Salt as an Industrial Chemical
An understanding of chemistry grew out of the experiments of the alchemists and the skills of the apothecary. Alchemy tended to involve metals and substances which changed their form when heated, or mixed, etc and in this respect salt was not a promising raw material.
Throughout history salt had been primarily associated with food as a preserver and flavour enhancer. Its preserving properties for meat had been extended to the tanning of leather and the embalming of dead bodies. Only in the eighteenth century was salt to become an industrial raw material for making other chemicals. In the first part of the century sulphuric acid began to be made on an industrial scale, replacing buttermilk as a souring agent in textile bleaching. The 1770’s saw the first use of salt as an industrial chemical with the manufacture on Tyneside of hydrochloric acid from salt and sulphuric acid. The hydrochloric acid proved to be superior to sulphuric acid as textile bleaching agent. However its life as such was cut short by the introduction of chlorine bleaching in the 1780’s, this followed Scheele’s discovery of chlorine and its bleaching properties in 1774. A weak solution of chlorine in alkali was first used, the chlorine gas being produced from salt, sulphuric acid and manganese dioxide. Chlorine bleaching expanded rapidly after Charles Tennant’s invention of bleaching powder in 1797.
Although based on the same decomposition reaction of salt with sulphuric acid, the utilisation of the sodium in salt was come by a different route and remained divorced from the chlorine bleach industry until the mid-19th century. From the late 1760’s the utilisation of salt as a raw material for the production of alkali was explored in Britain by Watt, Black, Keir and others but it was Nicholas Leblanc in France whose process of 1783 and patent of 1791 formed the basis of the 19th century alkali industry bearing his name. By the end of the 18th century salt had thus become firmly established as an industrial chemical. Its importance in this role, together with the gross inefficiency of the Salt Office, was the main reason for the abolition of the Salt Tax in 1825.