This section is from the "Architectural Iron And Steel, And Its Application In The Construction Of Buildings" book, by WM. H. Birkmire.. Also see Amazon: Architectural Iron And Steel, And Its Application In The Construction Of Buildings.
Metals are bronzed by chemical agents or by the application of bronze powders. The latter are principally applied upon metals to imitate bronze. To give the object a gold bronze, ochre is triturated with linseed oil and red lead, and some blue or black mixed with it, so that a dirty green color results. Two or three layers of this color are applied. The last coating should not be allowed to become entirely dry. The bronze powder is laid upon this layer with a badger brush or otherwise, and adheres strongly. After drying and dusting off the superfluous bronze powder with a stiff brush, the whole is covered with a coat of transparent and colorless varnish.
Silver bronze, copper bronze, red bronze, and green bronze are frequently used for finishing iron work.
An enamel for cast and wrought iron is obtained by fusing together finely pulverized crystal glass, calcined soda and boric acid. This enamel is glass-like, transparent, and lasts well upon sheet iron.
Electro-Plating in gold, silver, nickel, copper and brass is a favorite method of finishing iron. The articles should first be ground or polished to free them of grease and make their surfaces purely metallic.
They are then dipped in dilute sulphuric acid and water, and pickled in nitric acid, sulphuric acid and common salt. After being thoroughly rinsed they are brought without delay into the bath, which is connected to a dynamo-electric machine.
The metal to be coated requires to be freed of oxide and impurities of every description before it will take a proper coating of zinc.
The first step in the process is to pickle the sheets in a tank containing sulphuric acid diluted with water. In from one to two hours the scaling of the iron is effected; the sheets are then withdrawn and transferred to large, shallow washing vats of wood, in which they are washed in a stream of fresh water. They are next subjected to an inspection, and such patches and scales as may yet adhere in spots to the pickled sheet are scraped and brushed with a stiff brush; they are then passed into a second tank filled with clean water and allowed to remain for 12 to 24 hours, which removes all traces of sulphuric acid and basic sulphate of iron. From the clearing tank the sheets are arranged on edge in a rack which is mounted on a truck and rolled into a drying chamber. In about 20 minutes they are removed and are then ready for dipping.
The galvanizing pot is a rectangular vessel of heavy boiler plate, riveted in the most substantial manner, about 4 feet deep, 12 feet long, and 15 to 24 inches wide, and capable of containing 20 to 30 tons of zinc. A fire space is kept around the tank, and the heat of the bath is about 10000 F. The sheets are passed one by one into the melted zinc on a pair of rollers, and as they emerge on the other side are seized at one. end by an iron gripper and drawn through a layer of sand strewn on the surface of the melted zinc, the object of which is to remove all the superfluous metal.
In the setting of the zinc coating, which takes place almost instantly after the withdrawal from the bath, the beautiful crystallization of the metal ensues which is so much admired.
On exposure to the atmosphere all metals soon lose their metallic lustre. They oxidize by absorbing oxygen from the air. Iron becomes coated with a layer of rust. Every drop of rain causes a rust stain, and hence only iron perfectly free from rust can be successfully painted.
When the oxidizing process has once commenced, its progress may for a time be interrupted by painting, but it progresses slowly even under the paint, the latter finally peeling off together with the layer of rust.
The principal point in painting iron is the priming color. If this is defective or applied incorrectly, the efficacy of the entire work is doubtful, even if the succeeding coats are properly laid on. In order that the priming color shall adhere firmly to the iron, three conditions are required: (1) it must be capable of drying quickly and thoroughly; (2) it must be thinly fluid; and (3) it must be applied in a thin layer only. The priming color is linseed oil with red lead. For painting use 1 part of verdigris, 1 of white lead and 3 of linseed oil; or 1/2 of verdigris, 1 1/2 of white lead, and 2 1/2 of linseed oil; the iron to receive three coats, the first before the iron is used, the second after the first is thoroughly dry, and the third three days later.
Malleable Castings are formed by subjecting the castings to a process of annealing in boxes with hematite iron ore or black oxide of iron. The boxes are kept in an annealing oven under an equable heat, the duration of the process depending on the form and size of castings.
A lacquer protecting the iron from rust and presenting a beautiful black appearance is com posed of asphalt, pine oil and colophony. For coating iron on a large scale asphalt tar is used, first freeing the scales from the iron by immersion in diluted hydrochloric acid.