This section is from the book "Safe Building", by Louis De Coppet Berg. Also available from Amazon: Code Check: An Illustrated Guide to Building a Safe House.
All parts which are planed and are to make exact joints, and cannot, therefore, be painted, should be kept carefully covered at all times until erection with a heavy coating of lard and white lead mixed.
All iron and steel pieces should be so designed that all parts are readily accessible for painting. For this purpose it is usual to use any of the so-called metallic paints, which are made mainly from earths rich in iron ore (red oxide of iron). A heavy coat of this should be put on immediately after finishing, first removing all loose scales. In practice, however, this is rarely done, as it is found cheaper by the iron contractor to do his own painting, rather than to pay the mill price. The architect should insist, however, that every speck of rust and all loose scales be cleaned off before any painting be done, and should not be talked out of it by the so called practical man (who always has plenty of "pocket judgment").
In place of metallic paints, a mixture of red lead and linseed oil makes an excellent protector, but is rather more expensive than must metallic paints, as it. does not cover so much surface as the latter. Some authorities claim that lead paints set up galvanic action with the iron, and so injure it. On the other hand, if the metallic paint is made from a protoxide of iron, instead of red oxide, it is said it will rust within itself.
Protection from Rust.
There should never he less than two coats of metallic or red-lead paint on iron before attempting to finish same in colors, bronzing or gilding. It will take at least two additional coats of white lead and oil paint in colors to hide the color of the metallic paint. These two additional coats should also be put on under and before bronzing or gilding. Bronzing is done on in-terior work only, and is done by painting with a mixture of bronze powder and varnish. In gilding, the paint is covered with a coat of the usual oil gold-size, and then the hammered gold-leaf is put on. Iron is frequently galvanized, which gives it an effectual protecting skin so long as this remains intact, and is not cracked or broken by bending or blows. This skin consists of a thin coating of zinc. The iron is first cleaned by being soaked in a weak solution of sulphuric acid, is then sand-papered and washed. After this it is dipped, while hot, into a bath of chloride of zinc and then "plunged into molten zinc, the surface of which is protected by a layer of sal ammoniac."
In many cases, particularly with cast-iron pipes, the iron is tarred. This is a very good protection for the iron, but is apt to cover up and hide defects, such as sand-holes, etc.; for this reason the use of tarred pipes is sometimes prohibited, notably by the New York City Board of Health. The iron is heated to 700° F., and dipped into a mixture of coal-tar, pitch and five per cent to six per cent of linseed oil, heated to 300° F. The iron is left in it till it acquires the temperature of the mixture, when it is removed. In practice, the iron is usually dipped into the mixture without the preparatory heating.
Sometimes iron, notably registers and hardware, is japanned. This consists in painting the iron with a mixture of lead-paint, oil and copal varnish, with successive coats of copal varnish, all dried at a very high temperature.
Sometimes iron is protected and beautified by electro-plating. This consists of depositing on the iron in an electric-bath successive, but very thin, layers of brass, copper, bronze, etc. These effects are very beautiful, but expensive. The Bower-Barff process turns the outside skin of the iron into a magnetic oxide of iron, the color being of a very beautiful dark blue-black, and susceptible of a high polish. The iron is thoroughly cleaned and put into an air-tight chamber, and kept at a very high temperature. Superheated steam, or air heated to a very high temperature, sometimes as high as 1600° F., is then passed over the iron for from five to seven hours.
The effectiveness of this coating to resist rust depends upon the length of the exposure in the oven and the height of the temperature. A strong cement for filling poor joints between ironwork under compression is made of sal ammoniac, iron filings and sulphur. The more iron used in proportion to the other ingredients, the slower will the cement set.
It is frequently attempted to get extra-strong irons or steels by re-working them. In cast-iron, as a rule, re-melting, within reasonable limits, increases the strength of the iron. Box gives one case where the second melting added some forty per cent to the original tensile strength of the pig-iron; the third melting added some fifty per cent to this, or more than doubled the original strength; and the fourth melting added another twenty per cent to this, or the metal became about two and one-half times its original strength (tensile). On the other hand, Mr. Fairbairn found that the transverse, tensile and compressive strength of cast-iron was reduced gradually to the third or fourth melting, then increased with each melting till they reached their maximum at about the twelfth melting, and after that again decreased very rapidly. Accordingly, if the strength of the pig-iron were = 1,0 the third melting (second after pig) would give a minimum transverse strength of 0,82 and a minimum tensional strength of 0,77; while the maximum transverse strength 1,41 and the maximum tensile strength 1,32 would be reached at the twelfth melting.
With compression, the minimum strength 0,92 would be reached at the fourth melting, and the maximum 2,18 at the fourteenth re-melting. According to Gauthier's analysis, re-melting turns gray irons to white, decreases the graphitic carbon and the silicon, and increases the combined carbon, thus rendering the iron weaker in resisting tensile or transverse strains, but stronger in resisting compression.