What has a mill to do with urine?

This is Bourne Mill in Colchester which has been owned by the National Trust since 1936. It was built in 1591 from stone 'borrowed' from the nearby St. John's Abbey by Sir Thomas Lucas. He built it as a fishing lodge where he no doubt hosted many a banquet. It was then converted into a water mill and a fulling mill.

Fulling mills originated in Persia over a thousand years ago and by medieval times were widespread across Europe. Basically these mills were used to pound woollen cloth in water and 'Fullers Earth' to remove oil and dirt and then to 'fluff' it up and make the cloth thicker.

The process was invented by the Romans who instead used human urine which contains ammonium salts. The slaves would stand up to their knees in urine and tread it into the cloth. This of course made urine a sought after resource and the Emporer Vespasian put a tax on it! The urine was collected from the Cloaca Maxima sewer system and tax collected from whoever bought it. Vespasian's son, Titus complained about the disgusting source of income, however his father showed him a gold coin and made him smell it. Hence comes the saying 'Pecunia non olent' (money does not stink).