To what two objects that can be broken does Creon refer to support his belief that the will of Antigone also can be broken?

Theban King Creon believes that the will of Antigone can be broken. He fortifies that belief by reference to two objects that can be broken. One is 'the stiffest iron, baked to hardness in the fire, that thou shalt oftenest see snapped and shivered'. The other is the temperamental horse 'brought to order by a little curb'.