There's a solution for this: PATIMEER ©
Detect has found a manufacturer that can supply PATIMEER in small handy pots. There's a green paste for copper coins and
an orange-brown paste to repatinate copper objects. The brown paste creates a darker effect. With one pot, you can treat about 250 coins; it is very economical to use.

We're working on a short video that will demonstrate the entire process.

1. After purchasing a pot, try the product first on a Dutch Stuiver (not a Euro Stuiver, but a genuine copper Stuiver from the Gulden era).
2. Clean the Stuiver with a fine copper brush, meaning completely blank. It may feel like vandalism, but you need to push through.
3. A quick and radical cleaning method is to wrap the coin in aluminum foil, put a spoonful of caustic soda (drain cleaner) in a glass, and pour hot water over it. This should be done outdoors as unpleasant fumes will arise (do not inhale). Rinse the coin with clean water, remove it from the foil, rinse it again, and then use a fine copper brush on it until it's blank.
4. Then briefly heat the coin with a lighter; the heat accelerates the action of the Patimeer agent.
5. Dip a small cotton cloth into a very thin layer of Patimeer paste and rub it in well.
6. After a few seconds, you'll see a patina layer forming. By rubbing it with a cloth, the raised areas become darker, and the recessed areas remain light, making the image stand out beautifully and clearly.
7. Patimeer is not paint but a substance that undergoes a chemical reaction with the bare copper. It's an extremely fast natural method that would otherwise take years. The substance acts on the coin and is not removable.
8. After a few days, the patina layer becomes permanent.

Note: The Patimeer agent is not aggressive, but because you're working with copper and copper sulfide, it's advisable