We're just moving into a Georgian Colonial built ~1800; there was an addition added to one side of the house in the early 1900s that was unfinished (used as a workroom, storage, mudroom) that we want to finish. We've discovered, underneath some plywood and insulation, original (or near-original) clapboards that were on the exterior wall that the extension was attached to.

Because they're so unusual, and mostly intact, we want to keep the clapboards exposed on that one wall. The wood is very old, and very "soft." What are some things we can do to preserve it, yet keep its appearance? Or should we just leave it as-is, since it's inside and not exposed to the elements?