How to stabilize this kind of copper corrosion?

[Eleven Cents]

Today I dug an odd coin that looked like a green and crusty copper penny, but rang up a little lower than a zinc penny. Zincs are typically rotted in half here, and copper pennies are a rich dark brown. I thought I could barely make out a Memorial back, so I toothpicked the date and thought I saw 1979. I decided to try cleaning it with lemon juice as an experiment.

After only a few seconds, I gave it a rub with my fingers and brushed it with a soft toothbrush. Now I think it says 1919, which would make it my oldest coin yet. (Not very impressive, I know, but I've only been at this a few months.) I immediately rinsed it with cold tap water to neutralize the acid. I don't have any distilled water on hand.

The remaining corrosion is green and raised. My goal right now is just to stabilize it. Should I soak it in tap water? My water is pretty alkaline. So is the ground the coin came out of.

 

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[Dug]

If it has a memorial back then it is not a 1919. If it is so ate up that you cannot determine whether it is a wheat or memorial then you are fighting a lost cause to try to return any value to it.

 

[Eleven Cents]

To clarify, I only thought it was a memorial before I cleaned it. The wheat back becomes visible when it's wet.

I don't care about restoring collectible value (it's probably worth less than a dollar, anyway) but I'd like it to be visually identifiable for my collection. Right now I just want to stabilize it until I find more coins in similar condition to experiment on. My previous oldest penny was from the '40s, and it came out of the ground with a shiny brown patina and no green crust.

 

[oldsilver]

baking soda