Alkali works illustrating the Leblanc proces, late 19th century.
© Science Museum / Science & Society Picture Library
Model. Alkali, in the form of soda (sodium carbonate), has been used for centuries in the manufacture of glas, textiles and soap. Until the end of the 18th century, soda was extracted from the ashes of either seaweed or the plant barilla. As these sources became scarce the price rose, so an alternative method of manufacture was sought. Here, salt is heated with sulphuric acid, giving sodium sulphate and hydrochloric acid gas. This highly corrosive gas represented a big pollution problem in the early 19th century.