Here's a quick and simple test... next time you build a fire, place a candle where the clock would normally set. If the candle survives the heat intact, your clock should be ok too.

I'd just make one note of caution... the extra heat may cause the wood to dry out, over time. Cleaning the clock with Murphy's oil soap on a regular basis will help prevent that, as well as prevent a soot build up, heheh. If you only build a fire once in awhile, it shouldn't be a problem.

Have Fun

Should be no problem as they were made to be on the mantelpiece in the years when all folks had coal range fireplaces.