Never heard of alum? You’re not alone.Yet, for centuries, alum was vital to the production of coloured cloth. Alum was needed to fix dyes and colours to the cloth. Without alum, the colours ran and faded. With alum, they shone and were fast. As a result, as the Italian scientist Vanoccio Biringucci put it in the 16th century, alum was as ‘essential to dyers as bread is to man’. Corner the market in alum, and you could make a fortune.Manufactured from a rock called alunite that was mined in China, Egypt and Turkey, alum contributed to the fabulous wealth of the Chinese emperors, the Pharaohs, the Byzantines and the Ottomans. A prized commodity in international trade, alum funded the opulence of Genoa, Bruges and late Renaissance Rome. Henry VII cannily exploited it, but it nearly ruined Elizabeth I.But alum also allowed crooks to make debased coins and to adulterate bread, and was at the heart of one of the most spectacular bankruptcy scandals of the 18th century. And the Black Death used the shipping routes of the alum trade to spread through Europe.In this fascinating and eminently readable book, Sydney Thorne has picked out quirky and exciting stories of alum that range across the centuries and across the world, shedding new light on topics ranging from the slave trade to female entrepreneurs while re-discovering a fully-fledged industrial complex on the Yorkshire that was thriving two hundred years before the industrial revolution.