Complex sentence: The center's principal artifact is a 21 by 30 foot (6 by 9 m), two-story log slave pen built in 1830, which was used to house slaves prior to being shipped to auction.
Split into simpler sentences: The center's principal artifact is a 21 by 30 foot (6 by 9 m), two-story log slave pen built in 1830. It was used to house slaves prior to their being shipped to auction.