The Tudor Collection Rectangular Door Knocker Smooth Black Iron is ideal for rustic properties with wooden front doors. The knocker measures at 167mm x 40mm and is sold individually, 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.