Alloy are relatively similar in size, the atom exchange method usually happens, where some of the atoms composing the metallic crystals are substituted with atoms of the other constituent. This is called a "substitutional alloy". Examples of substitutional alloys include bronze and brass, in which some of the copper atoms are substituted with either tin or zinc atoms respectively. In the case of the interstitial mechanism, one atom is usually much smaller than the other and can not successfully substitute for the other type of atom in the crystals of the base metal. Instead, the smaller atoms become trapped in the
Liberty Bell Museum found a considerably higher level of tin in the Liberty Bell than in other Whitechapel bells of that era, and suggested that Whitechapel made an error in the alloy, perhaps by using scraps with a high level of tin to begin the melt instead of the usual pure copper. The analysis found that, on the second recasting, instead of adding pure tin to the bell metal, Pass and Stow added cheap pewter with a high lead content, and incompletely mixed the new metal into the mold. The result was "an extremely brittle alloy which not only caused the Bell