temperer

/ˈtɛmpərər/
noun
  1. A person or thing that tempers something, such as making it less extreme or strengthening it.
    • The machine is a temperer that treats glass to make it safer for use in buildings.
    • In the workshop, the temperer carefully controlled the heat to make the metal strong.
    • She acted as a temperer in the debate, softening harsh words between the two sides.