In France and Germany weak brines which could not profitably be used for salt making with the aid of artifieial heat are first concentrated by a natural process. The brine is pumped to the top of a tower-like structure of scaffolding enclosing brushwood and is distributed by means of a spray over the top of the brush wood. Exposure to air and wind during the trickling down rapidly concentrates the brine which is collected in a trough and sprayed again over the brushwood until the liquor is sufficiently strong to be used in the ordinary concentrating pans. The brines at Pomeroy, Ohio, containing notable quantities of barium chloride were formerly concentrated in steam jacketed vessels without treatment to remove this salt, but it is now customary to separate barium by a preliminary addition of "salt cake" (sodium sulphate) when the precipitated barium sulphate is removed by sedimentation or filtration. By this procedure the barium chloride content of the first-grade salt has been reduced from 0.2% to negligible traces.
(2) Manufacture from Rock Salt.—In the Middlesborough district of England and similar localities, fresh water is allowed to run down boreholes to the underground salt beds, remaining there until it is saturated with salt. This artificial brine is then pumped up and the salt crystallized from heated pans in the manner already described. White salt made from rock-salt is usually classified into boiled or fine, table, lump, stoved lump, superfine, basket, butter and cheese salt, unboiled and common, chemical, fishery, Scotch fishery, extra fishery, double extra fish ery and bay salt. These names are derived from the size and appearance of the crystals, from their modes of production and their uses. The small crystals of boiled salts are formed in a medium constantly agitated by boiling. The fine or stoved table salts constitute the familiar white masses. Basket salt takes its name from the conical baskets in which it is allowed to drain when first drawn from the pan. Butter and cheese salts are not stove-dried, but left in moist condition, being thus more easily applied to their respective uses. Of the unboiled salts, the first two show by their names the use to which they are applied, and the others merely depend for their quality on the length of time which elapses between successive "drawings," and on the tempera ture of evaporation, which varies from 55° to 18o° F (13° to 82° C). The time for unboiled salts varies from 12 hours to 3 or 4 weeks, the larger crystals requiring the longer time.
Where the rock-salt is of high degree of purity, as in the United States and in Galicia, the salt is ground and sieved, and comes on to the market without further treatment. A great drawback to this kind of salt is its tendency to revert to hard masses when kept in sacks. The mined salt in lumps of about I ft. diameter is first coarsely crushed and riddled. The coarser material is ground between rollers and the whole of the ground salt screened to four sizes, namely, oversize, no. 2, no. 3 and
C.F. (common fine). The last may be further screened into three sizes including "packers fine" while the oversize is frequently sent back to the mill. In Germany a similar process is adopted, but coarser fragments of impurities such as anhydrite and gypsum are picked out by hand as the coarsely crushed rock salt is carried past the workers on a travelling band or belt. The less pure forms of German rock-salt are purified by fusion, either alone or with soda ash and silica or with chalk and salt petre.
In some cases the fused mass is subjected to a blast of compressed air to burn away carbonaceous matter, leaving a clear, white melt, which crystallizes on cooling. After separation from the slag by concussion, the salt is ground and sieved. Alterna tively, an impure salt is digested with a saturated solution of pure salt in dilute hydrochloric acid, whereby impurities such as gypsum, magnesium salts and iron oxide are dissolved. The treated salt is filtered, washed with a saturated solution of pure salt, dried, ground and graded for market.
The salt workings in Great Britain represent the annual ab straction of a mass of rock rather more than a foot in thickness spread over a square mile. This displacement leads to grave subsidences so that in certain places—Northwich and Winsf ord the damage to property is so great that the houses have to be keyed up with "shaps," "face-plates" and "bolts." Saltmaking is not an unhealthy trade, an occasional slight soreness of the eyes being the only ailment, whereas the atmosphere of steam saturated with salt seems specially preservative against colds, rheumatism, neuralgia and similar troubles.