(i)Bronze and Copper[139].

Fig. 28.Krefting’s method. Iron spear-head wrapped with strips of zinc.

Fig. 29.Iron spear-head before and after treatment by Krefting’s method.

Fig. 30.Iron pin from “Danes’ Graves,” Yorks. [Cp. Yorks. Phil. Soc. Report, 1897.]

Fig. 31.The same after treatment by Krefting’s method, still showing chalky accretions.

Fig. 32.Iron object from Lamel Hill[130], York. It appears to have been originally rivetted to wood or leather.

Fig. 33.After treatment by Krefting’s method.

Fig. 34.Piece of iron sword-blade showing inscription, after treatment by Krefting’s method.

Hartwich’s Reduction Method[131]. This method is only applicable to small objects, because it necessitates the subjection of the objects to red-heat in a glass tube in a current of hydrogen. By these means the hydrogen combines with the oxygen of the oxides, which are thus reduced to metallic iron. Owing to the explosive nature of a mixture of hydrogen and air, this process should only be carried out by one who is conversant with chemical methods, for results which are equally good can be obtained at less expense by Krefting’s method. For Hartwich’s method a strong core of metal is essential, for although objects which are entirely oxidized may be thus reduced, the result will be the formation of a more or less loose iron powder which is frequently insuch a fine state of division that by union with the oxygen of the air, in consequence of the great amount of surface presented, it becomes red-hot with the formation of ferric oxide as a combustion product.

It is advisable to apply a combination of Blell’s or Krefting’s method with one of the first group (under certain conditions) to such iron objects as are found, during the process of preservation, to be penetrated by black stable rust to such a degree that the complete removal would only leave a kind of iron skeleton. Fig.35represents such an iron dagger-sheath[132], the dark spots upon it being rust. After heating and cooling down and a short treatment with acid the removal of the rust was proceeded with mechanically, but was not completed. The object was then well steeped, and when dry was warmed in the varnish-petroleum mixture[133].

Fig. 35.Iron dagger-sheath after treatment by a combination of Blell’s and Krefting’s methods.

Iron objects, the size of which is inconsiderable, such as arrow heads, small rings, etc., can be very quickly reduced, if they still have a well-preserved core, by heating them for a short time in molten potassium cyanide[134]. The cyanide may be melted in a porcelain crucible supported by wire gauze on a tripod over a good-sized Bunsen burner, and the object introduced by the aid of tongs. The reaction is accompanied by vigorous effervescence and is soon complete. It is then taken out and dropped into cold water. By repeatedly boiling in fresh quantities of water it is thoroughly cleansed, then treated with paraffin wax, or the water may be expelled by alcohol. It is then dried, and finally impregnated with zapon. If the cyanide treatment is insufficient, any remaining rust may be removed by drills or other suitable tools. Hitherto this method has only been applied to a small number ofobjects, but there is no doubt that its use may be largely extended. Owing to the poisonous nature of the cyanide this method should be left to those who possess chemical knowledge. The disadvantage of the process lies in the difficulty of fusing large quantities of the potassium cyanide[135].

(4)Preservation of Medieval Iron Objects.

A complete treatise on this subject would be beyond the limits of a handbook, the following observations, therefore, will be sufficient for our purpose. The rust spots on objects of this kind are frequently only superficial and can be removed either mechanically by rubbing with pumice or emery, etc., or chemically by a concentrated solution of sodium sulphide[136]. To prepare this, sodium sulphide is dissolved in water, or flowers of sulphur are boiled in a solution of caustic soda. If the object is too large for immersion, the solution may be applied with a brush, and if the layer of rust is thick, the application must be repeated. After treatment the object must be rinsed in water and dried.

Small articles can be freed from rust by immersion instrong fuming nitric acid[137], for strong acid dissolves the rust only, while it induces in the iron the so-called “passive[138]” condition in which it is not acted upon even by dilute acids, and can be safely washed in water. When thoroughly cleaned, the most suitable protective is some neutral substance such as paraffin wax, vaseline, or paraffin dissolved in benzine, but any of the numerous forms of oil or fat may be used.

Well-preserved bronzes with a stable patina, such as the highly esteemed glossy stable or “edel” patina, or that which, although not glossy, covers the bronze with a rough and often crystalline coating, should not be interfered with. Such bronzes as need treatment should be subjected either to simple cleaning or to some appropriate method of preservation.

The Cleaning of Bronzes.Bronzes, the metallic substance of which is more or less intact, while the surface is hidden under earthy or sandy material cemented together by copper compounds, may be cleaned either by mechanical or chemical means. When the materials forming the incrustation are more firmly cemented together than they are to the material beneath (which often still retains a polished surface), a small hammer may be used, but more adherent portions require the use of small chisels, which can be made to order in different shapes or sizes. I have used with advantage hammers with striking surfaces like those shown in Fig.36. The two on the right are rounded sothat they touch the object at one point or on a line only. The process may be facilitated by the use of Springer’s method. A warm thick solution of glue should be spread upon the incrustation covering the bronze. As the glue dries and becomes cool it scales off, carrying with it some portion at least of the crust, thus leaving the metal clean. That part of the glue which remains can then be readily detached by gentle strokes with a hammer. The eyes should be protected when using the hammer, whether on the incrustation or on the glue.

Fig. 36.Hammer heads, natural size.

Other Methods.Since metallic oxides are scarcely, if at all, soluble in water, washing with water, even when a brush is used, will remove only earth or soil which is loosely attached. Compounds containing oxygen or oxygen and chlorine are, however, more or less soluble in ammonia, and, if they are thin and not too compact, after immersion for some time can be removed with a brush. Thick compact layers are loosened with difficulty.

Immersion in 2-5% hydrochloric acid acts more effectively, while sulphuric acid, nitric acid, and concentrated acetic acid have the same action. The frequent use of these reagents is, however, strongly to be deprecated, for it is impossible to remove the acid by simple washing with water after the incrustation has been removed. The bronze should be washed and placed in a very dilute soda solution or in dilute ammonia, after which it should be again well washed with distilled water. As has been explained inPart I., it is to chlorine compounds that the destruction of bronzes is chiefly due, and these are actually produced by the hydrochloric acid treatment. If the bronzes are not thoroughly washed, and this is no easy matter, sooner or later efflorescences will make their appearance, and the process of preservation must be repeated if the destructive action is to be arrested.

Various attempts have been made to remove the incrustation by raising the bronze to a red heat. This process is notrecommended; for not only does it give to the bronze an unpleasant appearance, but it detaches any inlaid metal (gold or silver) or enamel which may be present.

In conclusion, it may be stated that, although the process is slow and laborious, the best results are obtained by careful removal of incrustations by mechanical means.

Preservation of Bronze and Copper Objects.

(A.)Methods of Impregnation.The impregnation of bronzes, as of the majority of antiquities, has for some time been carried out by the use of solutions similar to those already enumerated for iron. These are applied directly or after the specimen has been either steeped in water or treated with dilute acids. This latter treatment, as has been already stated, is to be avoided, and if used all acid must be washed out before the object is dried. Steeping in water is of little use, because compounds containing oxygen or chlorine are often insoluble in water, which will at most only wash off loosely attached dirt or earthy material. The impregnation process may therefore be applied directly, and this should be done in all cases in which the surface is much corroded, warty (Figs.7and8), or cracked (Figs.37and38), or in which there is little or no core of metal. Impregnation is also the only means of preservation when the formation of oxides has raised inlaid metals or enamel in such a way that the removal of the oxides would detach them. The “Merkbuch[140]” recommends poppy seed oil and benzine mixture (p.70) or the gum-dammar solution. To obtain thorough impregnation this should be carried out by extraction of the air, as has been already recommended in the case of limestone (p.68). The object must also be perfectly dry, which may be insured either by exposure to moderate heat or by keeping it forsome time over anhydrous calcium chloride[141]. The object is placed under a glass bell jar, the edges of which are smeared with vaseline to ensure contact with the glass plate upon which it rests. The calcium chloride should be placed in an open glass vessel, beneath the bronze, but care must be taken that they are not in actual contact.

Fig. 37.Osiris showing cracking and destructive patina.

Fig. 38.Boeotian bridle with cracking patina.

Immersion of bronzes in paraffin wax at 240°F. [115°-120°C.] gives results which are as good, if not better, than those obtained by the use of solutions.

Should efflorescences make their appearances upon bronzes which have been impregnated, their further spread may often be successfully prevented by smearing fish-glue on the parts affected. Fish-glue, however, has not proved a satisfactory material for the complete impregnation or coating of bronzes which are in the last stages of decay.

(B.)Preservation by Reduction.It has been previously explained (pp.28et seq.) that the efflorescences upon bronze known as creeping or malignant patina which may in time cause the complete destruction of the metal are due to the action of sodium chloride. It is found upon all Egyptian bronzes and upon those from some other localities.

The metal, especially the copper, is converted into the so-called basic chloride. In the reduction processes an attempt is made to reduce these compounds again to metal, while the chlorine thus liberated forms chemical compounds, which may be subsequently washed out with water. There are two methods which effect this reduction, viz., that of Finkener (Berlin) and that of Krefting. The principle of both is electrolytic, and both bring about the complete removal of the patina and the restoration of a clean metallic surface.

To complete this portion of the subject a third method may be mentioned, viz., reduction by heat in a stream of hydrogen. This method[142]is, however, only applicable to small objects.

Finkener’s Method.Care must be taken when examining the bronze that the metallic-looking mixture of cuprous oxide with other copper compounds is not mistaken for metallic copper. When it has been ascertained that the bronze still has a good metallic core, and that any inlaidmetals which may be present rest on the metal itself and not upon a crust of oxide, a platinum wire should be tightly wound round it. This should be connected by an insulated copper wire to the zinc or negative pole of the first of 3 or 4 Daniell cells, or, better, of two accumulators arranged in series. The object should then be immersed in a 2% aqueous solution of potassium cyanide. In the same solution, as near as possible to the bronze without actual contact, should be placed a piece of platinum foil connected first by an emerging platinum wire, and then by an insulated copper wire to the positive pole. The potassium cyanide completes the electric circuit and electrolysis takes place, whereby the water is split up into its constituents. The oxygen appears in small bubbles upon the platinum foil, but the hydrogen does not immediately make its appearance at the other pole, for, by combination with the chlorine and oxygen contained in the bronzes, free hydrochloric acid and water are formed. The hydrochloric acid in turn acts upon the potassium cyanide to form potassium chloride and hydrocyanic acid, both of which substances are dissolved in the water of the bath. The hydrocyanic acid can often be recognised in the room by its characteristic smell of bitter almonds. The process may be expressed by the following equations (neglecting the water produced by the oxygen of the oxide, which is of no importance in the process):

CuCl2+ 2H = Cu + 2HCl,HCl + KCN = KCl + HCN.

Although the chief portion of the potassium chloride and hydrocyanic acid are dissolved in the bath, the remaining traces of these substances must be removed by very carefully washing the bronze in water, after which it should be dried, and if necessary finally subjected to impregnation.

Some further observations may be made in connection with the practical application of this process.

Of course, other primary batteries may be used instead of the Daniell cells, but these latter may be specially recommended for the ease with which they can be procured and for the steadiness of their action. Information concerning the method of filling and using them may be obtained at any shop where they are sold. The copper wire and platinum wire should not be too thin, but must be at least from 1 to 2 mm. in thickness: they should be fastened together by binding-screws, and care must be taken that both the wire ends and the screws have clean surfaces. Glass vessels or glass cylinders are most suitable because the process of reduction can be watched, but large objects will of course require glazed earthenware baths. If wooden boxes are used they must be coated inside with paraffin wax. The strength of the cyanide solution should be 2%. Having a large number of reductions to carry out, I keep a 20% stock solution in a large bottle, one part of which is diluted with nine parts of water when required for use. Potassium cyanide is, as is well known, a strong poison, and care should therefore be taken to prevent access to any sore or cut on the hands; this can be done by the use of india-rubber finger stalls or gloves.

If the bronze object is neither too large nor too heavy it may be suspended in the bath by looping the platinum wire over the edge of the vessel. It is a convenient plan to use different coloured wires to distinguish the negative and positive poles of the battery, but should any doubt arise as to which wire should be connected with the bronze or which with the platinum, the following test will readily decide the question. Moisten a small piece of white filter paper with a drop of a solution of potassium iodide[143], and touch the twoconducting wires with it simultaneously: a brown spot will be seen on the paper at the point of contact with one of the wires; this is the positive wire, and must therefore be connected with the platinum. If the current is passing through the cyanide bath and the bronze, bubbles of gas will appear upon the platinum foil, or the products of the decomposition of the potassium cyanide may change the colour of the bath near the platinum to yellow or brown, while at the same time cloudy streaks under the bronze will show where the potassium chloride and hydrocyanic acid, resulting from the reduction of the copper compounds, are meeting with the cyanide of the bath. If the platinum wire is not firmly fixed round the bronze, hydrogen may be formed upon it, and should this occur the wire should be drawn tighter.

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 bronzeunder 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 withwater 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 objectswith 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.]. Yetthe 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.

Fig. 39.Bronze bull showing warty patina.

Fig. 40.The same after reduction by Finkener’s method[147].

There is no doubt that these bright-blue efflorescences are the result of an incomplete reduction, which in many cases can scarcely be remedied, for it is often impossible thoroughly to wash objects of great thickness. Thin bronzes, bronze plate, and copper plate remain free from efflorescences. Moreover, many bronzes, especially Egyptian ones, have a hard, non-metallic core, which in the casting has been partly fused or at least hard-burnt, and resists the effects of the washing.

Fig. 41.Bronze axe-blade before treatment by Finkener’s method (Aeg. 13203).

Fig. 42.The same side after treatment.

Fig. 43.Reverse side of axe-blade after treatment.

It is occasionally found that a bronze cannot stand the process of reduction, either because there is only a thin layer of metal over a stout core, or because the metal is permeated with cuprous oxide, which when tested with a file has a metallic appearance. The bronze must therefore be continually watched whilst it is in the cyanide bath, and ifnecessary should be taken out even before the reduction is complete. This should be done if large pieces or large quantities of a powdery precipitate fall from the bronze, or if it is found that a needle readily pierces the oxidized layer. A specimen of this kind must be taken from the bath, carefully steeped, dried, and impregnated[148].

It is not to be expected that bronzes which are in an advanced state of decomposition (e.g. Figs.9-12) can be so transformed by reduction as to appear as they did when they left the artist’s hand. For, although the decomposed oxidized layer is now reduced to metal, this no longer forms a coherent mass, but a loose powder, which, being deprived of its essential constituents, chlorine, oxygen and carbonic acid, no longer retains its coherency, but falls to the bottom.Only in the interior and in the pores is the reduced metal retained.

In addition to the preservation of articles by the removal of the injurious chlorine compounds (as is also the case with Blell’s and with Krefting’s method for iron antiquities), the process may result in the discovery of inlaid work, inscriptions or ornamentation, the presence of which was not suspected. The accompanying illustrations (Figs.39and40) show bronzes before and after the preservation process, while the axe-blade shown in Figs.41-43illustrates equally clearly the advantages which accrue from the treatment. Not less striking is the result of the treatment in the case of the dagger-sheath shown in Figs.44and45by which the design was discovered.

Reference may here be made to a case described elsewhere[149], in which reduction proved that what had been thought a single bronze object consisted in reality of two pieces whichdid not belong to each other, but were fitted together by means of a bottle cork of modern date! In another instance a bronze was found upon reduction to be brazed with a hard solder containing zinc, which was thus quite inconsistent with the age ascribed to the object.

Fig. 44.andFig. 45.Dagger sheath before and after treatment by Finkener’s method.

A short digression may be here made in order to discuss the question whether the composition of the bronzes undergoes any alteration. Three analyses[150]of Egyptian bronzes before and after reduction by Finkener’s method show that the change in composition is so slight as to be immaterial. It is of course obvious that greater differences will be seen in the results of the analyses before and after reduction of bronzes which are in an advanced state of oxidation, for in this case chlorine, oxygen, water, and carbonic acid constitute an appreciable proportion of the total weight. But even in these cases the analysis made after the reduction shows very slight variation from that of the original metal.

The two latter bronzes were tested qualitatively only for arsenic and antimony, and when the three objects werewashed the hydrated tin-oxide described on p.130was only found in the case of the Ibis. In this connection it should not be forgotten that slight differences in the quantities may be due to errors in the analysis as well as to a want of homogeneity in the alloy.

Krefting’s Method.This method is similar to that used for the reduction of iron (see page108). The layer of oxidized material is removed in several places by filing, hammering, or rubbing with emery cloth until the metal is exposed. The object is then wrapped round with strips of zinc, and placed in a 5% solution of caustic soda. The hydrochloric acid produced in the process of reduction acts upon the soda to form sodium chloride. Here too the greatest care must be taken that the steeping is sufficient.

Personally I prefer Finkener’s method, for potassium cyanide is more easily washed out than soda, and also, although poisonous, is less caustic.

Krefting’s method however has proved of considerable success in some cases, notably in the treatment of some 40-50,000 Roman copper coins at the Berlin Museum. These were, with few exceptions, covered with a crystalline layer resembling green malachite or blue azurite and were quite illegible. Various unsatisfactory attempts were made to clean them with ammonia, with warm and cold acids of different kinds, with acid and iron nails, and by electric current both in an acid solution and in a solution of potassium cyanide. The following method finally proved satisfactory[151]:

Krefting’s Method Applied to Oxidized Copper Coins.

“A thin plate of zinc with a bright metallic surface is perforated with a brad-awl, having a diameter of from 2 to 5 mm., until there are about 50 or 60 holes in each square metre. This is placed with the sharp edges of the holes uppermost on a row of glass rings (or crystallizing dishes will serve the purpose) 20 mm. in height resting upon the bottom of a large glass vessel. The coins, which in this case were 20 mm. in diameter, were then placed on the zinc plate, so that 7 or 8 of them occupy a space of 1 square decimetre. Another similarly perforated plate is laid upon them, and upon this more coins are arranged in the same way, and so on until there are six or eight double layers. A perforated zinc plate is then placed on the top with the sharp edges of the holes turned downwards, and over this a few zinc plates which have been previously used. The whole pile is surmounted with weights or stones resting upon glass rings or inverted glass dishes in order to press the sharp edges of the holes into the closest possible contact with the coins. A 5% solution of caustic soda is then poured over the whole, the immediate result of which is an evolution of gas. The reduction of the coins is usually complete in fifteen to eighteen hours, after which they should be well washed. After several rinsings in cold water they are placed, about 1000 at a time, in a large vessel fitted with a perforated false bottom containing hot water, which should be renewed three or four times every day. After four days the coins are wiped with a cloth and thoroughly dried on a warm oven plateor in a drying chamber at a temperature of about 212°F. [100°C.]. They are then brushed with a bristle brush before a dust extractor, a procedure rendered necessary by the fine metallic dust from the coins, which then assume a light or dark brown colour such as is seen on copper coins which are in actual circulation. The practice of placing the coins whilst still wet into melted paraffin wax at 260°F. [120°-130°C.], which gives a dark appearance even to the brightest, has the disadvantage that the wax prevents the use of sealing-wax for taking impressions, and is therefore not recommended.The reaction is analogous to that which occurs in the reduction of iron. The copper of the coin forms in the alkaline solution an electric couple with the zinc, and the hydrogen which forms at the copper end reduces the copper compounds covering the coins to metallic copper, and thereby loosens them, while the zinc oxide which is simultaneously formed is dissolved in the soda solution. In actual practice a part only of the zinc oxide is dissolved, while the remainder forms a white coating on the zinc[152]. Experience shows that a 4-5% solution is the most suitable for this method of reduction, which gives the most favourable results when these details are followed. If for example the zinc plate is laid immediately on the bottom of the glass trough, if the coins are laid too close together on the plate, or if there are more than 6 to 8 double layers in a trough, the processof reduction is often incomplete, and it is then necessary to treat the coins a second time. It is scarcely necessary to mention that larger coins must be placed at proportionately greater distances from each other.The 40-50,000 coins which were thus treated had originally been tinned, but the tin only remained at a few places. When the coins were washed immediately after the reduction, this tin could still be clearly distinguished, but on further washing, drying, and brushing, it ceased to be visible on account of the dark colour imparted to it by the finely powdered copper. In oneor two cases lead appeared on the surface of the coin, but was easily removed by mechanical means.”

The reaction is analogous to that which occurs in the reduction of iron. The copper of the coin forms in the alkaline solution an electric couple with the zinc, and the hydrogen which forms at the copper end reduces the copper compounds covering the coins to metallic copper, and thereby loosens them, while the zinc oxide which is simultaneously formed is dissolved in the soda solution. In actual practice a part only of the zinc oxide is dissolved, while the remainder forms a white coating on the zinc[152]. Experience shows that a 4-5% solution is the most suitable for this method of reduction, which gives the most favourable results when these details are followed. If for example the zinc plate is laid immediately on the bottom of the glass trough, if the coins are laid too close together on the plate, or if there are more than 6 to 8 double layers in a trough, the processof reduction is often incomplete, and it is then necessary to treat the coins a second time. It is scarcely necessary to mention that larger coins must be placed at proportionately greater distances from each other.

The 40-50,000 coins which were thus treated had originally been tinned, but the tin only remained at a few places. When the coins were washed immediately after the reduction, this tin could still be clearly distinguished, but on further washing, drying, and brushing, it ceased to be visible on account of the dark colour imparted to it by the finely powdered copper. In oneor two cases lead appeared on the surface of the coin, but was easily removed by mechanical means.”

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