Manufacturer and builder 3, 1888
Cast and wrought iron begave very differently under atmosperic influences, and require somewhat different treatment. The decay of iron becomes very marked in certain situations, and weakens the metal in direct proportion to the depth to which it has penetrated; and although where the metal is in quantity this is not very appreciable, it really becomes so when the metal is under threequarters of an inch in thickness. The natural surface of cast iron is very much harder than the interior, occasioned by its becoming chilled, or by its containing a large quantity of silica, and affords an excellent natural protection; but should this surface be broken, rust attacks the metal and soon destroys it. It is very desirable that the casting be protected as soon after it leaves the mold as possible, and a priming coat of paint should be applied for this purpose; the other coats thought requisite can be given at leisure.
In considering the painting of wrought iron, it must be noticed that when iron is oxidized by contact with the atmosphere, two or three distinct layers of scale form on the surface, which, unlike the skin upon the cast iron, can be readily detached by bending or hammering the metal. It will be seen that the iron has a tendency to rust from the moment it leaves the hammer or rolls, and the scale above described must come away. One of the plans to preserve iron has been to coat it with paint when still hot at the mill; and although this answers for a while, it is a very troublesome method, which iron-masters cannot be persuaded to adopt, and the subsequent cutting processes to which it is submitted leave many parts of the iron bare. Besides, a good deal of the scale remains, and until this has fallen off or has been removed, any painting over it will be of little value.
The only effectual way of preparing wrough iron is to effect a thorough and chemical cleansing of the surface of the metal upon which the paint is to be applied; that is, it must be immersed for three or four hours in water containing from one to two per cent of sulphuric acid. The metal is afterwards rinsed in cold water, and, if necessary, scoured with sand, put again into the pickle, and then well rinsed. If it is desired t okeep iron already cleansed for a short time before painting, it is necessary to preserve it in a bath rendered alkaline by caustic lime, potash, soda, or their carbonates. Treatment with caustic lime water is, however, the cheapest and easiest method, and iron which has remained in it for some hours will not rush by a slight exposure to a damp atmosphere.
Having obtained a clean surface, the question arirses: What paint should be used upon iron? Bituminous paints, as well as those containing variable quantities of lead, were formerly considered as solely available, but their failure was made apparent when the structures to which they were applied happened to be of magnitude, subjected to great inclemency of weather, or to constant vibration. Recouse has, therefore, been had to iron oxide itself, and with satisfactory results. A pound of iron oxide paint, when mixed ready for use, in the proportions of two-thirds oxide to one-third linseed oil, with careful work, should cover twenty-one square yards of sheet iron, which is more than is obtained with lead compound.
Oxide of iron paint endures a very great heat without material alteration, and keeps both its color and preservative qualities well. There is this difference to be noticed between painting iron and wood - that with the former, when the painter comes to spots of rust that cannot be removed, he should endeavor to incorporate them with the paint rather than paint over them. The repainting of iron involves carefully washing down and removing all dust, dirt, and so on from the entire surface, every particle of rust being scraped and chipped off, the work receiving from two to four coats in oil properly applied. The real value of any paint depends on the quality of the linseed oil, the quality and character of the pigment, and the care bestowed on grinding and mixing; and as all this is entirely a matter of expense, cheap paints are not to be relied upon.
- London Carpenter and Builder.