. Journal . d to produce dynamite, the necessary siliciousmaterial was obtained in the ashes of Boghead coal. VAPOUR-GALVANISING. BY S. COWPER-COLES. Vapour-galvanising is distinctive from all other formsof galvanising inasmuch as the vapour of zinc is employedfor coating metal surfaces instead of dipping them intomolten zinc or into an aqueous solution thi-ough whicha current of electricity is passed. Vapour-galvanising can be subdivided under twoheads, namely, the zinc dust process and the moltenzinc vapour process. Zinc dust, which is an essential feature of the firstprocess, is the product
. Journal . d to produce dynamite, the necessary siliciousmaterial was obtained in the ashes of Boghead coal. VAPOUR-GALVANISING. BY S. COWPER-COLES. Vapour-galvanising is distinctive from all other formsof galvanising inasmuch as the vapour of zinc is employedfor coating metal surfaces instead of dipping them intomolten zinc or into an aqueous solution thi-ough whicha current of electricity is passed. Vapour-galvanising can be subdivided under twoheads, namely, the zinc dust process and the moltenzinc vapour process. Zinc dust, which is an essential feature of the firstprocess, is the product formed during the distillationof zinc from its ores, but, as the demand now exceedsthe supply obtainable, by the ordinary process of dis-tillation from the ore. it is made by distilling zinc fromthe crude metal. Zinc dust usually contains about 80per cent, of metallic zinc, coated with a film of oxide,which prevents the metallic zinc from running into amolten mass when heat alone is applied. It is therefore. Fig. 1.—Furnace tor vapour-galvanising. particularly suitable for the process about to be described,as all risk of fusing and coating the iron under treatmentwith molten zinc is thus obviated. Zinc dust volatilisesat a low temperature, due no doubt to its mistablephysical condition. This fact was observed when someexperiments were being made in connection with theannealing of cast iron in 1902. Zinc dust, when in contactwith another metal, such as iron or copper, volatilisesat a much lower temperature than it would otherwise;a piece of iron placed in zinc dust and heated to a temfjer-ature several hundred degrees below its melting pointis quickly coated with zinc; if a similar piece of iron issuspended just above the zinc dust, no deposit of zinctakes place. In the former case an electric currentis generated which probably reduces the volatilisingpoint. The process of vapour-galvanising witli ^inc dustin practice is carried into effect in the following manner :—The zin
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectchemist, bookyear1882
