What kind of liquid is the glaze on the ceramic surface?

Glaze is a glassy coating on pottery, and its main uses include decoration and protection. The composition of glaze varies greatly, but it usually includes a mixture of many materials, which are usually fired in a kiln at a lower temperature than the pottery they cover. An important function of glaze is to make ceramic containers prevent water and other liquids from leaking. Glaze on pottery can be added by kneading, spraying, soaking, dragging or brushing a mud film composed of glaze and water. The method of brushing glaze often produces uneven coating, but it is as effective as decoration technology. The color of glaze will change obviously before and after firing. In order to prevent glazed pottery from sticking to the equipment in the kiln during firing, one method is to unglaze a small part, such as the foot, or support it with a particularly refractory spike. They will be removed after launch. Sometimes a special glazing method is carried out in the kiln. One example is salt glaze, where salt is added to the kiln to form mottled orange peel texture. Besides salt, other substances will also be added to the kiln, including sulfur. The coal ash in firewood kiln fuel can form gray glaze on the surface of pottery.