Tunnel Kilns are continuously fired kilns used in larger factories. It is used for flow line production methods were the truck of loaded ware is loaded every hour or so into one end and removed from the other. A pusher device gradually pushes the newly introduced truck and a row of others in front of it - one truck length and then retracts to permit another load to be charged. The firing zone is at the centre of the tunnel and is maintained at temperature for months or year, or until maintenance forces a shutdown. Such kiln structures absorb a lot of heat in bringing up to temperature, which is then wasted when the kiln is cooled hence the need to keep them in continuous operation. Most tunnel kilns are gas-fired or oil fired. Tunnel kilns have made the type obsolete except in the largest concerns.