Is this BD?

[maridvnvm]

I have just been updating some photos of coins in my collection and have what looks like BD developing on one of them. This is one of my nicer coins and the first time I have ever noticed anything like BD in my collection. So my simple question is:-
Is this BD?
Regards,
Martin

[areich]

I'm sure it is.

[maridvnvm]

Unfortunately, so am I, and I have never seen it there before. It is one of my prize coins, so I will have to learn how to treat it. I know the process has been discussed here many times so I will have to find it!
Aarrggghhh!
Martin

[maridvnvm]

Here is an image of the coin from last year some time and there was no sign of it then:-



Regards,
Martin

[areich]

Sometimes it is not visible in hand, but shows up quite clearly in a scan. The bd on your coin you could just brush off. Unfortunately I have never been able to treat BD, even with the quite clear instruction found here.
The only thing that happened was that some coins got ugly red corrosion from the distilled water.

[awl]

Try heating it.

[moonmoth]

The chances are that the coin has corroded invisibly beyond the area you can see green encrustation on.  If you boil it in a mix of sodium carbonate and bicarbonate, it will remove the BD and probably leave hollow areas with a soft red deposit which will brush off.  This is painful to see, but as far as I have been able to discover, the best way to go to prevent more corrosion.  Of course, it's your decision.