Cast Iron should be painted directly it leaves the mould, in order to preserve the hard skin which is formed upon the surface of the metal by the fusing of the sand in which it is cast. After this a second coat will be required, and will generally be sufficient for the preservation of the iron from atmospheric influences.
Before painting wrought iron, care must be taken to remove the scales or film of oxide formed upon the surface of the iron during the process of rolling, and which, by the formation of an almost imperceptible rust, becomes detached from the iron itself.
Paraffin may with advantage be substituted for the oil.
"But somewhat the same results may be obtained by allowing the ironwork to rust, and then scraping the scale off preparatory to painting. If some rust remains upon the iron, the paint should not be applied lightly to it, but, by means of a hard brush, should be mixed with the rust."1
The paints made with oxide of iron (some of which will be described in Part III.) are therefore preferable for this purpose; but they must be used alone, and not laid upon a priming containing lead, or the two metals will set up a galvanic action as above described.
Bituminous paints are said to adhere better than others to the surface of the iron, and to form a plastic film which yields without cracking when the iron expands and contracts under changes of temperature.
1 Matheson's Works in Iron.
All tooled surfaces in ironwork should be coated with tallow and white lead.
Gilding is of two kinds - "burnished " bright or left "dead." The latter description is most usual in the decoration of buildings.
The painted surface is covered with "oil gold size." When this is dry but sticky, the gold leaf is laid on in narrow pieces, overlapping slightly at the edges. These are pressed down to the surface with cotton-wool, and the loose portions brushed off.
In gilding varnished work, white of egg beaten up in water is applied to those parts where the leaf is not required to stick.
When woodwork is to be gilt with burnished gold, a different size is used, called " burnished gold size.*' The leaf adheres to this, and when the size becomes hard the surface of the "leaf is rubbed bright with a dog's-tooth or other burnisher.
In gilding ironwork the surface of the iron must be very carefully cleaned, and then painted, first with two coats of oxide paint and then two coats of lead paint of light colour. It is then ready for sizing and gilding.