I have questions about a couple issues: 1. I heard that some coins may have a patina, but may be corroded underneath... how can you tell if that's the case? And is that really bad, or is the patina essentially protecting the coin from further damage? 2. I know there are coins out there that have had an artificial patina applied to them. How do you tell if that's the case in a certain coin, what do you look for? The reason why I want to know about these topics is because of the coin below. I haven't seen many coins with that dark of a tone on it, and I was reading online about patinas and saw that a couple sites mentioned the issues above. I just want to make sure that this coin wasn't tampered with or has something wrong with it. There may be nothing wrong with it, maybe it's just me being paranoid, but I'd like to educate myself. Thanks!
Generally speaking patina is nothing but another word for toning. And yes it is true that sometimes there is corrosion underneath the toning. As for how can you tell - well there really isn't a way to tell for certain without removing the toning. But a person with the necessary experience can often look at a coin and surmise that it is corroded. It all depends on the look, the color of the toning. You see, in effect toning itself is corrosion. It is the metal oxidizing and that oxidizing is what causes the coin to change color. And as those colors progress through the spectrum (just like a rainbow) the degree of corrosion progresses also. And once it has reached a certain point it begins to pit the metal. That pitting is what we call corrosion. That point is when the toning turns a dark brown or black. Once it has done that then you can reasonably assume that the coin is corroded. It's a double edged sword. Yes the toning protects the coin, but at the same time the toning, if allowed to progress, harms the coin. It's kind of like the saying - too much of a good thing. Think of it like this. You go out in the sun. If you stay out in the sun for too long you get a burn. But if you go out in the sun for a little bit at a time, you get eventually get a tan. That tan then protects you from the sun so that you can stay out in the sun for longer periods. But there is still a limit to how long. If you stay in the sun long enough, even with a tan, you eventually still get damage to your skin. Toning on coins is exactly the same. It will protect the coin by slowing down the effects of further corrosion, but it will not entirely stop it unless you stop what is causing the toning/corrosion. With you, that means staying out of the sun. With coins, it means controling the temperature, humidity, and how much air the coin is exposed to. Again it's an experience thing. With enough experience you know what a coin with artificially applied toning looks like. In these cases we are talking about what they call darkeners - a liquid chemical that acts much like a coat of stain on a piece of wood. Just as stain soaks into a piece of wood and changes the color, darkener soaks into the pores of the metal and changes the color of the coin. And if that color is too even, too consistent - then you you pretty much know it is artificial. By the same token if it is not consistent enough, or too blotchy, then you again know it is artificial. This is where the experience comes in - you have to able to tell one from the other. There is really no other way to do it without sophisticated machines. The machines can do it because they can detect the chemicals (the stain) on the coin's surface. From what I can see of the coin in the pictures - it's fine.
My expierance with bronze corosion is when the pitting appears on the surface, its too late. Corrosion starts inside the metal and by the time it reaches the surface, the underlineing stucture is severly damaged. Think of it like a cone shape. The small pointed end is the surface, the damage below it the large diameter end. Once the surface is cracked, many times a powder will just fall out (oxide).
Sorry Ripley but I have to disagree. Corrosion always starts on the surface and works its way in. It cannot start inside and work its way out.
Exactly. Corrosion is driven by exposure to corrosive chemicals, i.e. air and water. It needs those to feed the reaction or the process halts. It ALWAYS starts on the surface.
