Whilst the reduction is going on it is advisable to renew the potassium cyanide at least once, or even several times, if large and greatly oxidized bronzes are under treatment, for otherwise all the potassium cyanide may be consumed by the changes in progress; this can be ascertained with certainty by a smell of chlorine. When the bath requires renewal the bronze may be taken out with a pair of metal tongs, or if too large, two strong copper wires should be passed underneath it, the ends of which are wound round a strong glass rod or wooden stick. The bronze should then be well rinsed or brushed with a soft brush before it is put into the fresh bath.
Bronzes are frequently met with which are much deformed by an earthy or sandy layer cemented by oxide. These incrustations can be partly removed by a preliminary treatment with dilute hydrochloric acid, but the bronze must be afterwards carefully rinsed with water or even steeped to prevent unnecessary decomposition of the cyanide by the acid. Before reduction it is useful to secure thorough penetration by placing the vessel containing the solution and the bronze under a bell glass attached to an air pump, as has been previously explained (p. [68]).
During the process of reduction small whitish-green crystalline needles often collect on the platinum foil, but although in large numbers they are so minute that it has not been possible hitherto to determine their composition; they seem to contain copper and cyanogen. After some time the platinum becomes covered with a whitish-green or brownish deposit, which should be removed by rinsing in water and brushing; if this should not succeed the platinum must be dipped in hydrochloric acid, rinsed with water, and rubbed with fine sand. The glass vessel may be cleaned in the same way.
The reduction is complete when all the chlorine, previously combined with the metal, has combined with the hydrogen produced by the electrolysis of the water. There being no further chlorine with which the hydrogen produced by the continued action of the current may unite, the completion of the process is marked by the appearance of bubbles of that gas upon the surface of the bronze. The bubbles which rise from beneath often mark out the outlines of the object upon the surface of the bath.
Before the bronze is washed it should be placed in a fresh cyanide bath, but of 1% strength only. For large and especially for thick objects, this bath must be renewed several times, so as to allow the washing process to begin in the bath itself whilst the current is still passing through it. Care should also be taken that every side of the object in turn faces the platinum foil for some time, for if one side remains turned toward the platinum throughout the process, it will sometimes assume the red tint of copper, while the rest of the bronze retains a somewhat dark colour.
When finally removed from the reducing bath, after the black metallic powder has been thoroughly cleaned off with water and a soft brush, the object should be suspended for a short time in water at the ordinary temperature, or so fixed that there is a good depth of water beneath it; it should then be washed in hot water. When the bronze is first placed in water, whether hot or lukewarm, small bubbles of hydrogen will continue to rise for some time, while at the same time a whitish, or sometimes grey, gelatinous precipitate, consisting of a hydrated oxide of tin[144], will often fall from it. The grey colour is caused by the admixture of small particles of lead or copper.
At first I renew the water two or three times a day, then once in twenty-four hours, and finally at longer intervals, using distilled water throughout for small objects, but for larger specimens for the final washings only. For the earlier washings at any rate I use warm water. Cyanides as well as chlorides give a white precipitate with silver nitrate; this reagent will therefore serve to indicate the progress of the operation. If at the end of a fortnight in the case of small bronzes, or in three to six weeks for large objects, the water shows no cloudiness, or if upon the addition of yellow potassium chromate it instantly assumes a red colour (p. [62]), the steeping may be considered complete. Some Egyptian bronzes, especially those which contain a large proportion of lead, after steeping exhibit a whitish crystalline coating of lead carbonate or small hemispherical groups of crystals scattered over the surface of the metal, especially where the pores are large; when dry these can easily be removed.
An extended experience points to the conclusion that bronzes should be dried at once, and as quickly as possible. They should be wiped with soft cloths and then dried in a drying chamber or upon glass or metal rings on a stove. A simple form of drying chamber can be made with copper or iron plate of sufficient thickness, with a loose lid provided with a hole fitted with a cork, through which a thermometer passes. This can be heated over a Bunsen burner, but the temperature should not exceed 230°F. [110°C.]. Small objects may be freed from water by immersion in alcohol for twenty-four hours before drying.
The completion of the process may be gauged by the yellowish or reddish yellow colour which the bronzes should assume when they have been dried and wiped with a cloth or brushed; brushes made of the finest steel wire may be used for this purpose. A bright colour is but rarely seen on bronzes which contain lead. Egyptian bronzes frequently contain as much as 20% of lead, and such bronzes have nearly always a dull-grey or blackish appearance. A similar colour is seen on bronzes which contain no lead, but which are very porous, and are in an advanced state of decomposition. In such cases the finely divided particles of reduced metal are retained upon the rough surface of the bronze, and as all metals, when sufficiently finely divided, form a blackish powder without any metallic lustre, the whole object then appears almost black. It is difficult, and in many cases impossible, to remove this dust, especially that retained in the pores. Metal dust is injurious to the lungs, and if recourse is had to brushing, an efficient extractor for the removal of the dust-filled air is required[145]; but brushing and the use of bellows in addition frequently prove insufficient. Washing the objects with benzine is more effectual, but a trustworthy method of giving the bronze a better appearance is to place it into melted paraffin wax[146] at 250°to 285°F. [120° to 140°C.]. Yet the use of paraffin wax should be avoided if possible, for in spite of the most careful washing blue efflorescences may sometimes appear upon thick bronzes in the course of a year. If this should happen they must be washed out at once, and the bronze can again be submitted to the cyanide-reduction process. If however paraffin wax had been applied an attempt would have to be made to remove it by immersing the bronze in benzine or a mixture of ether and alcohol, or by heating, before the reduction process could be repeated.