Cone 6 "Oil spots" are created by layering two glazes on top of each other. The base glaze is a high iron glaze called Vickery Oil Spot 1. The cover glaze is the VC Satin Black. As the kiln climbs in temperature, the red iron oxide in the base glaze decomposes into black iron around 2,232° (F). As the red iron oxide decomposes into black iron, oxygen is released and pulls a small amount of iron to the surface of the cover glaze and deposits the iron in the shape of a circular spot. The cover glaze is pushed to the side of the circular spot when the bubbling action begins and is deposited as a colorful halo around the black spot. Because the iron bubbling is random, the spots appear all over the surface.