It's often difficult to say if a coin has BD without looking at the coin in person. In general, the "is it soft and powdery; does it flake off when scraped with a fingernail" test is good. However, I've had some coins which had non-flaky light or medium green patches which were BD... those coins had been waxed at some point in the past so the topmost layer of green corrosion wasn't flaky. It was held firm with the wax or whatever coating was on the coin while beneath that tenuous layer rapid corrosion was taking place. @Evan8, your coin looks like it had metal loss from BD at some point in its past. I'd keep an eye on it and of course store it in a dry place or with desiccant. If in the future it shows any signs of accelerated corrosion, I'd soak it in a solvent to remove whatever coating might be on the coin, then treat with Verdicare or a more aggressive protocol. Wouldn't be a bad idea to apply some Verdicare now anyway. @Nathan, your coin worries me (the spots marked by the arrow plus various tiny spots). As noted above, sometimes oils or waxes can keep BD powder from flaking off. If this were my coin I'd soak it in a solvent and treat with Verdicare to be on the safe side. @Roman Collector, I have the same concerns for your coin. When I see that lighter but non-flaky green I wonder if it is BD powder that has been "solidified" with a topical treatment. The usual disclaimers apply: I am not an expert; I may be wrong; etc I have however had and treated many coins with BD.
@TIF since Verdicare has been mentioned how do you like to apply it to bronze? The instructions say not to let it dry on the coin, but when I attempt to wipe it away after a few minutes I feel like I’m just removing the Verdicare. Do you let a thin layer dry on the surface?
If I'm "treating" the coin, I flood the surfaces with Verdicare and let it sit on a plate, flipping it occasionally so the top side can soak more. After whatever period of time (minutes, hours, a day) I rinse it with distilled water and let it dry. I have on occasion skipped the rinse, if I was only applying a thin coat. There is no science behind my method
