Wood and gas kilns depend on air movement to create a greater amount of heat within the kiln. This is done by leaving gaps for air to enter the heating area. Once the air enters, it must also have a place to escape. Spaces are left at intervals on the top of the kiln as well as the bottom. Depending on how the gaps are placed, the wood or gas will burn hotter and more efficiently. Some kilns are designed for the draft to go up and others for the air drafts to go down. It is the choice of each individual kiln operator to design and build their kiln for the type of firing they need.