Wrought iron as an engineering material passed its peak in the 19th century. In line with other manufacturers the Whitechapel Bell Foundry replaced wrought iron components with those made from either steel or grey cast iron but with the notable exception of clappers where the material remained in regular use until the early 1970s. The relatively high cost of the material coupled with the lack of consistent quality were disadvantages offset by the convenience of its forging characteristics and its essentially ductile properties; a necessary feature for withstanding the impact stresses during ringing.

Even by the early 1940s supplies of genuine wrought iron were fading fast as were the number of companies able and willing to forge the bottom end sections of clappers ready for use. As an experiment, Whitechapel produced composite clappers comprising grey cast iron balls and flights cast on to round section wrought iron stems. The experiment was however unsuccessful as the ball ends tended to work loose. The experimental design was abandoned within two years and although most of these composite clappers have long since been replaced a few still survive to this day.

In 1959 Whitechapel's supplier gave notice that wrought iron would probably only continue to be available for a further 10 years and that in its place they recommended EN16 steel which could be forged, machined and welded. One such clapper was immediately manufactured and fitted to the tenor of a ring in North London where it is still in regular use. Although the clapper has proved successful, the cost of production was considered unacceptably high.

Whitechapel continued its search for a replacement material until, in 1970, and when designing special 18th century style ringing fittings for full size replica Liberty Bells, the company was introduced to ductile iron (also referred to as spheroidal graphite iron) for the manufacture of some of these special components. The techniques involved in causing the graphite in iron to freeze in spherical balls within the iron matrix rather than into thin razor like flakes had been researched and perfected by the British Cast Iron Research Association (BCIRA) during the late 1940s. Initially, the discovery was put to no commercial use, however by the early 1950s the American-based Ford Motor Company was employing the material for the manufacture of petrol engine crank shafts as both the material and the manufacturing processes were far less costly than the steel forgings that it replaced. By 1970 the material had become commercially available and today well over half of all iron castings produced worldwide are made in ductile iron.

A number of grades are available and one or two of these have mechanical properties that are remarkably similar to those exhibited by the best of genuine wrought iron which is perhaps surprising in that wrought iron is associated with the forging process whereas ductile iron is employed only in castings.

Working closely with BCIRA Whitechapel established the precise nature of clapper failures through a series of destructive tests, and from the results of these tests was able to select the grade of iron most suitable for clapper production.

As a casting material, the opportunity presented itself to produce clappers of a radically different design in which stronger components could be produced employing less material. In spite of this obvious advantage, it was eventually decided to model the new clappers on their wrought iron predecessors so that they were visually indistinguishable. This would allow new clappers to be interchanged with old without any significant change in appearance. As interchangeability was considered to be a major requirement, several of Whitechapel's change ringing installations from the 1970s have a mixture of the two types of clappers.

Although most clappers now supplied by Whitechapel are of ductile cast iron the company has not overlooked its knowledge and experience with wrought iron, EN16 and the alternative design possibilities of ductile iron. Whitechapel continue to make daily use of their skills in the working and reworking of genuine wrought iron clappers and at the time of writing are investigating the supply of irons similar and interchangeable with wrought iron but with higher levels of mechanical consistency.

Clappers manufactured from EN16 are still provided for larger bells where their relatively high cost is justified by improved mechanical properties in resisting the reversed impact bending fatigue failures associated with most clapper breakages.

The clapper fitted to Great Paul at St Paul's Cathedral London, the largest swinging bell in Britain, illustrates how a beam type stem section may be more efficiently employed in making best use of the mechanical properties of ductile cast iron.

By 1977 Whitechapel was not only supplying ductile iron clappers to its own customers but additionally to all other UK bell companies. What had started as a Whitechapel design had become, in less than a decade, the accepted industry standard and, over a quarter of a century later, this remains true today.