How did the ancients purify alcohol

Distillation.

Archaeology has seen the discovery of a strange circular-shaped remnant that, at first glance, looks a bit like a water well. Archaeologists have finally concluded that this is the earliest identifiable physical object for producing distilled spirits in China.

Then on the pedestal there was a huge heavenly cauldron, which was divided into two layers, the lower one containing the mother liquor, and the upper one containing cold water. The wood fire on the pedestal flourished, steaming the mother liquor, and the gas containing alcohol was cooled by the cold water above, condensed into liquid, and flowed out of the pipe, which was distilled liquor.

Expanded information:

Purification is actually to increase the alcoholic strength, and in ancient times, it was also done through the distilling method to increase the alcohol content and remove impurities. Archaeology at ShuiJingFang confirms that China had a very sophisticated distilled spirit brewing technology by the end of the Yuan and the beginning of the Ming Dynasty at the latest, and that the traditional process was accomplished using a still commonly known as a tianpian (heavenly pot).

In ancient China, due to the long history and geographical variations, there are many names of distilled spirits passed down, but the name "baijiu" in ancient literature does not refer to distilled spirits. It is a kind of brewed rice wine, as mentioned earlier. It is only in modern times that baijiu is used to represent distilled spirits.

China's ancient literature in the distilled spirit of the main title: Shochu, Shochun, began to be used in the Tang Dynasty, but the Tang Dynasty, Shochu, Shochun whether it refers to the distilled spirit is still debated. After the Song Dynasty, shochu, shochun is the real distilled spirits.

Baidu Encyclopedia-Distilled Spirits