Let's Define Another Loosely Used Term Thrown Around Here - Terminal Toning

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Back to the original topic. Just because a coin appears to have black toning does not necessarily equate to terminal or end toning. IF THERE IS LUSTER PRESENT, it has not reached that stage.
It makes sense, if you think about it. Terminal toning is when the toning has ceased to provide a protective layer on the coins original surfaces, and starts to actually eat into those surfaces....thus impeding the luster.
Flat black? Flat brown? Terminal.

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To put it another way, isn't "good toning" essentially a corrosive thing from the beginning, but it just hasn't started to do visible damage to the coin surface yet? How can it be protective, then corrosive, if it's the same thing? Forgive me if this is a stupid question; I'm no scientist.

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Is there a fairly simple way that you can explain how the toning goes from being "a protective layer," as you call it, into being something that "actually eat(s) into those surfaces"? It seems to me that it would be a progression, rather than a shift. To put it another way, isn't "good toning" essentially a corrosive thing from the beginning, but it just hasn't started to do visible damage to the coin surface yet? How can it be protective, then corrosive, if it's the same thing? Forgive me if this is a stupid question; I'm no scientist.

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As opposed to gold, silver is a highly reactive element. When exposed to the environment it will react with a number of contaminants to form tarnish. Sometimes this is attractive, especially when a thin film of tarnish reflects (refracts) light in a certain way. All sorts of pretty colors can result and the exact color depends (mostly) on the thickness of the tarnish layer. Once the tarnish forms, it is more difficult for contaminants to attack the metallic silver underneath the toning layer.



The toning itself forms a protective layer and acts as a partial barrier to further toning. Some coins tone more nicely than others (Morgans tone beautifully and Peace dollars don't). This is probably due to the specific alloy, surface treatments prior to striking, or the exact crystalline structure of the surface on a molecular level. Different designs and die surface preparations result in different strain hardening of the coin's surface.



The problem with dipping is that the protective layer is stripped away, leaving fresh, raw surfaces. In many cases even freshly struck coins originally left the mint with some sort of surface layer - oils, oxides, leftover chemicals from acid baths, etc. Putting the coin back in the environment with raw surfaces invites a second round of aggressive attack. Sometimes this works out (nice secondary toning) and sometimes it goes sideways (ugly secondary toning). Either way, the original surfaces are slowly disappearing, along with the microscopic flow lines that are responsible for luster. Anything that speeds this up is harming the coin. That's why originality is so highly praised by those who value true protection and conservation of the coin.

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For the sake of clarity, who are you quoting and where can it be found?