In the late 17th century it was discovered that soda and potash, important chemicals in the soap and glass industry, could be extracted from burning seaweed.
After being cut, the seaweed was carted up from the shoreline and dried on the machair. It was then burned in large, often stone-lined trenches (or circular pits in Orkney) for four to eight hours. The men would then beat the weed into a mass using 'kelp irons' (long-handled iron mallets or hooks). This would then be covered with stones and turf, to protect it against moisture, and left to cool overnight. The following morning the pieces of kelp ash would be broken into lumps and transported by ship to Leith, Dumbarton, Glasgow, Newcastle, Liverpool or Bristol.
The practice afforded landlords huge incomes. In today's money, it would be worth the equivalent of seven and a half million pounds a year to the Hebrides. It was so lucrative that in some areas it was not unheard of for all the people of an estate to be set to work on the seaweed, much to the detriment of the land.
The industry was crippled in the early 19th century by the discovery of mineral deposits of potash in Stassfurt, Germany. There was a short period of recovery when a process for extracting iodine from the kelp ash was discovered but this was short-lived. By the mid-20th century, it was confined to a few places in the Outer Hebrides. A government report into crofting conditions in 1954 noted there were 133 seaweed collectors working in Benbecula and South Uist. In later years the processing of the seaweed was done elsewhere.
Seaweeds have also been collected for use as fertiliser as they are nutrient-rich and alkaline and are therefore particularly suited for use on Scotland's generally acidic soils. There are also a number of small businesses currently making use of seaweed in Scotland - producing mainly foodstuffs and cosmetics.
Although the precise date of the photograph is unknown, it was registered with the publisher, Valentine, in 1890.
The major archive of monochrome topographical views by James Valentine & Co. is held by the University of St Andrews Library. For further details of this collection please contact the Library, or refer to Special Collections at St Andrews
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