How did ancient people make ice?

The saltpeter ice-making method was first seen in the Tang Dynasty. As mentioned earlier, the population of the Tang Dynasty increased dramatically, and the way of ice storage was far from meeting people's needs in summer. Therefore, a new method is needed to increase the amount of ice.

At that time, in the process of mining gunpowder, people inadvertently found that saltpeter dissolved in water would quickly absorb a lot of heat in water and freeze the water, so there was saltpeter ice-making method.