Reduction fired stoneware
The story goes that a chinese potter was once opening a kiln which he had loaded with copper green glazed pots. He couldn’t believe his eyes, the pots had all fired blood red. Nobody had ever seen red glazed pots before.
The astonished potter quickly sent one off to the emperor to show him what the kiln gods had done. The emperor sent back an order for a thousand more bright red pots.
The potter did his best but to no avail. After 2 kiln loads of green pots he lost his nerve, instead of facing the wrath of the emperor he simply jumped into the kiln during the next firing. The next day the potters assistant opened the kiln and to his surprise found the potters ashes surrounded by bright, blood red pots. The assistant became the most famous potter in the empire for his copper red glaze. His secret was to always throw a pig into the kiln half way through the firing.
We now know that to get copper oxide to turn red instead of green or to have a great number of profound colour changes in ceramic glazes, all that is needed is a manipulation of the kilns atmosphere. Carbon monoxide is introduced into the kiln just before the glaze starts melting, this reduces the number of oxygen molecules attached to oxide molecules and changes their colour.
The glaze then melts and encapsulates the oxide molecules in a glass matrix which means that they can no longer reoxidise. This process is done in a flame kiln as opposed to electric and achieved simply by blocking off the chimney to a point where there isn’t any oxygen in the kiln.
Too much “reduction” and unburned carbon from the smoke and flames can trap in the glaze and look ugly. Not enough “reduction” results in “oxidised” reduction-glazes which invariably look flat and bland.