Kelp (Fr. varéch) is the crude alkaline matter produced by the combustion of seaweeds, of which the most valued for this purpose are Fucus vesiculosus, F. nodosus, F. serratus, Laminaria digitata, L. bulbosa, Himanthalia lorea, and Chorda filum. These are dried in the sun, and then burned in shallow excavations at a low heat. About 20 or 24 tons of seaweed yield one ton of kelp. The kelp met with in commerce consists of hard, dark-gray or bluish masses, which have an acrid, caustic taste, and are composed of chloride of sodium, of carbonate of soda, sulphates of soda and potash, chloride of potassium, iodide of potassium or sodium, insoluble salts, and colouring matter. It used to be the great source of soda (the crude carbonate); but, as this salt can now be obtained at a lower price and of a better quality from the decomposition of sea-salt, it is prepared in far less quantity than formerly from kelp. A ton of good kelp will yield about 8 lb. of iodine, large quantities of chloride of potassium, and additionally, by destructive distillation, a large quantity of volatile oil, from 4 to 15 gallons of paraffin oil, 3 or 4 gallons of naphtha, and from 1½ to 4 cwt. of sulphate of ammonia.
Till 1825, before the remission of the duty on salt and on Spanish barilla, the kelp manufacture (introduced into Tyree in 1746) was carried on to a very large extent in the north and west of Scotland, and the value of many estates in the Highlands and Hebrides greatly increased in consequence. About the beginning of the 19th century some 20,000 tons, worth from £200,000 to £300,000, were made annually on the western coasts of Scotland alone. Now the total annual produce in the United Kingdom will hardly exceed 7000 tons, at about £4 a ton. The greater part comes from Ireland, the remainder from the West Highlands and the Channel Islands. The French supplies do not exceed those of Britain. See SODA, IODINE, SEaweeds.