Thank you to everyone for your suggestions! You've given me the courage to spot-clean it, using a tiny dot of dishwashing soap, distilled water, and a soft brush. I will know more when it's completely dry tomorrow, but this evening I think the stain has at least faded. If I think it's not come out completely and needs more work, I'll try the RetroClean. This was a smudge of soot from the hearth, nothing more. But it did make a bit of a mess!