Research
Of joined pine boards, oil painted to appear as a bricked chimney aperture.
A fireboard, or chimney board, was a wooden panel designed to cover a fireplace during the warmer months when it went unused. They were especially popular in the late 18th and early 19th centuries. Not only did the board provide a decorative covering for the dirty interior of the fireplace, but it also prevented insects or birds from entering a home through a damperless chimney.
Full research available on request.