故宫内的殿宇楼阁均为砖木结构，很容易着火。铜大缸价格，一旦发生火灾，如果不能及时扑救，火势就会迅速蔓延，把这座价值连城的辉煌建筑顷刻之间化为乌有。为此，紫禁城的建设者们对防火特别重视，
在宫殿前设置了这些大缸，称之为“门海”。门海即门前的“大海”，门前有了“大海”，以水克火，砖木结构的故宫就不怕火灾了。因此，这些铜缸又被称作吉祥缸。每尊缸可贮水3000多升，如同一座座小水库。
The buildings in the Forbidden City are all brick and wood structures, which are easy to catch fire. In the event of a fire, if it is not dealt with promptly, it will spread rapidly, destroying the priceless edifice in an instant. For this reason, the builders of the Forbidden City pay special attention to fire prevention, in front of the palace set up these tanks, known as the "door sea." Door sea namely before the door "the sea", before the door had "the sea", with water gram fire, the imperial palace of brick and wood structure is not afraid of fire. Accordingly, these copper crock are called auspicious crock again. Each tank can hold more than 3,000 liters of water, like a small reservoir.