The bottoms of the several flues have a gradual ascent above the level of the fire-grate. A special chimney o, rises above the ash-pit, to carry off the smoke, which may chance to regurgitate in certain states of the wind. p, p, are iron pipes laid upon each side of the ash-pit (see [figs. 966.] and [967.]), into which cold air is admitted by the flue q, r, where, becoming heated, it is conducted through iron pipes s, and thence escapes at t, into the stove-room. Upon both sides of the hot flues in the stove-room, hurdle-frames u, u, are laid, each of which contains 11 baskets, and every basket, except the undermost, holds 60 pounds of salt, spread in a layer 2 inches thick. v, v, show the pipes by which the pan is supplied with graduated brine.
Description of the Steam-trunk, in [fig. 968.]
In front of the pan a, a, there are two upright posts, upon which, and in holes of the back wall, two horizontal beams b, b, are supported. The pillars c, c, are sustained upon the bearers d, d. At e, e, a deep quadrangular groove is made in the beams, for fixing down the four boards which form the bottom of the steam-way. In this groove any condensed water from the steam collects, and is carried off by a pipe f, to prevent it falling back into the pan. Upon the three sides of the pan not in contact with the wall, there are three rows of boards hinged upon planks b, b. Behind the upper one, a board is hung on at g, upon which the boiled salt is laid to drain. The two other rows of boards are hooked on so as to cover the pan, as shown at h. Whenever the salt is sufficiently drained, the upper shelves are placed in a horizontal position; the salt is put into small baskets, and carried into the stove-room. i, k, is the steam-trunk; l, m, is a tunnel for carrying off the steam from the middle of the pan, when this is uncovered by lifting the boards.
In proportion as the brine becomes concentrated by evaporation, more is added from the settling reservoir of the graduation-house, till finally small crystals appear on the surface. No more weak brine is now added, but the charge is worked off, care being taken to remove the scum, as it appears. In some places the first pan is called a schlot-pan, in which the concentration is carried only so far as to cause the deposition of the sludge, from which the saline solution is run into another pan, and gently evaporated, to produce the precipitation of the fine salt. This salt should be continually raked towards the cooler and more elevated sides of the pan, and then lifted out with cullender-shovels into large conical baskets, arranged in wooden frames round the border of the pan, so that the drainage may flow back into the boiling liquor. The drained salt is transferred to the hurdles or baskets in the stove-room, which ought to be kept at a temperature of from 120° to 130°, Fahr. The salt is then stowed away in the warehouse.
The graduation range should be divided lengthwise into several sections: the first to receive the water of the spring, the lake, or the sea; the second, the water from the first shower-receiver; the third, the water from the second receiver; and so on. The pumps are usually placed in the middle of the building, and lift the brine from the several receivers below into the alternate elevated cisterns. The square wooden spouts of distribution may be conveniently furnished with a slide-board, attached to each of their sides, to serve as a general valve for opening or shutting many trickling orifices at once. The rate of evaporation at Moutiers is exhibited by the following table:—
| Number of Showers. | Total Surface of the Fagots. | Specific Gravity of the Brine. | Water evaporated. | |
|---|---|---|---|---|
| 1·010 | 0·000 | |||
| 1 and 2 | 5158 | square feet | 1·023 | 0·540 |
| 3, 4, 5, 6, 7, 8, and 9 | 2720 | 1·072 | 0·333 | |
| 10 | 550 | 1·140 | 0·062 | |
| Total evaporation | 0·935 | |||
| Water remaining in the brine at the density of 1·140 | 1·065 | |||
| Water assigned at the density of 1·010 | 1·000 | |||
From the above table it appears that no less than 10 falls of the brine have been required to bring the water from the specific gravity 1·010 to 1·140, or 18° Baumé. The evaporation is found to proceed at nearly the same rate with the weaker water, and with the stronger, within the above limits. When it arrives at a density of from 1·140 to 1·16, it is run off into the settling cisterns. M. Berthier calculates, that upon an average, in ordinary weather, at Moutiers, 60 kilogrammes of water (13 gallons, imp.) are evaporated from the fagots, in the course of 24 hours, for every square foot of their surface. Without the aid of currents of air artificially warmed, such an amount of evaporation could not be reckoned upon in this country. In the schlotting, or throwing down of the sediment, a little bullock’s blood, previously beaten up with some cold brine, promotes the clarification. When the brine acquires, by brisk ebullition, the density of 1·200, it should be run off from the preparation, to the finishing or salting pans.
The mother-water contains a great deal of chloride of magnesium, along with chloride of sodium, and sulphate of magnesia. Since the last two salts mutually decompose each other at a low temperature, and are transformed into sulphate of soda, which crystallizes, and muriate of magnesia, which remains dissolved, the mother-water with this view may be exposed in tanks to the frost during winter, when it affords three successive crystalline deposits, the last being sulphate of soda, nearly pure.