This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
The value of red-lead as a preservative for iron has been generally accepted. Wrought iron requires a hard and elastic paint, which will hold itself together even if the scale beneath gives way. The following experiments, made under the auspices of the Dutch State railroads, may be instructive. Iron plates were prepared for painting as follows : 16plates were pickled in acid (hydrochloric), then neutralized with lime (slaked), rinsed in hot water, and while warm rubbed with oil. The same number of plates were cleared of scale, so far as it could be removed by brushing and scraping. Plates from each set were then painted alike - namely, 4 with coal-tar and 4 with iron oxide A; another set with iron oxide B, and the remaining set with red-lead. They were then exposed 3 years, and the results observed were as follows: The coal-tar on the scrubbed plates was quite gone, that put on the pickled plates was inferior to the others. The iron oxide A on the scrubbed plates was inferior to the other two, while on the pickled plate it held well. The oxide B was found superior to that of A, but inferior to red-lead, while the plates covered with red-lead stood equally well on both prepared plates, and were superior to all others.
From these results it is evident that pickling the iron removes all the black oxide, while scrubbing doe3 not. It is also shown that the red-lead unites with oil to form a hard, oxy-linseed-oil aeid soap, a harder soap than that given by any other combination. The red-lead is shown by those experiments not to give way under the scaling; it is more adherent to the surface, more elastic and cohesive. On the Cincinnati Southern Railroad, experience extending over some years has shown that red-lead has proved the most durable paint in the many miles of iron trestle and bridgework. It is found that the iron oxide is washed away by the rain and perishes in spots, although a valuable paint if frequently renewed. Red-lead, on the other hand, is more expensive than iron oxide, and is difficult to be obtained pure. Referring to white-lead as a material for painting iron, one authority observes that white-lead should not, if possible, be used in priming iron, nor in any priming coat; moreover, it is a less desirable overcoat than iron oxide. The class of iron paints compounded of ores of natural iron rust, combined with clay or some other form of silica, are very useful, as they contain no water nor sulphuric acid.
Magnetic oxide, or pure iron oxide, is an excellent protection for iron, says one writer; it is impossible to scrape it off. It is also of value in woodwork, and resists the action of salt water and sulphurous gases, so destructive to most paints. There is no doubt the great protective element in paint is the oil, and the conditions required for success are stated to be to prevent the drying part of the oil from becoming hard dry; the soft-keeping, non-drying acids must be kept from flying away in such a quantity as to reduce the oil to a brittle mass. In other words, the elastic qualities of the oil must be protected from the action of the oxygen. According to Louis Matern, red-lead possesses the following advantages for the preservation of the iron, which is the main object to be gained : - (1) It dries easily with raw linseed-oil, without an oil-destroying drier. All known driers decompose oil.
(2) After drying, it remains elastic, giving way both to the extension and contraction of the iron, without causing the paint to crack.
(3) It imparts no oxygen to iron, even when constantly exposed to damp.
(4) It hardens, where it has been spread thickly, without shrivelling, forming the toughest and most perfect, insoluble combination of all paints.