A very impure carbonate of soda, obtained by the incineration of sea-weed, and chiefly employed in the manufacture of glass. The cultivation of the marine plants for this purpose is now much encouraged, from the increased value it gives to those estates which have an extent of coast adapted to the growth of the peculiar kinds of weed best suited to the manufacture of kelp. There is a very great difference in the product of soda from different plants, some yielding as much as 5 per cent. of the alkali, while others, not even 1 per cent. Those parts of the coast which are exposed to the fury of tempests, or to a heavy rolling surf, prevent the plant from taking root. They thrive best in sheltered bays, where the retreat of the tide leaves uncovered an extensive surface of rocky ground, to which the plants adhere by their roots. The plant commonly called tangle is the only one to be obtained in exposed situations; these adhere with great force to the rocks, and are obtained at the low ebb of spring tides; they are however of a substantial nature, and are considered to repay well for the labour of collecting, which is usually effected by cutting them off with a sickle or reaping hook. The spring is the best time for making kelp.
The marine plants, or sea weeds, are collected without distinction of kind, under the general term of wrack, or verack (which are probably corruptions of the French word vraic), and are first dried in the air and sun precisely in the same manner as in the making of hay, being spread and made up into cocks and stacks, so as to keep it as much as possible from the rain • care is likewise taken to prevent its getting muddy; and such as may collect mud in dragging it up the beach, is washed in the waves by means of pitch-forks or rakes. Experience has determined that the kilns for burning vraic should not exceed about 3 feet in width inside, nor more than 2 feet 6 inches high, but they may be of any convenient length; usually they are about 18 feet. In some places holes are dug in the ground to form the kilns, which are lined with stone; but in these all the vraic is rarely completely burned, and the unburned portion yields no alkali. It is now generally deemed preferable to erect the kiln on a firm level piece of ground, of such rough stones as can be easily got together, and without mortar or cement, but the windward side of the kiln is generally covered up with turfs on the outside, and if the wind be violent, on all sides.
The process of burning is commenced by igniting some furze or heath in the kiln, on which the vraic is then thrown lightly in small quantities at a time, until the whole body of the kiln is fully ignited. The additions are then continued to be made with care, by only throwing on small quantities at a time, where a red hole appears in the mass; and thus the feeding is continued until the collection of vraic is expended; then as soon as red holes appear, the less ignited portion is stirred into them. The want of due attention to the thorough and uniform burning of the vraic, causes a great deterioration in the value of the product. Towards the close of the burning, three or four men are usually employed in actively raking the ashes with the kelp-irons until the whole contents of the kiln become a semi-fluid mass. Sometimes a portion of the kelp will be found congealed to the sides of the kiln; this is then removed and worked up with the rest, that it may incorporate whilst hot. If after the raking is begun, the materials still continue hard and dry, they are allowed to burn a little longer.
Sometimes common salt and saltpetre are added to the ashes to increase the ignition and bring the ashe3 to the desired semi-fluid consistence; but this measure is seldom found necessary, except when the vraic has been wetted by rain prior to burning. When a new burning is commenced, the remaining unfused ashes from the previous operation are introduced into the kiln by degrees along with the fresh vraic, but not until the fire has become fierce, and the largest and hardest pieces should be put in a row along the centre of the kiln. The kelp, after being made, should be carefully preserved from moisture. In Scotland the kelp makers usually break the lumps into pieces of about 2 cwt. each, which are piled into conical heaps, covered with dry vraic, and over all a layer of turf; this preserves it well until the time of shipment. Kelp is esteemed of good quality when, on breaking a piece, it is hard, solid, and has some reddish and light blue shades running through it. When it has none of its peculiar salt taste, it is unfit for making ley, though it may be of use to glass makers.