Reduction is firing where the kiln atmosphere has insufficient oxygen for complete combustion (more fuel than air to burn it). At high temperatures, this produces carbon monoxide gas, which will steal loosely‐bonded oxygen from other materials, in ceramics mainly iron and copper.
Most ceramics glazes are simple to formulate and very easy to fire. But there are some glazes and glaze effects that have the reputation of being elusive or at least difficult to obtain. This was certainly the case early in the 20th century with copper reds and celadon blues. These glazes had a history of secrecy and myth surrounding them. One story even claimed that a Chinese emperor had slaves thrown into the kiln during the firing in order to produce the elusive copper red...
