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Copperas is also known as green vitriol and is a form of ferrous sulphate extracted from iron-pyrite rich nodules. These occur naturally within Eocene clay deposits and can be found off the coast of Essex in the London Clay (http://www.eng -h.gov.uk/archcom/projects/summarys/html97_8/2059.htm).
The copperas industry was particularly based in Harwich and Ramsey, near Walton. The copperas nodules were weathered for up to six years until they produced a dilute solution of hydrated ferrous sulphate and sulphuric acid which was and reduced by evaporation before being tapped off into a cooling tank to obtain the ferrous sulphate crystals.
Copperas is the foundation of the modern chemical and pharmaceutical industries (Williams and Brown 1999, 21). It was used in the metallurgical industry and extensively in the textile industry which was so important to this area in the post medieval period. Copperas was used to make chlorine, used as bleaching agent in the 17th and 18th centuries and as a dye fixative for woollens. Other uses include as printers ink, a tanning agent for leather and in the manufacture of gunpowder (http://www.english-heritage.org/). The copperas industry is considered to be the first heavily capitalised industry (Williams and Brown 1999, 21).