Rock salt

“ SALT. Geological Position of Rock Salt—Mines of Northwich—Their immense Excavations—Droitwich and Stoke—Wieliczka—Berchtesgaden and Reichenhall—Admirable Machinery—Stassfurt—Processes employed in the Manufacture of Salt—Origin of Rock-salt Deposits.
Common salt is so necessary to man, and of such vast importance to the manufacturer and agriculturist, that the processes by which it is obtained are justly reckoned among the chief branches of industry.
In many of the warmer countries of the globe it is procured simply by the evaporation of sea-water in shallow lagoons ”

“ The facts that the chief accompaniment of every known deposit of rock-salt is clay, and that clay is deposited in water, formed the basis of the erroneous theory that because salt is a deposit out of water, and sea-water contains salt, all salt beds must have been deposited in the sea. But salt does not mix mechanically with water and has not been deposited like sedimentary rocks; it forms a solution, and not until the solution becomes super-saturated does it crystallize out. Now sea-water rarely contains more than 31⁄2 per cent. ”

“ Most of the common salt (the mineral “halite”) of commercial value occurs in nature in sea or salt lake water; or in beds or strata of rock salt associated with other strata; or as natural brine in openings or pores in certain rocks. Considerable salt is obtained by evaporation of tidewater, as around San Francisco Bay, and of salt lake water, as at Great Salt Lake, Utah. It has been estimated that the Great Salt Lake, whose area is about 2,000 square miles and greatest depth 50 feet, contains several hundred million tons of common salt. ”

“ Man of the present derives his existing salt-supply chiefly from beds of rock-salt similarly laid down against his expected appearance some hundred thousand æons or so ago. (An æon is a very convenient geological unit indeed to reckon by; as nobody has any idea how long it is, they can't carp at you for a matter of an æon or two one way or the other.) Rock-salt is found in most parts of the world, in beds of very various ages. The great Salt Range of the Punjaub is probably the earliest in date of all salt deposits ”

“ Particles of chloride of sodium in volcanic ejections were no explanation of the formation of huge deposits of rock-salt, and since it was realized that salt in large quantity can only be obtained from salt water, and that it cannot be got naturally from the sea, it became evident that what man does in isolating tracts of sea-water to produce salt by solar evaporation, must have been practised by nature on an extensive scale in all ages. ”

“ Perfectly pure rock-salt is colourless and transparent; but one doesn't often find it pure. Alas for a degenerate world! even in its original site, Nature herself has taken the trouble to adulterate it beforehand. (If she hadn't done so, one may be perfectly sure that commercial enterprise would have proved equal to the occasion in the long run.) But the adulteration hasn't spoilt the beauty of the salt; on the contrary, it serves, like rouge, to give a fine fresh colour where none existed. When iron is the chief colouring matter, rock-salt assumes a beautiful clear red tint ”

“ The purer rock-salt is often simply ground for use, as at Wieliczka and elsewhere, but it is more frequently pumped as brine, produced either by artificial solution as at Middlesbrough and other places, or by natural means, as in Cheshire and Worcestershire. One great drawback to the use of even the purest rock-salt simply ground is its tendency to revert to a hard unwieldy mass, when kept any length of time in sacks. ”

“ In many places, through this rock-salt crust the water boiled up as clear as crystal ... but so salt that our hands, after being immersed in it and suffered to dry, became as white as snow. Thrusting the arm down into these holes, they appeared to be walled with salt as far down as one could reach. The cliffs which overhang this place are composed of red clay and gypsum, and capped with a stratum of the latter.... We found this salt a little bitter from the impurities it contained, probably Epsom salts principally. ”

“ Even in this country, when the old open-pan system of evaporating salt from brine produced only two tons of salt for the consumption of a ton of fuel, rock-salt could be raised, purified, and marketed in competition with white salt, but the modern boiling processes have effected such substantial improvements and consequent economical advantages, that the rock-salt industry appears to be doomed to decay. ”

“ The Winsford district, taking the beds of rock-salt at an average thickness of 65 yds., which is 5 yds. less than the figure given by Dickinson, we have 1,932,902,400 tons.
As the whole of the white salt has been manufactured from brine derived from the rock-salt, it represents so many tons of rock-salt pumped up. Now, as the specific gravity of rock-salt is 2·125, a cubic yard contains 32 cwts. ”

“ We have salt, or the chloride of sodium, presented to us in six different conditions, viz.: sea or salt water, salt or brine springs, salt lakes, salt mines, mountain or fossil or rock salt, and salt marshes. The characteristics of salt are just the same fundamentally, whether we extract it by evaporation from sea-water or salt lakes; whether we obtain it from salt-springs; whether we dig it out of the earth or by the excavation of salt mountains ”

“ Salt is rarely if ever ob- tained pure. The chief impurities in rock salt are sulphate of lime, oxide of iron, and clay; but besides these the chlorides of potassium, calcium, and magnesium, the sulphates of soda and magnesia, and bituminous matters are occasionally met with, and some varieties are even colored by the presence of infusoria. In salt made from sea water, the salts of magnesia with a little sulphate of lime are the principal impurities. All the varieties of salt occasionally contain minute quantities of bromides and iodides. ”

“ All these mineral waters come from brine-springs, and whether they are called chalybeate, carbonated, saline, or hepatic waters, whether they come from Spa or Tunbridge Wells, from Carlsbad or Ilkestone, from Püllna or Cheltenham, Buxton, Friedrichshall, Droitwich, or Wiesbaden, their common origin is rock-salt, and to that mineral alone their virtues are principally due; the iron, magnesia, lime, and the other salts which they collect on their upward course are merely accessories, and are more useful to the proprietors than to the credulous recipients. ”

“ The massive salt has a vitreous lustre. It is not so brittle as nitre; it is nearly as hard as alum, a little harder than gypsum, and softer than calcareous spar. Its specific gravity varies from 2·0 to 2·25. When pure, it is colourless, translucent, or transparent. On exposure to heat, it commonly decrepitates; but some kinds of rock salt enter quietly into fusion at an elevated temperature, a circumstance which has been ascribed to their having been originally subjected to the action of fire. ”

“ The salt fields are wonderfully beautiful in the moonlight, but not very agreeable to work in, for the mercury often reaches 140° F., and the air is so full of particles of salt that the workers feel an intense thirst, which the warm, brackish water does not satisfy. The work is done by Indians and Japanese, for white people cannot endure the heat.
A large portion of the salt used in the United States comes directly from rock salt strata, hundreds of feet below the surface of the ground. ”

“ The water of landlocked seas like the gulf of Mexico or the Mediterranean sea contains more salt than that of the open ocean, and it is also found that the water of the bottom of such seas is salter than that upon the surface.—Salt crystallizes in colorless, transparent, anhydrous crystals, belonging to the isometric system, and has a very perfect cubic cleavage, which generally displays itself even in the great masses of rock salt, parts of which however are frequently massive and granular, and rarely fibrous or columnar. ”

“ Here alone we have the beds of pure rock-salt. As we come nearer the surface, the presence of the earthy chlorides is more marked, and the product is less valuable, because, by reason of its attracting more moisture from the air, it is rendered unfit for the dairy or the table unless it goes through a process of purification. The conclusion that we must reach, therefore, is, that the deeper the boring goes, the purer the salt will be, whether it is in Michigan or in New York. ”

“ The deposits, which are found at a considerable height above sea-level, are of great but undefined magnitude, as the lowest strata has been bored through for a thickness of 72 feet, without penetrating the formation. The rock salt here was acknowledged to be purer than any previously encountered in Cheshire, and the brine derived therefrom, containing 26·100 chloride sodium by weight, yields on evaporation an exceptionally high class of white salt. ”

“ Common salt constitutes the mineral halite, the composition of which is sodium chloride. It is rarely found perfectly pure in nature, but is commonly mixed with other saline materials, such as gypsum and anhydrite, and occasionally with salts of potassium and magnesium. The general grade of rock-salt deposits, where not admixed with clay, is perhaps 96 to 99 per cent of sodium chloride. ”

“ Every shower of rain creates a certain amount of mud or sand, and every brook and stream running into the salt lakes during the rainy season brings in a certain quantity of the same material. The mud represents the wet season of the year, and the salt the dry season. The geological conditions must have been the same when salt was deposited in Cheshire, and with the instances of modern salt-forming regions before us, and the strata of the Cheshire salt country to guide us, it must be concluded that the genesis of rock-salt, modified by local circumstances, must have been the same in every case. ”

“ The primitive methods that characterized the brine industry have been adhered to with equal tenacity in the winning of rock-salt. It is extraordinary that, in the manufacture and in the mining of salt, each successive generation of salt-men, in inheriting their methods from their forefathers, or adapting them from the miners of another country, have always preserved the intense conservatism that appears to be inseparable from the industry, and have resisted all innovations that have promised to simplify or expedite their labours. ”

“ Pseudo-morphous crystals of salt are common throughout the whole formation, which, besides, contains two great beds of rock-salt, each 80 or 100 feet thick, which could only have been deposited in a lake that had no outflow, and from which all the water poured into it by the rivers of the country was entirely got rid of by evaporation induced by solar heat. ”

“ For a time, a cycle of dry seasons prevailed; a great change occurred, and a bed of rock-salt, 84 ft. in thickness, was deposited. In other parts, the bed of rock-salt varies from 80 ft. to over 100 ft. in thickness, none of which is perfectly pure, and not more than 20 ft. of it is sufficiently pure to be of commercial use. The greatly changed seasons are indicated by these formations. A portion near the bottom, containing less clay, shows a less copious or less protracted rainfall, and these periods were followed by wet seasons and the presence of much clay. ”

“ One is rock-salt, which has been heaved up by volcanic power, and the other is what is known as sea-water; the former has produced the mines, and the solitary mountains, and the Indian Salt Range, and that salt which generates mineral waters, and, it may be, those saline lakes like that which exists between Bombay and Nagpur.
According to Sir Charles Lyell, sea-water has access to volcanic foci. ”

“ The salt-mines of Cheshire, and the brine-pits of Worcestershire, according to the best authority, not only supply salt sufficient for the consumption of nearly the whole of England, but also upward of half a million of tons for exportation. Rock-salt is by no means confined to England, it is found in many countries, especially where strata of more recent date than those of the coal measures abound. Though in some instances the mineral is pure and sparkling in its native state, it is generally dull and dirty, owing to the matter with which it is associated. ”

“ One difference between the manufacture of salt from rock-salt brine as carried on in Britain and on the Continent lies in the use in the latter case of closed or covered pans, except in the making of fine salt, whereas in Britain open ones are employed. With open pans the vapour is free to diffuse itself into the atmosphere, and the evaporation is perhaps more rapid. When covered pans are used, the loss of heat by radiation is less, and the salt made is also cleaner. ”

“ It can be shown that a primitive
and more rapid solution of the original crust of the Earth by the
slowly cooling ocean would have given rise to relatively small
salinity. The fact is, the quantity of salts in the ocean is
enormous. We are only now concerned with the sodium; but if we
could extract all the rock-salt (the chloride of sodium) from the
ocean we should have enough to cover the entire dry land of the
Earth to a depth of 400 feet. It is this gigantic quantity which
is going to enter into our estimate of the Earth's age. ”