The introduction of iron - Babcock

It was with great reluctance that piano makers began to use iron in their instruments, and this was particularly true of the English makers. It was thought that the use of iron would affect the quality of the piano’s tone detrimentally. Wood was used but this added great mass and weight. Broadwood claimed that they had used tension supporting bars of iron soon after 1808. From three to five bars were used in grands of 1821, and a combination of metal bars and metal hitch pin plates were used by 1827. This latter combination was referred to as the composite iron frame. The use of iron was necessary to cope with the gradually increasing string tensions that piano makers were using. Ultimately this was to increase the volume of tone these instruments produced, during the early nineteenth century. 

The full cast iron frame was the invention of Alpheus Babcock of Boston USA in 1825. This had the hitch pin plate as an integral part of the iron frame. The frame was first employed in a square piano.