I will tell you same thing as shylaah...
I use "Ammoniac" in french.
Easy,display some fluid in a closed box.
Then display the brass part above,no contat with fluid.
will act with vapour.
Depending on temperature of fluid and part,it can take from a few seconds to a few hours.
If you want a fast action,easy,display the piece in the freezer a few minutes,and put the container with ammoniac on radiator,then,put the brass part in the container,will work in a few seconds...
Advantages are the short time needed for operation,and the quality of results,really neat and regular.
Of course,you can leave brass above ammoniac for a long time,will give dark patina,from dark greenish brown on brass to a black-blue color on bronze.
It's possible to remove partially and do a new treatment,to add contrasts,or create damaged items.