My knowledge, which might be patchy, tells me there's too many
confounding variables here. What's considered the metal's "natural
hardness"? Cooling by oil/water quenching versus cooling with
ambient/flowing air of various temps and wind speeds from liquidus to
solidus affects hardness. Give us torches and kilns and we can modify
hardnesses all day long. Aren't metal hardnesses measured in Rockwell
and/or other units? Brasses/bronzes vary by hundreds of degrees in
melt point, depending on alloy mix.
Maybe concentrating on fine versus sterling silver and the effect of
alloying on melt point would be a more manageable hypothetical.
With metals prices rallying, a piece on acid testing would be fun.
That was my sci-fair project that never was.
Maybe just giving different brasses the comparison?