I'm sure this is not "natural toning" but I am curious to know how they get like this? Any thoughts? I've seen this not just on copper but also on silver coins...
Definitely artificial toning. Copper coins rarely tone with this color pattern. This toning job is so sloppy—it looks like it is coated, and baked on.
The reason they don't like them is because the toning is artificial. There are plenty of collectors who will say that they don't care whether toning is artificial or not, but few collectors are able to maintain that indifference when it comes time to sell them.
We did have a CT member that did an unplanned experiment several years ago. He had put his proof set and forgot about it for several weeks left in his car in the Dallas summer sun. The plastic package was twisted and looked that it had been in a fire and the coins did develop that rainbow looking toning.
The early PCI slabs and some of the cheap plastic holders are known to tone coins blue. I don't believe that's the case with the OP coin. I'll guess heat caused it's blue look.
Simple answer - chemicals. And chemicals can be applied in liquid form, gaseous form, and even powdered form. And if you know which chemicals to use and how to apply them properly you can make coins any color, or combination of colors you want.
I love blue toning! I have some NT mostly from English collectors/dealers. I think those ones have something to do with storage methods/album types. The US blue coins I have mostly appear to be NT, but I do have an AT Franklin half.
So do I, especially on Rosies and Franklins, but I prefer toning that looks natural as opposed to the coins in this thread which is best case scenario were treated with MS70 coin cleaner. To me, it is the combination of vibrant blue & violet together that screams AT across almost every series of coin.
Some series and dates are known for blue toning such as the 1960s proof nickels. I cut this one out of the original mint proof set package myself. Before and after pictures.
While I understand your point re "best case scenario", and while MS70 is notorious for turning copper a shade of blue or purple, I think it's important for others to be aware that it's a near certainty the OP coins in this thread were not treated with MS70. MS70 pretty much always results in a natural looking toning when used on copper. A look that is as different from these coins as night is from day.
Different colors can be, and in the case of “natural” toning almost always are, produced by the same chemical. Usually it’s hydrogen sulfide. The different colors are due to diffraction differences which, in turn, are due to differing thicknesses of the sulfide layer on the coin. Cal
Given that it is a 1958, I would lean towards NT, those double mint set created some wild color schemes.