Hi all – Any metallurgists about? Or anyone who knows a thing or two about silver tarnish... I'm stumped. Observation 1: American "junk" silver circulation coins don't tarnish much, e.g. pre-1964 quarters, dimes, etc. These coins are 90% silver, 10% copper. While they aren't completely immune, they don't tarnish as much as pure silver bullion, or Sterling. Observation 2: Sterling silver is notorious for its propensity to tarnish. Sterling wares have to be laboriously maintained, polished, etc. People buy all sorts of products to fight its propensity to tarnish. Strangely, Sterling's composition is very similar to coin silver: 92.5% silver, 7.5% copper. Observation 3: Pure 999 silver bullion readily tarnishes. If I leave it in a random flip in a sock drawer for a couple of months, tarnish. I don't know how pure silver's propensity to tarnish compares to Sterling's. My hunch is that Sterling is the worst of the three. Anyone know why 90% coin silver tarnishes less than both Sterling and 999? It's not the linear relationship I expected. 90% barely tarnishes, 92.5% badly tarnishes, but 999 tarnishes more than the 90%? That doesn't make sense to me. Is there something metallurgically special about the 90/10 silver/copper recipe? Is this why it was chosen for coins? Why does Sterling behave so differently, given its very similar composition? Is there a magic silver/copper recipe that never tarnishes? The tarnish-resistant silver alloys I've seen lately tend to add other metals like germanium, silicon, etc. (Argentium Silver is a good example – great stuff, wish they made bullion from it.) Thanks for your help.
We can start by saying that all coins begin toning the day they are struck and exposed to the environment. As far as silver and the compositions .999 is the most reactive. 92.5 is second and .90 is last. The difference in my opinion is that sterling gets handled cleaned etc, exposing it to many more environmental elements than silver bullion or US coins.
One of the world's mysteries. I wonder why some silver coins eg, CC mint bags, were opened after many decades in storage and some of the coins were pristine like the day they left the mint. Go figure.
Actually the opposite is true. "Pure silver resists tarnish. However, one almost never encounters pure silver. It is far too soft for use in items that require durability, such as coins. Therefore, silver is generally mixed with other metals in an alloy form. This makes the metal much harder and more suitable for day-to-day wear. Most commonly, it is mixed with copper, and copper is a highly reactive metal. Thus tarnishing is usually because the other metals in the silver alloy are reacting to the surrounding environment. It’s the natural chemical reaction metal has when exposed to air and the elements it contains." Why Does Silver Tarnish (gainesvillecoins.com)
Yeah, but that's not actually true, as far as pure silver not tarnishing. It tarnishes all the time. Tarnish is one of the biggest complaints people have about silver bullion. I know people who focused on gold because they were so annoyed by how their pristine silver would tarnish.
“Strangely, Sterling's composition is very similar to coin silver: 92.5% silver, 7.5% copper.” That’s why the British named their currency the “Pound Sterling”.
"Chemically, silver is not very reactive—it does not react with oxygen or water at ordinary temperatures, so does not easily form a silver oxide. However, it is attacked by common components of atmospheric pollution: silver sulfide slowly appears as a black tarnish during exposure to airborne compounds of sulfur (byproducts of the burning of fossil fuels and some industrial processes), and low level ozone reacts to form silver oxide. As the purity of the silver decreases, the problem of corrosion or tarnishing increases because other metals in the alloy, usually copper, may react with oxygen in the air. The black silver sulfide (Ag2S) is among the most insoluble salts in aqueous solution, a property that is exploited for separating silver ions from other positive ions. Sodium chloride (NaCl) or common table salt is known to corrode silver-copper alloy, typically seen in silver salt shakers where corrosion appears around the holes in the top." This comes from... https://en.wikipedia.org/wiki/Sterling_silver
But why does 90% silver tarnish less than 92.5% silver (Sterling)? That's the key question. The article's claim that tarnish increases as purity decreases appears to be incorrect. And it seems to be a big difference – 90% silver tarnishes much less than 92.5%. Can you imagine if coin silver tarnished as much as Sterling? All these coins would be black within five years of minting...
Is it possible that the items you've observed were stored differently? I know from personal experience that if you store a silver coin in a box of kitchen matches, it will tarnish up pretty fast.
I was just trying to rule out differences in environmental conditions. Reactive substances in the air may be different between the open air of a dining room hutch...compared to a plastic flip in a sock drawer. For instance, your sock drawer may be higher in chlorine (washing machine detergent residue) and the dining room hutch could be higher in sulphur. It might be interesting to do a test with each composition exposed to equal environmental stresses.
Good points. Isn't tarnishing of pure silver bullion a pervasive phenomenon? I see people complaining about it all the time on various forums, and the solutions come up a lot, like the aluminum foil and baking soda cleaning method. I ended up refining that method to restore the coins, and now I don't put anything in sock drawers or exposed to ambient air – it's all in Ziploc snack bags now, or capsules.
