Difference in toning on silver bars - 1oz and 10oz

I bought these two silver bars about 45 years ago and always stored them in a box or in a safe, basically exposed to the air. However, for about 35 of those years, the 1oz bar was stored in a flip PVC holder (not sure it was actually PVC, but you get the idea). I've never cleaned either bar, but I have handled them with my bare hands dozens of times over the years.
For the past 10 years, the two bars have sat alongside one another, on my desk, fully exposed to the air 24x7x365.
As you can see, the 1oz bar shows significant toning and tarnishing, while the 10oz bar shows no signs of anything. In fact, it looks exactly the same as when I bought it all those years ago. Why do the two bars differ so much? Is there something that is preventing the 10oz bar from toning?
Comments
maybe the people on the precious metals forum might also have input
The 10 oz has a different surface finish than the 1 oz does, the 10 oz has a grained texture to it whereas the 1 oz looks stamped and was probably somewhat prooflike. The plastic flip probably gave off some slightly acidic vapors. These things change the way that films grow on the metals surface.
Mr_Spud
That's a long hold. Looking at the spot price chart, in feb of 1980 silver spiked in feb to around 34.
I found my original sticky note, which says that I bought the silver in 1986, for $5.31/ounce.

Toned bars don't sell in my area. The dealers dip every one they get.
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