Is the gold part what was cleaned? If so, I think it would look great cleaned, but if you hate tht look, then I would just stop.

Yes that's the part that was cleaned and it never looked like that even when 'new' although it was already old when my parents bought it. It just looks wrong to me but thanks for saying it would look good that way.

Is ti metal or gilded gesso? What did you clean it with?

The mirror is metal. I always thought it was tin but this one on 1st Dibs is either brass or copper - why they don't know seems odd. I used a metal cleaner - obviously too strong.

A 19th century Dutch Baroque style mirror with bevelled central glass and gold coloured metal frame with embossed, sculptural decoration. The raised designs were handcrafted using a technique known as “Repoussé” in which the metal (in this case either copper or brass) was shaped by hammering from the reverse side, creating decorative elements in low relief when viewed from the front.

https://www.1stdibs.com/en-gb/furniture/mirrors/wall-mirrors/19th-century-dutch-baroque-style-repousse-metal-framed-mirror/id-f_18732072/

Beautiful mirror! Maybe there’s a dark Rub n Buff color that would match? Or look close to the darker color?

Jilly that's the solution I have in my back pocket. It appears now that the color is oxidation over time. I'm going to do some research and see if there's an option to brighten it a little without removing ALL the patina.

I think patina is over-rated. If you have a really valuable and very old piece of furniture, it makes a difference, but for most things, a cleaning improves them. Few people want grungy decor.

Well, it is old, if not antique by definition then close, and it does have value although it couldn't fund retirement or anything. It's clean, not grungy but the metal is oxidized. I like natural brass and the way the color softens with age but this is unrecognizable as brass - if that's what it is. It's possible there was some kind of finish applied to the metal - whatever it is - to mimic age when it was made. I was hoping one of the antique 'experts' here would know if that's possible and chime in.

Well if it's brass you're fine. Brass does not rust like something with iron in it like tin.

Both copper and brass are that way. They turn dark by oxidizing. It's like a brass bed frame, will turn dark, but can be turned to bright brass again with a metal polish. Especially a mild acid will remove the oxidation and brighten it up. So it looks like you removed oxidation. When it was new it may have been dark as they deliberately applied something to give it a uniform oxidation layer.

I would try applying an oxidizer to darken the bright area. Or else you could clean the entire thing to bright brass then oxidize the whole thing if you're having trouble making it look uniform.

Do a search for "brass darkening agent" and you will find some products.

Or maybe this: METAL DARKENING SOLUTION – | Brass | bronze

Used to antique or darken brass, copper, bronze, or tin. Easy to apply and causes no dimensional change, the gradual color change lets you control the color.

Brass Ager

Unless it originally had a coat of paint on it? I can't really tell.

@Michael Hilber I think you hit on it. I'm betting the original mirror had a darkening agent applied somewhat selectively to highlight details and it has continued to darken through the years to a largely uniform and very dark finish. Except where I polished it down to bare metal. I'm going to look into those products and see what might be the best fit. Thank you very much for chiming in.