Sentence:  Many of the later style fireplaces, most especially of the better class, showed firebacks. These were of iron, and were designed to keep the back of the fireplace from cracking. ... In the Pickering house on Broad Street, Salem, is a quaint <WSD>fireback</WSD> which was made in the first iron foundry at Saugus, now Lynn. 
Meaning of the word marked with <WSD> tags:  A piece of iron that fits into the back of a fireplace to distribute the heat and keep the brick from cracking.