Sheet iron, iron castings, and the like, are first cleaned and scoured by immersion in a bath of water, acidulated with sulphuric acid, heated in a leaden vessel, or used cold in a wooden one, to remove the oxide. The pieces are then thrown into cold water, and taken out one at a time to be scoured with sand and water with a piece of cork or the husk of the coconut, the ends of the fibres serving as a brush. The pieces are then returned to cold water. Pure zinc, covered with a thick layer of sal ammoniac is then melted in a bath, and the iron, if in sheets, is dipped several sheets at a time in a cradle or grating. The sheets are raised slowly to allow of draining, then immediately thrown into cold water; on removal, the work is finished by wiping dry. Thick pieces are heated in a reverberatory furnace before being placed in the bath, to prevent cooling the zinc. Chains are similarly treated, and on removal from the zinc are shaken until cold to avoid soldering the links together. Nails and small articles are dipped in muriatic acid, and dried in a reverberatory furnace; next, thrown into zinc covered with sal ammoniac, left for a minute, and taken out slowly with an iron skimmer; they come out in a mass soldered together, and to separate them are placed in a crucible surrounded with charcoal powder, then heated to redness and shaken about until cold for separation.
Wire is reeled through the zinc, into which it is forced to dip by a fork or other contrivance. The zinc is melted in a crucible just a little beyond the point of fusion, and is always covered with a thick coat of sal ammoniac, for the purpose of preventing waste of zinc and preparing the metal to be covered. Wrought-iron baths welded at the angles succeed much better than cast-iron, lined with clay. By another system the sheets of iron are pickled, scoured, and cleaned just as for ordinary tinning. A large wooden bath is then half filled with a dilute solution of muriate of tin, prepared by dissolving metallic tin in concentrated muriatic acid, which takes 2 or 3 days, and 2 qt. of the saturated solution are added to 300 or 400 gal. of the water contained in the bath. Over the bottom of the bath is spread a thin layer of finely-granulated zinc, then a cleaned iron plate, and so on - a layer of finely-granulated zinc and a cleaned iron plate alternately, until the bath is full; the zinc and iron, together with the fluid, constitute a weak galvanic battery, and the tin is deposited from the solution, so as to coat the iron with a dull uniform layer of metallic tin in about 2 hours.
Whilst this is being done a wrought-iron bath, containing fluid zinc, is being prepared, the melted metal is covered with sal ammoniac, mixed with earthy matter, to lessen the volatilisation of the sal ammoniac, which becomes as fluid as treacle. Two iron rollers, immersed below the surface of the zinc, are fixed to the bath, and are driven by machinery to carry the plates through the fluid metal at a determined velocity. The plates are now received one by one from the tinning bath, drained for a short time, and passed at once, still wet, through the zinc, by means of rollers; the plates thus take a regular and smooth layer of zinc, which, owing to the presence of tin beneath, assumes the natural crystalline character, giving the plates the well-known appearance.
In Cleveland, Ohio, the galvanising of sheet iron is carried on in the following manner: The plates are first placed in a bath of dilute sulphuric acid, into which steam is led. The acid is contained in a wooden vat, and the plates are placed on their edges and kept separate from each other. They are removed from this bath and placed in another vat, through which water is flowing. From this rat they are removed one by one, examined, any unevenness is scraped off them, and they are again washed in another vat with water, and are then placed edgewise on a wagon, which carries 20 plates, leaving a space 1/2 in. between each plate, and are run on this wagon into a drying kiln. This kiln takes two such wagons at a time, and the wagons are run in at one end and out at the other. It is heated by flues passing under the floor and round the sides. The drying and heating takes about 20 minutes. The zinc bath is made of forged iron, 8-12 ft. long, 2 ft. broad, and 3 ft. 6 in. to 4- ft. 2 in. deep, and holds 18-25 tons of molten zinc. This pan is set upon masonry, and is surrounded by a 12 in. wall of firebrick, leaving a space of 8 in. between the wall and the iron sides of the bath, the height of the wall reaching just to the height of the pan.
In this space between the pan and the brickwork a coke fire is maintained, two rows of air-holes being let into the lower part of the wall, the lower row serving to remove the ashes. A bar of iron divides the bath longitudinally into two halves; this bar dips a few inches below the surface of the zinc, and projects about 3 in. above the surface. On one side of the bar the zinc is covered by a J in. coating of sal ammoniac, and on the other side of the bar by a 2 in. coating of damp sand. The iron sheet is taken from the drying kiln, and is dipped perpendicularly, its long edge first, slowly into the bath on the sal ammoniac side of the dividing bar, and is brought up on the other, side of the bar through the sand. It is then gripped by a pair of tongs by one corner, and drawn slowly out of the pan by a rope passing over a pulley in the roof, and wound upon a hand-winch. The adhering drops of zinc are removed, and the plate is laid upon a table; the sand is rubbed off with a piece of sacking, and then, if found to be perfect, it is stamped and packed.
It remains in the bath about one minute.