On seeing the new pen one immediately notices the case-hardened metal parts and the fine engravings. Until the late 19th century, case hardening was the sign of a high-quality gun, with the inimitable shimmering coloration it provides. For centuries, the technique was handed down by word of mouth and was something of a secret art. The metal parts are usually packed in carbonized leather and heated to convert the surface into steel; the colours appear on cooling. Some 20 parameters contribute to the beautiful visual effect, including the thickness of the metal, the temperature, and the rate of cooling.