If I were you, I would post this on the Old House forum.

Use Lime-Away. This takes years of tarnish off copper, brass, or bronze in seconds.

Thanks to both. I did list the question under the old house forum.
Is lime away available at grocers or home centers?
Thanks again.

Lime-away is available at grocery stores--usually along with the bathroom and kitchen cleaners.
Before you clean the firebacks, consult with a local museum or house restoration firm. You need to know what the metal is before you put anything on it.
You may be dealing with soot build-up, in which case a good brushing with a stiff plastic brush will bring out the detail (and make a mess, so spread plastic sheets and wear a face mask and goggles. Believe me, I know!)