Iron clad copper so they don't dissolve in the flux.
As stated above they are magnetic which holds the relay in when under temperature. On reaching the Curie point the field drops and switches the heater off.
My new Weller double iron goes to sleep when no motion is detected and the tips last for ages. The only issue I have is soldering surface mount stuff when there is very little movement going on and iron goes to sleep. All of a sudden there is no heat and you have to wave your hand around.
regards Martin