In France, often 2 fires only are put under each pan. The general construction of a French salt-works is rather more regular than in those of this country, and the pans are usually placed side by side in sheds, while a common flue connects with the outlet-flue of each pan, and such arrangements are made that, when required, any one pan can be cut off by a damper. This common flue is made to pass beneath one or more long deep pans fed with cold brine, and from these the brine is fed, already more or less warmed, into the evaporating-pans. English pans are always set on brickwork, and their bottoms stand about on a level with the ground, overlapping their sustaining walls by some inches, and reposing on longitudinal flues. These latter are usually 4, corresponding in number with the fires, and run straight nearly the whole length of the pan, sometimes entering a chamber at the far end, and passing thence to a low chimney serving one or two pans; but sometimes they converge simply into one common flue, running the whole length of a row of pans, and having an exit to the main chimney. At times the flues do not continue the whole length of the pan, which is then supported here and there by pillars or bits of wall built in parallel lines.
Sometimes no flue? at all are employed, the pan being merely sustained by pillars of brickwork, sandstone, or cast iron. The whole space then beneath the pan constitutes one large flat flue, through which the heated gases find their way unencumbered. This plan is common in Worcestershire. On the Continent, other dispositions of flues are often adopted. At Nancy, and pretty well throughout France, the flues from each fire (often only 2) run down to the end of the pan, returning towards the fire-end, and back again once more to the chimney or main flue, each flue thus forming 3 parallel lines. This plan has been tried in England, but is not now usually employed, the simpler form of straight flues leading from each fire right away to the chimney or common flue seeming generally to be preferred. Here in England also they usually have 2 "dead" flues, as they call them, one on each side beneath the pan, these being spaces like flues, but completely walled up at each end, so that no gases can enter them. The flues are usually 2-3 ft. deep, of a capacity in fact to admit a man or boy; and between the entrance of the flues and the fire-place, is built a wall of fire-brick, reaching to within 18 in. of the bottom of the pan.
Over this "bridge," as it is called, the heated gases pass before entering the flue, and as the bricks of the bridge become red-hot, they tend to induce a more perfect combustion of the smoke before it enters the flues, where it would become too rapidly cooled by contact with the bottom of the pan, and soot would fall.
In Cheshire, and other places in England, the evaporating-pans are at times employed quite open and exposed to the sky, but nowadays they are mostly surrounded with sheds, these being furnished with ventilating openings in the roof, to facilitate the escape of steam. On the Continent, all except the fine and butter-salt pant are generally covered in with wooden trunks, flat on top with sides converging upwards, thus forming an elongated truncated cone about 5 ft. high over the pan. All along the lower parts of the sloping sides of this cover, and on both flanks of it, are frames fitted with shutters removable by hand. By removing one or other of these, the progress of the crystallisation may be watched. A shelf is sometimes made, running along the whole length of this cover of the pan, just above the shutters; and when the pan is drawn, the workmen fish out the salt with rakes and scoops, and let it drain a bit on the drainers alongside of the pan, corresponding to what our salt-makers call "hurdles," and then pitch it overhead on to this shelf, on which it is allowed to drain pretty completely, the drippings falling back into the pan; thence it is shovelled on to the flat top of the cover of the pan, which is set with tiles.
On these tiles, which are kept hot by the steam within the trunk during the time the pan is at work, the salt becomes dried, and is then on a level with the bins (magasins) into which it is tipped from wagons fur storage. From that end of the trunk farthest removed from the fires, rises a wooden chimney 10-15 ft. high, for carrying off the steam from each pan; it passes through the roof of the building in which the work is carried on. Sometimes fan-blowers are placed in this and the main chimney, to expedite the exit of the steam. It is asserted by many of the French salt-makers that notwithstanding the greater cost of covering in the pans in this manner, the lessened facility of egress for the steam, the inconvenience, and the somewhat larger amount of labour involved in drawing the pans, they are compensated by a considerable economy in the combustible employed, through the diminished loss of heat by radiation; certainly they obtain cleaner products than English salt-makers. At the Dombasle salt-works, one of the best-managed and best-organised in France, on the contrary, with 100 lb. of the small, poor coal from Saarbruck they only produce 160-170 lb. of common salt.
This coal is, however, far inferior to the slack used in Cheshire and Worcestershire, and it is not employed for fine or butter-salt, being unable to maintain a pan in continued ebullition, so small is its heating power. It is used on account of its low price, and its yielding a gentle diffused heat suitable for the work.
Both in England and abroad, attempts have been made to reduce the loss of heat, chiefly due to the scale in the pans and the soot of the flues, by heating by steam. Whatever economy there may be in this method, it has not made much progress among English salt-makers, though the system is a common one for other purposes in the salt districts. The steam-pipes get covered with scale, which is difficult to detach without injury to them, and they are rather in the way of drawing the pans. So-called " machine-pans " are sometimes employed. They are usually worked in pairs, standing 20-30 ft. apart, with a small engine between, or a shafting running above several of them driven by an engine at one end; this shafting is geared by bevel-wheels to the stirrers, and is so arranged that any one or more of the pans can be thrown into or out of gear at will. The depth of the pans is 2 ft., and an opening is left in one side of each down to the bottom, this opening being closed with outside troughs riveted to the sides of the pans. The bottoms of these troughs go lower than the bottoms of the pans, ,so that any salt swept out of the openings falls into the troughs, and cannot return into the pans.