The Tudor Collection Centre Door Knob Smooth Black Iron is ideal for rustic properties with wooden front doors. The knob has an 80mm diameter and projects 80mm from the door. It is sold individually and comes complete with fixings. Smooth Black Iron Centre Door Knob The smooth black iron finish does not have deep surface indentations like many other black finishes - this gives it a more modern feel. It is made from malleable cast iron, treated with a zinc phosphating process and then powder coated to provide a durable finish.