These are silvered
bronze and lemon juice would not be a good
idea. Only mechanical cleaning will really help
. You will see more details if you look under the cleaning silvered
coins sticky in this topic.
If the dirt is soft you can use a knife to carve small wooden or plastic chisels from bamboo skewers, popsicle sticks or plastic knifes. You can then use these to scrape the dirt off. If greasy you may need to clean the "chisel" on a paper towel each time.
If the dirt is harder you may have to use a scalpel.