The Tudor Collection 3622 Cupboard Knob Smooth Black Iron has an attractive design featuring square detailing on the face of the cupboard knob. The diameter is 32mm and the projection is 27mm - it is sold individually and comes complete with fixings. Smooth Black Iron Cupboard 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.