The bell furnace produces high quality products, and all the additional investment can be quickly recovered due to the high quality products. The control of temperature and atmosphere, the design of the kiln and the control of the air flow in the kiln are all perfectly combined to ensure a uniform temperature when the product is heated and cooled. The control of the furnace atmosphere during cooling is directly related to the furnace temperature and guarantees an oxygen content of 0.005% or even lower.