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New cat was apparently nervous and used an old horn as a urinal (sounds funny, but is not, in person). I did not see this for a while. I used some dish detergent to take off the caked-on residue, which was green. It left some pitted-looking areas, also green.
I am concerned that anything green will turn into a hole in the brass. I know we have some chemists in the house. Can I take advantage of the ph of cat urine (whatever that may be, I suspect acidic) to clean the stuff off of the brass with something opposite of that ph?
My goal is to protect the horn as gently as possible, so I am hoping that it is smart to avoid steel wool, brasso, and attachments that fit into drills (yikes). I know this thread is the all-time repository of straight lines for comics, but I am more interested in avoiding tuba damage than comedy at the moment!

I believe you are seeing the ammonia from the breakdown of urea start to eat away at the brass. I would wash well with dilute vinegar to clean it up. (my aunt loves to clean up cat pee with vinegar, she claims it's the best way to get rid of the smell). Dilute vinegar should not harm your horn.

Just a chemists 2c.