This article first appeared in Practical Engineering 1940 Vol1 No25. The information contained within the article is accurate as of 1940. This article describes new methods in Engineering at the time.
Most metals can be coloured by chemical processes, and these are preferable to processes involving the use of coloured lacquers. Most metals change colour on being left exposed to the atmosphere, and real antique finishes result from this. There are, however, various methods of producing the "antique" appearance by quicker artificial means.
One of the advantages of colouring is that, in most cases, the finish prevents further oxidation and rusting, provided that the metal is not left out of doors. In fact, most of the widely-used rust-proofing processes change the colour of the steel treated.
Probably one of the most popular colour finishes is the "blueing" applied to steel. One method of obtaining this finish is by dipping the steel into molten saltpetre and holding it there for a few seconds. Another is to coat the metal with a paste made from thick oil and fine sawdust, and then to heat it until the-paste has all been burnt off. This can be done in the case of small objects by holding an iron plate over a gas-ring and then placing the steel objects on it. After the paste has been burnt the gas should be turned out and the metal allowed to cool slowly.
A simpler method of obtaining a-blue-black coloration on iron or steel is by immersing the metal in a hot solution of sodium thio-sulphate (photographer's hypo). The latter should be made by mixing about one part by weight of the chemical with 50 parts of water. Immersion should be continued for between 10 and 15 minutes. The actual colour can be varied over a fair range if one part by weight of lead acetate is added to the above solution. After about 30 minutes' immersion the darkest colour will be reached, but the metal may be removed at any time immediately the desired shade has been reached.
A modified and better method of blueing steel is by boiling it in a strong solution of hypo to which a little lead acetate has been added. Here again the metal should be removed when it has reached the desired shade. Iron and steel can be made a pleasing grey shade by boiling them for a period of up to half an hour in a fairly strong solution of iron phosphate.
Coslettising is essentially a rust-proofing process, but it is convenient for giving a black coloration to steel. The process consists of immersing the metal for about 30 minutes in a hot bath made by adding 1/2 ounce of phosphate of iron to a quart of water; to make the phosphate dissolve it is necessary to add a few drops of phosphoric acid.
"Antique" Brass and Copper
Brass and copper readily lend themselves to colour treatment, and can be chemically coloured in various ways. For example, an "antique" finish is produced if the object is put in an airtight box containing a saucer of water and also a saucer of hydrochloric acid. A little broken chalk or marble should be added to the acid about twice a day, the process being allowed to continue for a few days until the required shade of green is obtained. It is possible to obtain a very rich green colour by this means.
A range of shades right up to black can be produced by immersing brass and copper articles in a dilute solution of ammonium sulphide or ammonium sulphydrate. The stronger the solution is made the darker will be the colour produced in a given time. It is possible to find the most suitable strength of the solution for any particular colour only by experiment. The best method is to make the solution fairly strong initially and then to make tests with odd strips of metal similar to that to be treated. The solution can gradually be weakened until the coloration is most pleasing.
A "Shot" Effect
An excellent "shot" effect can be obtained on copper by first washing it well with cold water and, before the surface is properly dry, rubbing over it with a rag dipped in ammonium sulphide. Rub lightly and move the rag in circular strokes.
To produce a pale green colour on copper the metal should be immersed in a cold solution made by dissolving 250 grains of ammonium chloride and 250 grains of common salt in a quart of vinegar, and then adding 1 ounce of liquid ammonia.
Brass can be coloured to almost any shade. For example, a bronze finish may be obtained by immersing it in a hot solution made by dissolving one part of ammonium chloride, three parts of iron oxide and three parts of copper acetate in 100 parts of water. To make it steel blue, immerse the metal in a warm solution made by dissolving 4 ounces of hypo crystals and two ounces of lead acetate in a quart of water.
Zinc can be made a brownish black by dipping it into a cold solution of one part by weight of copper nitrate to five parts of water. After dipping, the metal should be well washed in running water to remove all traces of the solution. A colour between grey and black may be obtained by using a bath made by dissolving 2 ounces of ammonium chloride and 2 ounces of copper acetate in one pint of water. A few seconds after immersion the surface of the metal will be seen to be covered with a black deposit. At this stage the metal should be removed from the bath and well rubbed with a soft rag; the process of immersing and rubbing should be repeated about six times, after which the required colour will remain despite hard rubbing.
It is seldom required to colour tin-plate, but it may be made crystalline (the finish is described as moire metallique) by heating the plate until the tinned surface just reaches melting point and then dipping the sheet in an acid solution. This is made by very slowly adding 4ozs. of sulphuric acid to 2-1/2 pints of water and then adding 1/2oz. of nitrate acid. Since considerable heat is generated when sulphuric acid and water are mixed, great care should be taken that the acid is added to the water very gradually, preferably using a glass or porcelain dish; a photographic developing dish is suitable.
Aluminium can be given a pleasing "frosted" appearance by washing it in warm water containing a pinch of potash, and then immersing it in a l-in-20 solution of caustic soda to which is added loz. of common salt to each quart of solution. This will soon turn the metal black ; it should then be removed, washed in cold water, and dipped in a l-in-10 solution of nitric acid in water. Finally, wash the metal thoroughly in hot water.
Another method of finishing aluminium is to dull the surface by immersing it in a l-in-10 solution of caustic soda and then, if desired, dipping it in an aniline dye after first well rinsing in cold water.
The above methods of colouring are given on the assumption that the metal to be treated is perfectly clean and well polished. If not, it should be freed from grease by wiping well with a rag moistened with carbon tetrachloride or methylated spirit, or by scrubbing in strong soda water. If soda is used, wash the metal with hot soapy water before drying. Always wash and dry the metal after the chemical treatment.
The prime object of lacquering is to prevent subsequent tarnishing due to exposure, and, therefore, a transparent lacquer is most suitable. There are two kinds of lacquer : one for use when the metal is cold, and the other for use after heating the metal. The latter is always to be preferred since it is difficult to avoid a "milky" and "treacly" finish with cold lacquers until a fair amount of experience has been gained. There are on the market, however, various cold lacquers with an amyl-acetate base which are often satisfactory. These are sold under a variety of trade names. Shellac varnish may be used, but this is not recommended.
Hot lacquering is better in every way. The procedure is to place a sheet of iron over a gas-ring and heat this thoroughly. After that, the object to be treated may be laid on the metal and heated through by frequently turning and moving it. When it has been heated uniformly to such a temperature that it can just be handled without discomfort, a thin coat of lacquer should be applied quickly with a camel-hair brush. After covering, replace the object on the hot tray and turn out the gas so that cooling is gradual.
One of the most convenient colourless hot lacquers is made by thoroughly mixing 11ozs. of shellac and 1oz. of gum sandarac with 1quart of methylated spirit. After mixing, allow the varnish to stand for four or five days and then strain through a muslin rag. It can be stored for indefinite periods in an air-tight bottle, which should preferably be kept in a dark place.
Various coloured lacquers can be made for use on warm metal. As an example, green is made in exactly the same manner as colourless but 1 1/2ozs. of turmeric is substituted for the 1oz. of sandarac. An excellent bronze lacquer is made by mixing 1/2oz. of shellac, 1/2oz. of sandarac, 1/8oz. of gum acaroides and 1/8oz. of gamboge in 1 quart of methylated spirit. After thorough mixing the solution should be placed in a warm— not hot—place for a few hours; near a window during strong sunshine or in a cool oven, for instance. Allow it to cool and then strain ready for use.
A gold lacquer is made by grinding together 2-1/2ozs. of fine shellac and -1/8oz. of red sanders and dissolving this in 1 quart of methylated spirit. As in all other cases, the lacquer should be strained before use. If the straining is not done, small pieces of shellac will be applied to the metal so spoiling the smooth polished surface.
As a simple alternative to lacquering it is possible to obtain a pleasing effect on most copper and brass objects by rubbing them with a good wax polish of the kind used for woodwork. This will prevent tarnishing of objects kept indoors, but is not suitable if they are exposed to the weather.