I would like to detect the change of state in copper and copper alloys
from liquid to solid state while casting. The current system
works on a timer (pre calculated based on control paramters - Temp,
freezing point etc)
I am looking at a cost effective, more accurate alternative.

Casting is done using a metal mould instead of sand mould. The metal
mould / insters need to be removed as soon as the copper changes
state.

Have you considered a thermometer connected to a PC that takes the
temperature at consistent intervals and looks for a hiccup?

I'm not sure what the heat of fusion of copper is, but in most
materials it is large enough that you could see the change in dT/dt
(the rate of change of temperature with time).  I used to work with a
piece of equipment called a differential scanning calorimeter (DSC)
that used a reference sample that didn't have a phase change to make
it more sensitive, but I don't think that is necessary for large
amounts of material.

Good luck.