Salt and the salt industry

by Albert Frederick Calvert

From an Old Print

Published in 1556·

In Italy most of the salt is made by solar evaporation. The salt grounds, which occupy extensive areas, are furnished with reservoirs for the preparation of the sea-water by saturation and for the deposit of salt. The former are known as condensers and the latter as crystallizing beds, and in both the work is carried on by solar evaporation only. Every salt-ground, or salt-garden, as it is called, has a feeding channel for the inflow of sea-water, a drainage channel, and a network of internal channels at low and high levels, as are required for immission or drainage purposes. In Portugal and Spain, salt is made by solar evaporation from sea-water, and although there are differences between the several methods, they apply only to details regarding the areas of the salt-grounds or the sizes of the reservoirs.

Let it be clearly understood that all commercial salt is produced either from the sea or from rock-salt. Sea-water is evaporated to precipitate its salt either by the heat of the sun or by artificial heat. Rock-salt is mined and refined for market purposes, and it is resolved into brine from which the salt is extracted by solar heat or the process of boiling, but whether the salines are obtained from salt lakes or from natural brine springs, or are prepared by flooding salt deposits with water and pumping it out in the form of fully saturated brine, rock-salt is the foundation for them all. And in all the processes of manufacture the basic principle is the same, and consists of applying heat to drive off the liquid which contains the salt and collecting the crystalline deposit which remains.

The principle of what is described as the boiling process is fundamental and unalterable, and for thousands of years the plant and utensils employed in the process underwent no material change. Since the sixteenth century in England, variations in the shape, size, and capacity of the pans have been introduced, and experiments have been made in the re-arrangement of the receptacles and redistribution of the furnaces, while coal fuel has been substituted for straw and wood, but it is only in the past twenty-five years that any material success has been achieved in the matter of economizing and accelerating the process of production, controlling the heat in order to regulate the grain of the salt, producing more than one grade of salt in one operation, or of automatically and continuously collecting the salt as it is precipitated from the brine.

The earliest exact and detailed description that we have of salt-making appears in De Re Metallica, a famous work by Georgius Agricola, of Saxony, which was published in 1556, and which for the following 180 years, remained the standard text-book on mining and metallurgy. In Chapter XII of this work, the preparation of which occupied Agricola for a quarter of a century, he gives the exhaustive particulars relating to the boiling process from which the ensuing account is compiled.

After explaining the method by which sea-water is received into the first series of prepared trenches, in which the first stage of evaporation takes place and is thereafter carried into the second basins, where it is thickened by further evaporation to the constituency in which it is ready to be converted into salt, Agricola tells us that the liquor is then boiled in pans placed in sheds arranged for the purpose. Each shed is divided into three parts. In the first part is stored the firewood or straw, and in the second is the fireplace on which is placed the caldron. To the right of the caldron is a tub for the brine that is to be converted into salt, and on the left is a bench upon which the salt is placed before being removed to the third compartment, where it is moulded into cones or tablets and left to dry in the warm air.

The fireplaces are made 8½ ft. long and 7¾ ft. wide; if wood is burned in them they are nearly 4 ft. high, but if straw fuel is used, they are 6 ft. in height. The caldrons are rectangular, 8 ft. long and 7 ft. wide, and 6 in. deep. They are made of sheets of iron or lead, “not very thick so that the water is heated more quickly by the fire and is boiled away rapidly.” To prevent the brine from leaking out at the points where the metal plates are fastened with rivets, the caldrons are smeared over with a cement of ox-liver, or ox-blood, mixed with ashes. As soon as the first dipperful of brine is poured from the brine tub into the caldron, the wood or straw is ignited in the fireplace. If the firewood consists of faggots or brushwood, the salt will be white, but if straw is burned the salt is not infrequently blackish from the sparks which rise with the smoke and settle upon the water.

In order to accelerate the condensation of the brine, the salt-maker adds and mixes into it bullock’s blood, or calf’s blood, or buck’s blood, which dissolves and is distributed into all the corners of the caldron. When the boiling water seems to be mixed with scum, it is skimmed with a ladle, and from the firing of the furnace to the skimming of the boiling scum is the work of half an hour. After this it boils down for another quarter of an hour, and thereafter it begins to condense into salt. When the brine commences to thicken with the heat, it is stirred assiduously with a wooden spatula, and then allowed to boil for an hour. At this stage beer is added to the contents of the caldron, which is protected from the wind by boards, and the salt is then withdrawn with a shovel and thrown into baskets. The remaining brine is allowed to boil for another three-quarters of an hour, when the salt is again removed and placed in the drying compartment. In this manner the salters alternately boil the brine and collect the salt, “day and night, with the exception only of the annual feast days.” No caldron is able to stand the fire for more than half a year. New caldrons are washed out three times in the first two weeks, and afterwards once a week. In this manner the incrustations fall from the bottom of the caldron, and if this is not done the salt would have to be made more slowly over a fiercer fire, which not only requires more brine but burns the plates of the caldron. If any cracks make their appearance in the caldron, they are filled up with cement. The salt made during the first two weeks in a new caldron is usually inferior in quality, being stained by the rust at the bottom where incrustations have not yet adhered.

Agricola’s description is full of technical exactness in regard to those parts of the apparatus and the process which are of comparatively insignificant interest, but it is, unfortunately, silent about details on which fuller information would be useful. He tells us the capacity of the tubs in which the brine is conserved, but not of the caldrons in which it is boiled, and we cannot calculate the quantity by the dimensions of the receptacles, since he omits to mention the depth to which they are filled. He explains that it takes half an hour to fill the baskets with the salt that is drawn from the caldron, but as he does not give us the dimensions of the baskets employed, or the amount of wood or straw consumed, we cannot determine the length of time required to make a certain quantity of salt, or the cost in fuel. But, condensed and simplified by the elimination of extraneous particulars and complex technicalities, the foregoing enables us to obtain a fair idea of the methods employed by the salter of Halle, in Saxony, assisted by his wife as helper and a youthful stoker—working naked, on account of the great heat, save for a straw cap and a breech cloth—in the first half of the sixteenth century.