Millions of Chairman Mao badges were produced during China's Cultural Revolution, and were worn by almost all Chinese people, from Premier Zhou Enlai down to the smallest child. Made in a wide variety of materials (aluminium, plastic, bamboo, porcelain, gold, silver, copper, iron and lead) and with an extensive range of shapes, sizes and designs, they immediately became collectors' items. To give an idea of scale, in China today serious collections...