The most famous and desired shades range from a very pale green to deep intense green, often meaning to mimic the green shades of jade. The main color effect is produced by iron oxide in the glaze recipe or clay body. Celadons 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.