The best method of obtaining a coating resembling patina is to immerse the article in a solution of nitrate of copper, and then to place it while still wet in a chamber containing an abundance of carbolic acid. In this case the development of the green incrustation may be observed from day to day. If after a week the object has not yet obtained the proper colour, it must be again dipped in the above solution, and this operation is repeated till the desired shade has been acquired. As the formation of this green colour proceeds in the same way as in the open air, but more rapidly, a handsome and permanent coating of green can be produced by this means.
CHEMICAL METAL COLOURING
How to put a thin coating of colouring on the surface of metals by chemicals: A great variety of shades may be obtained simply through heat. The colours ordinarily produced on copper articles by means of heat come through the polishing. Any metal whose surface is highly polished will take on a number of different colours, beginning at a straw colour and changing to a dark straw, purple, dark blue, light blue, and steel or gray, by heating the piece to be coloured to a temperature of 630° F.
Barium sulphide and water (a very good composition for colouring metal): Immerse object until it assumes desired tint from light brown to violet to black. Heat gently over bunsen flame until a rich deep violet appears. After it is cool rub with soft cloth.
Patina imitation (a very good composition for the greening of copper):
- 1 part sal. ammoniac,
- 3 parts cream tartar,
- 6 parts salt,
- 12 parts hot water,
- 2 parts nitrate of copper.
Saturate soft cloth and rub or sprinkle the solution on the copper. Let stand in the open air ten or twelve hours. Wash the colour off with clear cold water. Tone down to suit.
Another way of colouring metal.
Silver.
To oxidize silver: The following is a solution commonly used for oxidizing silver: