If the coating is to serve only for a short time, instead of calcined magnesia, the same quantities of burnt lime, marble, or dolomite may be used. For packing and covering iron articles, such as wire, chains, files, etc, cloth or paper may be used, one side of which is coated with the last-mentioned mixture, whilst the opposite side is rendered impermeable by the application of a layer of chroma ted glue. (Chem. Rev.)
(12) In mixing paints for iron surfaces it is of the first importance that the best materials only should be used. Linseed-oil is the best medium, when free from admixture with turpentine. A volatile oil like turpentine cannot be used with advantage on a non-absorbent surface like that of iron, for the reason that it leaves the paint a dry scale on the out.side, which, having no cohesion.
can be readily crumbled or washed away. Linseed-oil, on the other hand, is peculiarly well adapted for this purpose. It does not evaporate in any perceptible degree, but the large percentage of linolein which it contains combines with the oxygen in the air and forms a solid translucent substance, of resinous appearance, which possesses much toughness and elasticity, and will not crack or blister by reason of the expansion or contraction of the iron with variations of temperature. It is, however, remarkably adhesive, impervious to water, and is very difficult of solution in essential oils, spirits, or naphtha, and even in bisulphide of carbon. Another important advantage of linolein is that it expands in drying, which peculiarity adapts it to iron surfaces; since cracks, however minute, resulting from shrinkages, expose enough of the metal to afford a chance of corrosion, which will spread in all directions, undermining the paint and causing it to scale off, besides discolouring it. With all its advantages, however, the best linseed-oil paint is but poorly adapted to long service as a protection to iron surfaces exposed to extreme variations of temperatnre and to all kinds of weather.
Even the continuous film of linolein, notwithstanding its compactness and the additional substance afforded by the body of the paint, gradually loses its toughness, curls up, and peels off. If chipped by accident before it has lost its hold on the iron, we find, if we carefully examine the exposed spot, that a thin film of oxide has formed under it. This fact accounts for its diminished adhesion. Iron, in uniting with oxygen to form a rust, increases its bulk in proportion to the amount of oxygen it has taken up, and necessarily occupies greater space. In a word, it swells, and in so doing pushes off from it the paint film, which sooner or later drops away from it. This undermining action of rust is the chief difficulty to be contended with in effectually preserving iron surfaces by means of paints or varnishes. It is not improbable that the linolein, itself an oxide, may impart oxygen to the iron, and thus promote rusting. This idea has been suggested by Prof. Williams in a recent treatise on the subject; and, whilst purely speculative, it may account for the oxidation of iron surfaces when to all appearance effectually protected by a coat of paint thick enough and continuous enough to exclude both air and damp.
In selecting a paint for iron, mechanical adhesion is a consideration- of the first importance. In this respect paints differ widely; but it must be remembered that in painting or varnishing a metallic surface, mechanical adhesion is all we have to depend upon. With absorbent surfaces it is different. Prof. Williams gives it as his opinion, based on observation and experiment, that pitchy or bituminous films are especially effective as regards their adhesion to iron: for example, solutions of asphalte, or pitch, or petroleum, or turpentine. These are also very effective as regards continnity, owing to the fact that in drying they form plastic films, which yield with the contraction and expansion of the iron, and show no tendency to crack. If the surface is rusty, they penetrate the oxide scale and envelope the particles very effectually, making them a portion of the paint. The solubility of such a film may be counteracted by mixing it with linseed-oil. The experiment may easily be tried by mixing about 2 parts Brunswick black with 1 of red- or white-lead or litharge.
Red-lead is the best for many reasons, if finely-ground and thoroughly mixed with linseed-oil. Any one of the several kinds of bitumen may be used; either natural mineral asphalte, pine pitch, or artificial asphalte, such as gas-tar or the residuum of petroleum distillation in cases where the crude oil has been distilled before being treated with acid. This gives a very hard bright pitch, which is soluble in "once run" paraffin spirit, and makes the base of an excellent cheap durable paint for ironwork in exposed positions. During the past few years have appeared many accounts of the preservative influence of paraffin when applied to iron surfaces, and recommending it in all classes of ironwork which can be treated hot. The most effective mode of applying it is to heat the iron in vacuo, in order to expand it and open its pores, when paraffin raised to the proper temperature is poured into it. By this means the iron is penetrated to a sufficient depth to afford a very effectual protection against oxidation, especially when a suitable paint is subsequently applied.
Any non-oxidisable substance would probably answer, but paraffin is as cheap as any, and quite as good, if not better; the exception as to quality being made in favour of some vitreous enamel, which, while costing more, would certainly be more permanent in its results. Brushed upon the outside merely, it is doubtful whether paraffin would hare much effect upon the iron, while it certainly would tend to lessen, if not destroy, the mechanical adhesion of a surface paiut. There is no reason, however, why bridge-work, iron fronts, etc, should not be treated with paraffin before they leave the shops where they were made, which would greatly simplify the problem of their easy and economical preservation from oxidation. In the absence of such treatment, a careful coating with the paint above described will probably prove the most effectual means of protecting iron surfaces. (Amer. Painters' Mag.)