The ancient methods of making ice are as follows:

1. Use ice cellar method and saltpeter ice making method. During the cold season every year, ancient officials led their subordinates to dig on the frozen river, transport the ice cubes to the ice cellar, and use fresh straw and reed mats as bedding. After placing the ice on the bedding, it was covered with heat-insulating materials such as rice bran and leaves. Seal the cellar door and enjoy it next year.

2. The ancients discovered that when saltpeter is dissolved in water, it absorbs a large amount of heat, causing the water temperature to drop and even freeze. Using the properties of saltpeter, you can make ice drinks.