The enamelled inner surface of the pan is essentially a glassy layer painted on a metal base, which is usually cast iron or steel. The main characteristic of this surface is that it does not react with food. As a result, acidic foods such as tomato sauces or dishes with wine can be prepared in the pan without the metal affecting the taste or damaging the surface of the pan. Unlike raw cast iron, enamel pans do not require the so-called 'firing' process, i.e. the formation of a protective layer of fat, and are ready for immediate use. Cleaning is easier because the surface is smooth and non-porous. Although enamel reduces food sticking, it is not a fully non-stick surface like Teflon pans and therefore fat must be used for most cooking. The surface is hard and scratch-resistant, but can crack if subjected to a violent impact or large thermal shock.