The Tudor Collection Ring Door Knocker Smooth Black Iron is ideal for rustic properties with wooden front doors. The knocker has a 107mm outer diameter and a 77mm inner diameter. It is sold individually and comes complete with fixings. Smooth Black Iron Door Knocker 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.