The floor of a pan is usually mado slightly arched upwards towards the centre, so that a new pan ie rather deeper at its aides than in the middle ; but they soon flatten out and warp in various directions under the influence of the firing. On the Continent, east-iron pans have been in some caeca adopted, and east-iron plates substituted for the smaller wrought-iron ones universally employed in this country in the part of the pan juet over the fires. Besides the advantage accruing from the letie tendency to buckle and warp, the caet-iron has a much higher conductive power than the wrought-iron, and the advantage of cheapness. The plates are not made much thicker than the ordinary wrought platee, and aro cast with exterior flanges all round their edges, by which they can be bolte,d together heneath the po.n. They also. have grooves cast in their edgee, to receive asbestos cord or cement, by which, when screwed up, they can be made watertight. Were it not for fear of their greater fragility and some difficulties of adjustment, they would doubtleso be employed in this country, thue avoiding leakages into the flues, and the consequent production of large etalactitee of ealt, technically termed " ado," an intolerable nuisance to the salt-maker. In Austria, ouch cast-iron pane are actually now in use, and their advantages will be manifest from the following comparative experiments made at Berchtesgaden under like conditions of firing, &c.
It is also sometimes the practice abroad to make the pans with plates riveted on t,o T-iron bars running across the width of the entire pan, the central flange of the T-iron standing up between the edges of the plates, and these latter having the rivets countersunk into them, This seems somewhat to prevent the buckling.
Fig. 1208 represents the usual mode of setting pans in Cheshire, the two ends only being given, showing the positions of the chimney and the fires. This may he taken as illustrating any of the ordinary pans, whether for boiled or unbeiled salts, the sole difference consisting in the respective lengths of the pans.
Wooden pans even have been and still are ern_ played. One belonging to Thompson, of Northwich, is 4 ft. 6 in. deep, 12 ft. wide, and 75 ft. long. The two ends are of sheet-iron, and a long sheet-iron cylinder, closed at the two ends by steam-tight doors, runs from end to end. This cylinder is about 18 in. diam., and is supplied frorn above at about the middle of the pan by means of a lateral pipe with waste steam from an engine and boiler near. By this, the pan is kept at a temperature of about 90°-100° F. This pan is said to produce 45-50 tons of extra fishery salt every 6 weeks or se. Figs. 1209-11 represent such a pan, of which there are several at Northwich, in plan and in longitudinal and transverse sections, as well as the house containing it.
In Cheshire and Worcestershire, the fire-places, usually 4 in number, measure about 4-5 ft. from the deer to the back, and are about 3i-1 ft. wide ; from the bottom of the pan to the grate-bars is usually about 3 ft. In the case of very long pans, this height may increase to 3 ft. 4-5 in. The grates are formed of square wrought-iron bars, it being found inconvenient in salt-works to employ the improved cast-iron " fishbellied " bars. This is en account of the great liability to choking with clinkers, and caking of the ashes with the brine which drips from leaks over the fires fusing into clinker, and clogging the grate-bars. The blows necessary to detach these masses would seriously endanger cast-iren bars ; but certainly the shape of the bars might.well be improved, and rocking bars, such as those employed in pyrites-kilns and elsewhere, might be more generally introduced with advantage. The firing is usually done in a stoke-hole with steps on each side leading up to the pathway around the pan.
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 re quired, 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 evaporatiug-paus. English pans are always set on brick-work, and their bottoms stand about on a level with the ground, overlapping their sustaining wells by some inches, and reposing on longitudinal flues. These latter are usually 4, correspond ing 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 pairs ; but sometimes they converge simply into one common fluo, running the whole length of a row of pans, and having an exit to the main chimney. At times the fines 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 flues at all are employed, the pan being merely sustained by pillars of brickwork, sandstone, or cast-iron. The whole apace then beneath the pan constitutes one large flat flue, through which the heated gases find their way unencumbered. This plan is common in Worcester shire.