Dear Chris,
It is an unfortunate start. As Areich said these crumbly
patina types are very tough. The first trick is to recognize them, and I did what you did to a few before I learned to recognize them.
Regular
bronze disease (or I should say a regular case of
bronze disease) normally leaves a few areas of light, bright green. When look at close up this will be soft and fuzzy. It is often in pits but can sometimes swell up and hide the pit. These coins are usually
still stable and you can tackle the BD
bit by
bit.
The situation your coin was in is a whole different kettle of
fish. The entire coin body is soft and crumbly, only a thin layer of the original surface remains like hard icing on a cake.
These
types usually show lots of damage at the edges. By looking with a magnifying
glass you can get a clue about the scope of the problem. Gently prod
part of the exposed green stuff with a hard toothpick, bamboo skewer or steel pin and you will see how soft it is.
As Tkon Nova says you then avoid cleaning and
flip it. I find that these coins can continue to crumble as friction in the
flip damages it. I have started to (gently!) wax these coins. I know that
seals in the BD but I think the BD has already run its course in these
types. The wax helps hold what remains of the surface together and reduces further flaking.
Shawn