At some stage in the nineteenth century, the rigid swords were replaced by flexible rappers in the coalfield. Very little is known about this major development in the tradition because of an absence of sufficient evidence, and it may have been entirely accidental. It used to be commonly thought that the flexible version was used for removing dirt from the backs of pit ponies, but there is no available documentation to verify this theory. It seems that two factors have influenced interpretations of when the rapper sword was introduced: firstly, the relocation of a steel works operation by Ambrose Crowley to the Derwent Valley, before which time it is believed that suitable steel for flexible swords would most likely have been unavailable; and secondly, the prohibitive expense of such instruments before the Bessemer process in 1855 that allowed steel to be made inexpensively.