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I used your Black Darkener on a copper piece, and it was a beautiful color and had a wonderful finish when I took it out of solution but most of the color has flaked off. What causes this?

There are two main reasons for the finish flaking off. The first reason would be a problem with how the metal was prepared and cleaned. The more likely (and very common) application error is that the object has been left in the solution for too long. The object should be removed from the solution as soon as the desired color is achieved and washed off well with water. If the metal stays in the solution for longer than that, the finish will just build up on top of itself, have nowhere to go, and easily flake off.