A Treatise On The Principles And Practice Of Dock Engineering

146 DOCK ENGINEERING. As a general rule, lead paints* are employed, but it has been suggested that preference should be given to oxide of iron paints, to avoid any tendency to galvanic action between two metallic substances. Care should be taken to reniove all rust and scale before applying the paint. Gast iron on leaving the mould has, or should have, a hard bluish skin, which should be kept intact by an immediate coat of (hydro-carbon) oil or paint. Wrought iron is also sometimes specified to be dipped in oil while hot, but the method is not a very successful preservative, and ironworkers dislike it on account of its messiness. Dock gates and other marine structures of iron and steel should be , thoroughly scraped, cleaned, and painted at frequent intervals—in some cases annually. The materials usually employed for the purpose include— red lead and oil paint, mineral tar, vegetable tar, black varnish, and siderosthen. The surfaces of ironwork in close contact should be painted before being put together. The interior walls of ballast boxes, and other generally inaccessible surfaces, are frequently floated with a thick wash of Portland cement. Galvanising consists in immersing the iron in a bath of molten zinc, whereby a skin of that metal is formed upon the surface. The process is successful so long as the zinc covering remains intact. When it cracks, or becomes defective in any way, rapid corrosion ensues in the presence of the least damp. The writer’s experience of galvanised iron, employed as a material for dock sheds, is that sea air, highly charged with salt and moisture, works havoc with it. Several such sheds, after being a few years in existence, have had to be completely coated with black varnish to preserve them from imminent destruction. The Anyus-Smith treatment, largely adopted for cast-iron pipes, consists in dipping them, at a temperature of 700° F., into a mixture of coal tar, pitch, linseed oil, and resin, at a temperature of 300° F. The process is an admirable method of preservation, and enjoys a considerable reputatiom TIMBER. The varieties of timber principally in demand for the purposes of dock engineering may be enumerated as follows : — Piles.—Greenheart, Jarrah, Karri, Mora, Pitchpine, Oak, Elm, Beech. *A very common constituent of modern paints is sulphate of barium, of which there are two forms, viz. :—(1) the finely-ground mineral barytes, and (2) blanc fixe, or precipitated sulphate of barium. While both these substances have the same chemical composition, there is a wide difference in their physical conditions, which results in the ground mineral being worthiess as an ingredient of paint, whereas the precipitate is just as valuable, owing to its covering power and unalterability. Examin- ation of a sample of each paint under the microscope will easily show the difference between the fragments of crystals in the first case and the amorphous condition of the other.