How to protect a mantle?

I have a beam of old butternut that has an oil finish over my fireplace. I am worried that it will split now that we are starting to use the fireplace. The beam is supposed to be old, but who knows, I bought it from through the web. Any suggestions for protecting it? Do I need to find out exactly what was used for the original finish? Should I put some kind of heat shield on the bottom?
-Steve W

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