Celadon is a term for ceramics denoting both a type of glaze and a ware of celadon (color). This type of ware was invented in ancient China, such as in the Zhejiang province. Celadon has since been spread to various regions in Asia, such as Northern Thailand. Celadon glazes can be produced in a variety of colors, including white, grey, blue and yellow, depending on several factors: 1) the thickness of the applied glaze, 2) the type of clay to which it is applied, and 3) the exact makeup of the glaze. However, the most famous shades range in color from a very pale green to deep intense green, often meaning to mimic the green shades of jade. The color is produced by iron oxide in the glaze recipe or clay body. Celadon are almost exclusively fired in a reducing atmosphere kiln as the chemical changes in the iron oxide which accompany depriving it of free oxygen are what produce the desired colors. As with most glazes, crazing (a glaze defect) can occur in the glaze and, if the characteristic is desirable, is referred to as "crackle" glaze. The Longquan kiln sites in China were especially well-known internationally. Large quantities of Longquan celadon was exported throughout East Asia, Southeast Asia and the Middle East in 13th-15th century. Large celadon dishes were especially welcomed in Islamic nations.