Is it real brass?
If it is real brass and looks like real brass chances are it has a lacquer on it. The reason I say this is that brass will tarnish fairly quickly and can turn black. What keeps the brass from tarnishing so fast(and keeps it shiny and gold-ish) is the lacquer which keeps the air from tarnishing. Eventually (takes years), that brass shine will tarnish even underneath the lacquer though. The way to deal with that is stripping the lacquer... then shine up with a brass cleaner once that lacquer is off, and then re-lacquer. Sometimes something will look like it is solid brass but is actually brass plated. I think you could still try the above method though, and you will eventually figure out what metal is all the way through. In either case, you could try shining it up with a brass cleaner, if nothing happens to make them look better, you will need to strip it. That means there is lacquer that needs to be stripped to get to the brass. Then try to brass cleaner it again. (TRY BRASSO cleaner) If still nothing looks better, then the metal is probably a cheap metal that has been painted to look like brass and can just be repainted with a rustoleum spray paint and finished off with another lacquer or polycrilic finisher.
Here is some info
Hi, this may help... https://www.stretcher.com/stories/04/04dec20a.cfm#:~:text=Dip%20half%20a%20lemon%20into,to%20buff%20the%20shine%20up.
I use Young Living's Thieves cleaner
Here's a helpful video: https://www.youtube.com/watch?v=DXuW1NEcHKM
Do you mean the firebox’s surround is brass or the actual fireplace has a brass insert?
Here is a fun idea: http://www.thegarciahouse.com/2017/06/06/clean-brass-one-simple-ingredient/
Use Brasso or metal polish.
Brasso cream cleaner is a very good cleaner for brass
Hi! If it has a clear sealer on it, you may just need to give it a good cleaning to remove any soot buildup. If it is real brass, and the sealer is gone, you will need to use Brasso or another metal cleaner. If it is only brass plated, it may be needing a refinish. Once the base metal starts showing through, the finish is wearing off. Good luck and stay safe!
Hi Simon: There are a zillion ways, but these 2 work for me: I use Brasso. It's a multi-purpose metal polish for a variety of metals including brass, copper, stainless steel, chrome, pewter, and bronze. Brasso Metal Polish is so gentle on most metals, it will not scratch or mar them, however, it is not recommended for use on aluminum. I also use Barkeepers Friend. Just wet a sponge, put a bit of Barkeepers Friend on it and rub the area gently. It cleans in a flash! I use it for my copper bottomed pots. Just rinse and dry.
Be aware that vinegar is acidic and can ruin brass if the solution is too strong with vinegar and the brass allowed to remain too long in the solution. If the solution turns blue, copper is being removed from the brass and the loss of copper will make the brass softer therefore weaker. Good luck