C25    Discuss why repaired amalgam restorations (if such a technique is used) should be retained by mechanical retention and not only by chemical bonding between the new and the old amalgam.

A set amalgam consists of unreacted particles and a matrix (mainly the gamma-1 phase, Ag₂Hg₃). The surface contains less than 50% unreacted alloy particles. When new amalgam is added to this surface, the mercury present in the newly mixed amalgam can react with the exposed unreacted particles of the old amalgam. However, because the number of such particles is limited, one cannot reach nor exceed  50% of the  strength of an unrepaired amalgam if an old amalgam is repaired.  This explains why any repair of an old amalgam needs to be mechanically retained like a separate restoration.