I know that my position on this subject flies in the face of conventional wisdom, but I've actually learned a lot from my conservation efforts. Toning will reoccur ... in exactly the same manner if its allowed to return (usually by poor storage...chemical laden holders, albums etc) In order to arrest it, you have to get it out of harms way. That's why I settled on the staple type holders with the mylar liner. It forms a barrier between the coin and the cardboard used to make the holder.
Once toning is allowed to go completely black, the surfaces of the coin are damaged forever. They become etched as badly as if they'd suffered PVC damage from long time storage in the old soft flips.
If the original luster has been abraded or otherwise compromised by polishing or simple circulation, its no longer a candidate for conservation. There's no path back from "wear". Once the skin's gone, its gone. You just end up with a coin that looks much worse than it would left in its natural state. You have to make sure that the chemicals are neutralized when you're done, or it ends up toning unnaturally, and quickly. Failure to allow the coins to dry thoroughly prior to placing them in holders is also detrimental.
I'm not trying to change anybody's mind on the subject, but mean only to explain that I have a pretty thorough understanding of it, and its not something that I recommend that somebody with no experience dive into with any coin that is of value. There are tons of
Roosevelt dimes in Whitman folders to practice on. There's no particular interest in those. They're worth melt, and likely always will be...during our lifetimes or those of or kids anyway.
I will add one more thing ... Unless you want a worthless coin, don't mess with copper coins.... period. That includes nickels since they're 75 percent copper. I have zero successes to share in that end of the field.
Chance