The Tudor Collection Bell Push Smooth Black Iron is designed to be used with a battery or transformer fed door bell. It measures at 103mm x 32mm and is sold individually, complete with fixings. Smooth Black Iron Door Bell 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.