How did the ancients make ice?

In ancient times, the methods of making ice were as follows:

1, using pit ice method and saltpeter ice making method. In the cold season every year, ancient officials led their subordinates to dig in the frozen river, transported the ice cubes to the ice room, covered the ice with fresh straw and reed mats, covered the ice with rice chaff, leaves and other insulation materials, and sealed the potholes for next year's enjoyment.

The ancients found that saltpeter absorbed a lot of heat when it was dissolved in water, which made the water temperature lower and even frozen. Using the characteristics of saltpeter, it can be made into iced drinks.