An anvil is not a lump of dead metal. It is a spring. Drop a 25 mm bearing ball onto a good anvil face from 250 mm and it should bounce back 70 to 90 percent of the way. That figure is the coefficient of restitution, and it remains the most honest single measure of anvil quality. The maths is trivial: e equals the square root of rebound height divided by drop height. The physics underneath is not trivial at all. When the ball strikes, the contact patch is loaded to enormous local pressure over roughly a hundred microseconds. Two things can happen to that energy: - Elastic strain, stored in the lattice and returned as the material springs back - Plastic strain, spent permanently deforming the surface, plus heat, sound and vibration A hardened face maximises the first and starves the second. Anvil faces run around 50 to 60 HRC, hard enough that the blow stays below the yield point, tempered enough that they chip rather than shatter. A soft or work-damaged face swallows the strike in...