The Chemical Colouring of Metals

This information was originally published in Practical Engineering 1940 Vol1 No18. The information within this article is therefore correct as of 1940. The publication of this attempts to provide historical insight on techniques employed in process industries during this time period.

If a metal is left alone for a sufficient length of time, it invariably tends to colour itself. Thus copper will either turn green or brown according to the conditions under which it has been exposed. Zinc and lead will usually acquire a whitish appearance; iron, of course, will turn brick-red in hue, whilst even gold itself will, under some conditions, acquire a deeper and mellower coloration.

The colouring of metals by chemical means is usually termed "bronzing," although, strictly speaking, such a term should only be applied to the browning of metals. If, however, we turn iron blue or copper green, we usually refer to the "bronzing" of the metal.

Polishing and Cleaning

It is of the greatest importance that all metal objects selected for chemical colouring be perfectly clean. The object should first be polished. Then it should be "degreased" by swabbing it over with methylated spirit or some other grease solvent. Finally, it may be advisable to dip the metal object in warm dilute hydrochloric acid or spirits of salts for a minute or two in order to scour it thoroughly. After this treatment, the object is rinsed in warm water, and is then ready for "bronzing."

If a metal object is not scrupulously clean, its subsequent colouring will very frequently be patchy and uneven. Also, the colouring may not be permanent. Hence, it will be clear that a thorough cleaning of the - metal object before" bronzing " or colouring is an absolute essential to the success of whatever process may be used, and in all the instructions for chemical colouring given in this section it will be assumed that the metal object undergoing the process has been previously thoroughly cleansed and, indeed, scoured.

A Dead-Black Surface

Most common metals can be given a dead-black surface coloration very readily by chemical means. For instrument work, such a coloration is very useful-and often, indeed, a necessity. The black colour, unlike many of the painted-on lacquers, does not flake off or chip away. Brass and copper articles can be blackened by immersion for a few minutes in the following liquid; 1oz. Copper nitrate, 3oz. Water. A small quantity of silver nitrate dissolved in the above solution is said to improve the black coloration produced upon the metal, but its employment is by no means essential.

Copper (but not brass) articles may be made to acquire a slightly shiny black surface by immersion in the following solution: 1 part Ammonium sulphate (liver of sulphur) and 4 parts Water.

Brass articles take upon themselves a steely-grey colour in this solution. By immersing iron articles in a solution of photographers' "hypo," they are given a blue-black colour, particularly if a little lead acetate or nitrate is dissolved in the hypo. Silver immersed in sodium-sulphide solution turns almost black, while a black colour on zinc can be obtained by immersing it in a solution of antimony chloride.

Cosletising

A pleasant grey colour is produced on iron by boiling it for half an hour in a weak solution of iron phosphate. This process is akin to that of "cosletisation," a thin film of iron phosphate and oxide being formed on the surface of the metal.

In order to colour brass or copper a variety of shades ending in black, the metals should be immersed in a very dilute solution of ammonium or sodium sulphide. Brass, for instance, placed in an extremely dilute solution of either of these sulphides will acquire a golden appearance, whilst copper, in the same solution, will be reddened. By making these sulphide colouring solutions stronger, or by allowing a longer time for them to act upon the metal, it is possible to obtain almost any yellow, red, brown, or black colour desired on these metals. Steel articles can be "blued" simply by passing them through a flame. Better still, they may be blued by boiling them for a short time in strong solution of hypo containing a little lead nitrate.

Antique Effects

The production of antique effects on articles of brass and copper will be of interest since by careful working beautiful effects of these metals can be obtained fairly readily. The green or brown coloration which an article of brass or copper usually acquires by age and from exposure to the elements is termed" a "patina," the word signifying an encrustation. Copper, bronze, and brass patinas can be divided into two varieties, namely, green and brown. The latter is the easier to imitate by chemical means. If, for instance, an article of copper is dipped in a dilute solution of sodium sulphide, it will instantly acquire a brown patina, the exact shade and depth of the coloration being dependent upon the strength of the sulphide solution. Brass acquires a good patina of the brownish variety when it is heated in a paste made of sulphur and lime.

The green patina which is often seen on brass or copper articles of great age, and which is often very beautiful in appearance, consists for the most part of a layer of copper carbonate. We may obtain such a patina on brass and copper articles by burying them in damp earth for a considerable period. Such a process, however, is a slow and an uncertain one.

Green Patina

An excellent green patina can be given to copper and brass objects by suspending them from some improvised wooden stand and then placing them in an airtight container. Within the container is placed, also, a small vessel containing some ordinary washing soda or bicarbonate of soda, together with a little water. The metal articles are brushed over with strong vinegar, or, better still with dilute acetic acid, and a little of the acid is poured into the soda-containing vessel, the container then being immediately closed up. The carbon-dioxide gas evolved from the soda-acid mixture will react with the acetic acid on the metal' articles, and gradually the latter will acquire a yellow-green coloration and a hard, shiny surface.

The operations mentioned in the above paragraph should be repeated every alternate day until the metal articles have been sufficiently coloured, a task which will occupy about two or three weeks.

A quicker method of obtaining a green colour upon brass or copper articles consists in painting them over daily with the following solution : 3 parts Copper Carbonate, 1 part Sal-ammoniac, 1 part Common Salt, 1 part Copper Acetate, 1 part Cream of Tartar and 8 parts Strong Vinegar.

This solution gives a blue-green coloration which takes about four complete days to develop. Quite a good yellow-green coloration may be obtained on copper and brass (particularly the latter) articles by brushing them over daily with a mixture of vinegar, common salt and ordinary sugar. Note that for the production of these antique green colorations the metals must not be immersed in the solutions, but merely brushed over with them.

Dulled Aluminium

The silvery appearance of aluminium is not always desirable. It may, however, be permanently and uniformly dulled by dipping the metal in a hot, moderately strong solution of caustic soda (sodium hydroxide) for a few seconds. The metal will thereafter have a matt appearance. If aluminium so treated is immediately rinsed in warm water and then immersed in a hot solution of an aniline dye, the aluminium surface will take up a little of the dyestuff, and will become permanently tinted. This constitutes an imitation of the now well-known process of "anodising" aluminium and the subsequent "dyeing" of the metal.

By immersing zinc in a hot solution of ammonium molybdate containing a little free ammonia, a deposit of metallic molybdenum-plated zinc has a very fine colour, ranging from an iridescent golden yellow to a steely brown. Aluminium articles can be made to acquire a dusky hue by the same process.

What is known as "oxidised silver" is simply silver which has been immersed in a weak solution of liver of sulphur (potassium sulphide) containing a little ammonia. Very weak solutions produce the best results, for in strong solutions the silver is merely blackened.

Similarly, nickel-plated articles may be " oxidised" by immersion for a few seconds in the above solution, in which they usually acquire a dark golden tint.

Brass articles may be made to acquire an extraordinary series of colorations ranging from pale gold to pink and pale blue simply by immersing them in a solution containing half an ounce each of lead acetate and "hypo" (sodium hyposulphite) to the pint of water.

Permanent Coloration

It is difficult to obtain good permanent colorations on tin objects. If, however, a sheet of tin is heated to near its melting-point and is then suddenly plunged into the following solution, nitric acid 1 part, sulphuric acid 10 parts, water 89 parts, the surface of the metal will acquire a very beautiful crystalline appearance to which the term moire metallique (watered metal) has been applied.

Metal articles which have been chemically coloured should invariably be well rinsed in warm water and then dried in warm sawdust before a fire. Afterwards their surface appearance may be heightened and improved by rubbing them over with a soft polishing cloth charged with a little light oil.