Reduction is the process of reducing the amount of oxygen in the atmosphere. The flaming hot pieces of pottery are taken out of the kiln and put into a container that can be sealed. The container may contain combustible materials that will catch fire when the pottery comes in contact with them. The container is seals while the fire still rages. When the fire consumes all the oxygen in the container and it starts gasping for air, it finds some stockpiles of it in the metals of the glaze, like FeO (iron oxide) or CuO (copper oxide). The flame attacks the glaze and clay bringing with the carbon to the party. The result in the metal is left behind while the oxygen is consumed leaving behind a spectacular show on the pottery piece. In the video I took out the base to this lid prematurely. While the glaze was still molten enough that it could re-oxidize and it happened before my very eyes, the beautiful copper and red flashes re-combined with the oxygen in the air to become the copper blue green patina color. Its pretty cool!