Before electricity, central heating, and down quilts, winter in ancient China was not simply a season but a test of class. Cold was shared by everyone, but warmth was usually not. During the Han dynasty (206BC–AD220), emperors used specially designed “warm chambers” to protect themselves from the winter cold. The walls were plastered with a mixture of mud and Sichuan peppercorns, thought to preserve heat while resisting dampness and insects; curtains and screens helped keep out draughts. These...