You can test if it is sulfur based oxidation by doing this, (it works primarily on sterling but you can give it a try...........)

You need.......

a ceramic bowl lined with aluminum foil, shiny side up

a tablespoon of baking soda

a tablespoon of salt and some hot water.

Put just enough water in the bowl to dissolve the baking soda and salt and cover the item you want to clean up.

stick the silver thing in it and see if the sulfur flakes off  after a few minutes and moves to the aluminum foil.  Dry whatever you stuck in there with a towel and see what you have.  I know some jewelry folk put the soda and salt on the item and then pour the water on it.......my sister used to add tiny bit of dish soap on her sterling Native American Jewelry at the shop she had years ago, then buffed it up. She said it takes a bit of practice to get the consistency correct but it works after about 5-15 minutes of soaking as I recall.

This is interesting. Tried looking into this more but don't know nearly enough about chemistry or physics to really grasp much. I might have to throw on the safety glasses and see what they sound before they break at what I assume will be a very low tension. I wish I had an ornamental instrument to put them on; they look much more like decor than like actual strings.

This is a form of oxidation since your silver plate is not sterling silver. Silver plate that is not sterling will show a rainbow effect in varying degree as it ages.  Ken is correct when he speaks of a chemical reaction.  Are the strings that were plated made of nickel?

I think what you are seeing is the result of a chemical reaction either from the proximity of other metals as already noted or from the envelopes in which the strings were packaged. Did the envelopes have printing on them? Even if they didn't, there are chemicals used in creating the paper that could have leached on to the strings.

Ken

"The dulcimer sings a sweet song."

what an interesting idea. They were both still in their envelopes but the tub they were in has 75+ random strings of random compositions. I can spot red blue and yellow spots on these strings and as much as I want to try tuning them up, they are spiraled like a slinky when I hold them up and I think they would probably break.

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It sounds like what happens to new chrome car exhaust parts when first subjected to very high temperatures.

My guess---and it is that---is some sort of change due to being in the presence of other metals for the multitude of strings in your bucket. They should be fine to clean them and use them if they are not rusted or corroded to the point of risking breakage.

I've had these  silver plated classical guitar strings sitting in their paper envelopes in a bucket of spare strings for probably 3 years now. Finally decided to sort through it a bit and I found two separate strings which had each turned rainbow colored! The variety of colors is crazy and I was wondering if anyone knows more about this. Ive heard of strings getting a duller color over time, but these look like a full on art project. Thanks in advance,
Nate

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