using silver polish, which removes the tarnished silver altogether, this process retains all the original silver.
Tarnished metals are usually sulfides. In this case, the tarnish is silver sulfide (Ag2S). The solution described above sets up a cathodic battery reaction, wherein the alkaline water (due to the baking soda) is the electrolyte. The salt isn’t strictly necessary, but it may help the electrolytic process.
Here’s where the magic happens. In a alkaline solution, the aluminum is electrochemically more aggressive than the silver, and actually goes in and displaces the silver in the tarnish and replaces it with aluminum atoms, forming aluminum sulfide (Al2S3). The silver atoms then return back to the plating on the object and the aluminum sulfide wipes off quite easily.
When I dumped the water out, it stunk of sulfur, with an odor that was very similar to methyl mercaptan, which is what they use to make natural gas smell bad. Given that we have a natgas stove, furnace, water heater, and dryer, I have a strong suspicion where the sulfur that tarnished the penguins came from 🙂 Removing the foil showed a lot of pitting where the metal had been removed to go join its sulfur buddies.
For grins, I stuck a voltmeter with the anode probe on the aluminum and the cathode probe in the water, and it started out at about .3V, and it steadily kept climbing. After about half an hour, the meter was reading about half a volt.
So, I got to have science fun, AND get my penguins shiny.