yes i noticed this when i tryed to make a potion mixing vat (might meddle around a bit and see what is possible) but you could allways try:
onactivate, self, ifvariable, player, tin, equalsgreater, 1,
variabletrue, self, ifvariable, player, copper, equalsgreater, 1,
variabletrue, player, addvariable, bronze, 1,
variabletrue, player, addvariable, tin and copper, -1,
variabletrue, client, displayvariable, bronze,
variablefalse, client, centerprint, You dont have enough copper or tin to make bronze,