1000
Z IK C. —Galvanizing.
turn has appended to it a beak of the same materialbut much smaller. The retorts are placed in thefurnace shown in section in Figs. 1 and 2, the latterin the line a b of the former. The stack consists offour distinct furnaces, each in the form of a semi-cylindrical chamber, 8 feet 8 inches from the highestpoint of the arched roof to the floor, the back of thechamber being constructed so as to recede slightlyfrom the bottom or floor upwards, as shown by b d,in Fig. 2, and the front, a c, being left open for theconvenience of introducing the retorts. The fire isshown at f, the combustible gases and other pro-ducts passing upwards through four apertures inthe grate, shown at e e e, and thence by a doubleflue, G G, finding an exit overhead into the chimney,C. The latter forms one massive construction, em-bracing the four outlets, each closed by a damper.In each of the chamber furnaces forty-two retorts,g g, are heated; these being placed in rows oneabove the other, as shown in the drawing, theposterior end resting upon a ledge made for thepurpose in the masonry or brickwork. In the frontthe distilling cylinders are supported by plates ofcast-iron, so fixed at each side of the face of thefurnace as to allow the cylinders to slope down-wards from the back to the front.
The furnace is brought to a bright red heat, andthe retorts which have been heated in a reverbera-tory furnace arc then fixed in their places. Thecharge is filled in by slow degrees for a period ofthree or four days, or until the furnace is foundto be in working condition. When the zinc vapouris evolved and inflames at the mouths of the retorts,the condensing tubes are fitted on and the distillationcommences. From time to time the beaks of theretorts are removed, and the fluid zinc is raked intoladles and cast into ingots. The oxide is returnedto the furnace with the next charge.
The bottom range of retorts is seldom filled in thiscountry on account of their liability to be broken.
Silesian Process .—The ore and powdered coal aredistilled in muffles made of fire-clay and brokenmuffles—baked and annealed after the fashion ofglass pots. These are set in rows in a furnace pro-vided with suitable openings. The front of themuffle has two apertures. The lower one serves toremove the residues after each distillation; theupper has a hollow clay arm fitted to it, bent atright angles, and open at the bottom as well as atthe top, and also a small door at the top of thebend through which the muffle can be charged.From six to ten muffles are set in a row. The fuelin the older works is coal, but all the new worksare provided with Siemens’ gas furnace.
Galvanizing .—The process of coating iron with
zinc, known under the inappropriate title of “ gal-vanizing,” was discovered by Melodin in 1742. Oflate years iron coated with zinc has come largely intouse as a roofing material, for making spouts, gutters,buckets, and in fact for all the purposes for which sheetiron can be used. Melouin’s process is followed witha few modifications even at the present day. Theiron is cleansed by immersion for a few minutes inan acid bath, composed of oil of vitriol and water, orof hydrochloric acid and water, or of equal volumesof the two acids and water; it is then scrubbed withsand, washed with water, and dipped in a concen-trated solution of ammonium chloride, taken out, andsubsequently introduced into a bath of melted zinc,covered with fat or rosin, to prevent oxidation, andstirred in it till the zinc alloys with its surface. Oflate years a few lbs. of tin have been added to thezinc to improve the appearance of the plates.
II. W. Ckawfurd in 1837 patented a processof coating iron with zinc, the chief characteristicof which is covering the zinc bath with a thicklayer of sal-ammoniac. Thick pieces of iron areheated before immersion, to avoid cooling the zinc.Nails, &c., are cleaned with hydrochloric acid, driedin a reverberatory furnace, and plunged in the zincbath for one minute. When fished out with askimmer they are generally soldered together, andare separated by heating to redness with charcoal,and then shaking till cold.
Wire is galvanized by reeling it through a zincbath covered with a layer of ammonium chloride.
Mallet prepared his iron for galvanizing by dip-ping it in a saturated solution of zinc chloride andammonium sulphate, after which he treated it witha hot amalgam consisting of 1 part of mercury withG} of zinc; 1 pound of sodium being added to eachton of the amalgam.
In Morewood and Rodger’s process the iron isfirst tinned, and then immersed in a zinc bathcovered with sal-ammoniac, to which some earthhas been added to prevent its too rapid volatilization.
A large wooden bath is filled with a dilute solu-tion of tin chloride, in the proportion of 2 quarts ofsaturated solution to 300 gallons of water. Overthe bottom of the bath is strewn a thin layer ofgranulated zinc; upon this rests a cleaned iron plate,then another layer of zinc, and again a plate ofiron, and so on till the bath is full. The zinc andiron form a voltaic couple, which in two hoursprecipitates the tin upon the iron in a dull uniformcoat. The plates are transferred to the zinc bathwhile still wet, and are passed through it by meansof iron rollers.
Zinc White, Zinc Oxide , Zinc, Carbonate,and their hydrates—see Pigments.