Raku is a type of low-firing process that was inspired by traditional Japanese raku firing. It involves removing pottery from the kiln while at bright red heat and placing it into containers with combustible materials. Once the materials ignite, the containers are closed. This produces an intense reduction atmosphere which effects the colors in glazes and clay bodies. The drastic thermal shock also produces cracking (known as crackling since it is deliberate). Any clay that does not have glaze on it during this process will turn black in the reduction process.