You could always run one grate through the self cleaning cycle and see what happens. At worst, you would have to buy a new grate that would be the only shiny one.

It may also be that the self cleaning cycle would not heat it as hot as the direct flame impingement does and nothing useful happens.

An alternative test would be to put a grate upside down and offset on a normally-placed grate so that the flame impinges on a gray area and see what happens after a period of being flamed. If that works, then broiling each grate at close proximity to the gas flame might work (given a gas oven or gas salamander).

Please note that I have no confidence that heating will restore the glossiness but I see little risk in experimenting to find out.

kas

Thanks for the suggestions, kas. Or I could find a friend with a ceramics kiln!