The Chemical Gazette 332, 15.8.1856
By A. O. Mathey.
In Switzerland, the galvano-chromic process has been for some time employed in colouring different parts of watches, and this has induced the author to institute a series of experiments upon this mode of colouring metals, the foundation of which consists in covering a metallic surface with an extremely thin coat of oxide, which then produces certain colours like those produced in the tempering of steel.
The galvanic apparatus employed by the author is a small permanent battery of two pairs; the electrodes and conducting wires are of iron or platinum. The oxides which the author has hitherto made use of as colouring agents are peroxide of lead and peroxide of iron.
Preparation of the Lead Solution.
425 to 450 grms. of caustic potash are dissolved in a litre of distilled water, about 125 grms, of oxide of lead (massicot is the best) are added, and the mixture is boiled for ten minutes in a flask with a narrow neck, so that there may be as little access of air as possible. After cooling, the solution is decanted from the oxide of lead remaining undissolved, and diluted with distilled water until it shows 24° to 25°. B., this density being the most proper to furnish fine colours. It is kept in a well-closed bottle, so that no foreign matters may have access to it. As the solution is used, carbonate of potash is gradually formed in it. It is then boiled with caustic lime, and left to settle, and the clear fluid is again employed. From time to time it must be boiled with oxide of lead. When this solution is employed for colouring objects which exhibit rough spots, they do not acquire a uniform colour. The author considers the probable cause of this to be, that the fluid does not conduct the electricity so well as the metal; and this defect can easily be got rid of by increasing the conducting power of the fluid by the addition of an acid. For this purpose bitartrate of potash is often added; but this, according to the author, is the least fitted for the purpose, and may be much better replaced by oxalic acid, acetic acid, &c. It is best however to add no acid at all, as its addition greatly injures the solidity of the colour, and the object may also be obtained without it. For this purpose massicot is better than litharge, as it dissolves more readily in potash. It may be prepared, when this is necessary, by heating minium in an unglazed earthen pot to a dull red heat, and constantly stirring it with an iron rod until a sample of the mass exhibits a citron-yellow colour on cooling. Too strong a heat must be avoided, as this would fuse the oxide.
Preparation of the Iron Solution.
Although this solution presents difficulties in its preparation and employment, it may still frequently be made use of, and is even indispensable in some cases, as it furnishes tints which cannot be obtained with solution of lead. Sulphate of iron, which possesses a pale green colour and has not begun to oxidize, is dissolved in warm distilled water; the solution is boiled to drive off all the air, and drawn off into a well-closed bottle. When it is to be used, the necessary quantity of it is poured out of the bottle, and mixed with ammonia free of air until the precipitate produced is again dissolved, which however does not take place completely unless an acid or an ammoniacal salt be added at the same time. The solution, thus prepared, cannot be employed for more than an hour, because peroxide of iron is thrown down from it by the action of the oxygen of the air. The colours obtained by means of this solution are much less changeable than those furnished by the lead solution. They are brighter, and as solid as the blue which is produced on steel by heating it.
Preparation of the Objects to be coloured.
The galvanic colouring is employed as much as possible for metallic surfaces which are not liable to oxidation, as the object must be attached to the positive pole; and when its surface consists of an oxidizable metal, it frequently loses its brightness. For the peroxide of lead separated from the lead solution, a golden, or gilt, or platinum surface fur nishes the best recipient. On platinum it produces a beautiful blue, but on gold a green, owing to the colour of the metal shining through. On German silver and the other white metals, the green colour only appears after they have become blue. The colouring of silver does not take place in the same way as in the other metals, because it soon undergoes an oxidation, which renders its surface dull and prevents the appearance of the colour. The alloys which contain silver, even only in small quantity, do not colour well in consequence, and change rapidly, on which account silver must be carefully avoided in this process.
The goodness of the result depends especially upon the proper cleaning and preparation of the object. The better this is polished, the brighter will be the colour; a surface polished with a steel burnisher becomes more beautiful than one polished only with oxide of iron. Before colouring, each piece must be carefully cleaned, and especially freed from all fatty matter; for this purpose it is dipped into a solution of potash (an alcoholic solution is the best), and washed afterwards in water. For large articles chalk may also be employed. After cleaning, the objects must not be touched with the fingers, nor even with a cloth.
Mode of performing the Operation.
If we suppose that watch-hands are to be coloured, six pairs of them are put upon a steel rake, the teeth of which have the proper form and elasticity for fixing the hands with their prongs. The rake is put in connexion with the positive pole of the battery, and then immersed in the fluid. It must be covered by the latter to a depth of about 25 millims.; if it be immersed to a greater depth, the colours produced cannot so well be observed, and the desired tint is not so easily obtained. When all is thus arranged, the negative electrode is moved about in the surface of the fluid with only its point immersed. In the course of five to six seconds the watch-hands are seen to change; the first order of colours are allowed to pass, and when they become gray the second order begins. The gray disappears to give place to a yellow, which them also disappears, and is replaced by red. This moment requires every attention not to allow the desired tint to pass away; and in this respect it must be observed that the colours do not appear so deep in the fluid as they really are. When the hands appear red in the fluid, they are violet in reality. When they are to be red, they must be taken out when they appear orange in the fluid. If the tips of the hands acquire the desired colour sooner than the heads, the tips are lifted out of the fluid, whilst the portions which are not yet sufficiently coloured remain immersed; and the current is then allowed to act interruptedly, that is to say, the tip of the negative electrode is immersed repeatedly for a moment in the fluid until the desired colour is produced throughout. The duration of the operation varies from 10 to 40 seconds. It is advisable to treat a large number of watch-hands at once, as they then turn out more uniform in colour.
If the current be too strong, hydrogen and oxygen gases are evolved at the electrodes. The object then acquires a grayish appearance, and the iron electrode becomes covered with spongy lead. Under these circumstances the current must be weakened, and the operation commenced afresh after the object has been again polished. A brass plate of a certain size, when exposed to the action of the current, remains passive, and acquires no colour whatever. If this be the case, a small portion of the plate must first be immersed, and in proportion as it changes colour it must be further immersed. If the object be large, it infallibly acquires several colours, because the portions most distant from the point of connexion with the conducting wire are coloured most rapidly. This becomes of more importance the less the conducting power of the fluid. In order to do away with this inconvenience, the object must be united in several places with the positive pole, and the negative electrode must be allowed to run out in several wires suitably arranged. A fresh bath always produces several tints upon the same plate, and the bath improves by use. The hands placed upon the rake are therefore allowed to remain some minutes in the fresh bath, and then coloured in an old one. If the colour do not turn out well, the object is cleaned in strong vinegar, and then coloured again. In this way the object may be coloured two or three times without a second polishing, when it consists of gold of at least 14 carats. When a gilt object has been submitted to the colouring process five or six times, the gilding is entirely removed, so that it must be again gilt and polished. If a coloured object be brought in contact with the negative pole in the lead solution, its colour disappears, the peroxide of lead being dissolved. This method of removing the colour is preferable to the use of vinegar.
Production of various Colours on the same Object.
When an object, such as a bouquet of flowers for a brooch or hair-pin, is to be furnished with several colours, it must first, if it be not made of gold, be strongly electro-gilt, and deadened according to circumstances. Those portions which are to retain the colour of gold are then coated by means of a hair pencil with black varnish (epargne noire liquide), and the object is connected with the positive pole, and put into the lead-bath. When all the flowers have become pale red, those which are to retain this colour are also covered with varnish; the object is again immersed in the bath, and the other flowers allowed to become violet. Those which are to retain this tint may then be coated with the varnish, and the remainder may be allowed to become blue. When the latter are coated in the same way, another immersion will render the leaves green. The green may also be shaded, as at first a dark green appears, which becomes lighter, and finally passes to yellow. The varnish is removed by treatment with cold oil of turpentine, and the object is cleaned first with soap and water and a soft brush, and afterwards with warm water and a cloth. These various colours, which resemble the natural colours of flowers, and are deposited upon a ground of dead gold or silver, have a splendid effect, and in brilliancy and lustre leave the paintings on enamel far behind them; but they unfortunately do not possess the permanency of the latter. Single silver flowers with gilt stamina produce a pretty effect in such a bouquet.
Causes of the Change in the Galvanic Colours, and the means of preventing it.
Dry air produces no alteration whatever in the colours produced by peroxide of lead; but this is not the case with moist air, especially when it contains traces of sulphurous acid or sulphuretted hydrogen. On this account the colours of watch-hands are changed by the exhalations from the body, if the case of the watch is not perfectly air-tight. The author frequently observed, that of two pairs of hands which had been coloured under similar circumstances, the one had entirely changed its colour in eight days, whilst the other was completely unaltered after the lapse of a year. He long tried in vain to discover the cause of this, but is now convinced that the rapid change of colour was to be attributed to the presence of a trace of potash, under the influence of which oxide of lead was reproduced, and this combined with the potash. This last cause of the destruction of the colour may be easily guarded against by washing the object when coloured with boiling water, so that all potash is removed, wiping it, and drying it on a hot iron plate. With regard to the alteration of colour produced by the action of moist air and air charged with foreign matters, Becquerel has recommended the application of a protective varnish to the coloured objects. This varnish must have as little reducing action as possible, so as not to decompose the peroxide of lead. For this purpose Becquerel recommends the following varnish: —
½ a litre of linseed oil,
4 to 8 grms, of prepared litharge, and
2 grms, of sulphate of zinc are put into a glazed pot, and the mixture is heated moderately for some hours. The clear varnish is then decanted from the undissolved portion, and mixed, when it is too thick, with oil of turpentine, which must be previously boiled with oxide of lead to remove all traces of acid. The object is verythinly coated with this varnish by means of a hair pencil, dried by a gentle heat, and then coated a second time. By the application of this varnish, the colours, as Becquerel remarks, lose something of their lustre, and also appear partly of a different tint, but they gain in durability. According to the author's experiments, Becquerel's varnish is inapplicable, and every varnish applied over the red colours causes them to appear yellow. If the varnish be removed, the red colour appears again unchanged. The action of the varnish therefore does not depend upon a change of the peroxide of lead, but upon the alteration of the thickness of the stratum laid upon the metallic surface upon which the colour depends.
- Polyt. Centralblatt, 1856, p. 612.
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