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It's the sulphur in the egg yolks which is the secret. It will take more time, but it's a lot less toxic than using Liver Of Sulphur, which is the standard oxidizer.

A few different methods described, including the vinegar + salt one.

Add a patina using Vinegar and Salt. A remark in the comments says that the mixture needs to stay on for at least two days. Vinegar-and-salt gives a different effect than the Ammonia-and-Salt patina (no wonder, one is a base and the other is an acid)\012However, the patina is very crumbly and fragile, and needs to be heavily laquered to stay on.

Add a patina using Ammonia and Salt. No need to go and get liver-of-sulphur.