These methods are founded upon the fact that the sunken areas of the coin are, by the pressure of the die in stamping, rendered denser than the raised portions, such as the inscription. The earliest method is that published by Brewster, reported by Süpke[155]. The coins when cleaned are placed upon red-hot iron, which causes the oxidation of the entire surface of the coin. The thin film of oxides varies in colour according to the duration and the intensity of the heat. The oxidation of the letters of the inscription differs from that of the surrounding parts, and is recognisable by a difference in colour. Drude[ [156], treating more especially of silver coins, remarks that the inscription is rendered legible by heating them to redness over a Bunsen-burner. It then, according to “Prometheus[157],” when viewed in a dark room, appears dark on a bright ground, especially if the coin has been previously polished and then roughened again by slightly etching it with acid. In conclusion, the method of Roux[158] may be quoted:
“The smooth-worn and polished coin is placed in a solution of copper sulphate or of some other metallic salt, and suspended between the electrodes of one or more cells of a battery (any other form of continuous current will serve the purpose). If the current is weak, the electrodes must be near to the coin. The stronger the current the more rapidly the impression appears. On the side which faces the anode or positive plate the impression is metallic; on the other side, after gently wiping off the less firmly adherent part of the oxide, the impression appears in grey lines. These markings can be fixed by varnishing them with a thin alcoholic solution of shellac. To render the impression legible on both sides, the coin should be placed upon the four upturned feet of an insulating stand. The larger the coin the deeper must be the layer of solution above and below the coin. The depth below should be equal to the radius of the coin.
This can perhaps be most conveniently carried out by placing that electrode in immediate contact with the coin which upon immersion in the solution becomes tarnished with the metal, i.e. the cathode or negative pole. Other portions which it is not intended to treat should be first covered with varnish.
The striking success of this method is due to the fact that that portion of the metal which has been compressed by the stamp is a better electrical conductor than the rest; no success could therefore be expected from the use of this process for the restoration of such objects as worn engraved copper-plates, etc.”
(j) Silver.
Preservative treatment of silver is scarcely necessary (cp. pp. [49]-[52]), except in those cases in which the silver is alloyed with a large percentage of copper, and which show efflorescences similar to those which appear upon bronzes containing chlorine. Electrolytic reduction will be found to be the most suitable method of treatment in such cases. To treat silver coins they should be placed in contact with iron nails in lemon juice. Instead of the citric acid, which is the active principle in this process, other diluted acids and other metals, e.g. zinc, may be employed. Flinders Petrie[159] has shown that the reduction can also be effected by a weak solution of common salt. Silver chloride is soluble in ammonia, and thin layers may be removed by the application of ammonia by means of a soft brush. Thorough rinsing with pure water, drying with soft cloths, and cautious warming are always essential.
An excellent reducing agent for single coins, the characters of which are rendered illegible by a layer of silver chloride, is molten potassium cyanide, or a mixture of this substance with sodium or potassium carbonate. In a short time the silver chloride is decomposed and removed from the smooth surface of the coin. After boiling out with water, steeping in alcohol, drying, and brushing with a soft brush, the coins may be coated with zapon. Coins treated in this way appear to be less brittle than those reduced by Krefting’s method. More troublesome but less dangerous, because potassium cyanide is not used, is the treatment of silver coins with a fused mixture of potassium and sodium carbonates. In this case the silver chloride is converted into silver carbonate, which is then decomposed with 50% acetic acid. Further treatment by washing, drying, and impregnation is carried out as previously described.
Silver which has become friable (p. [51]) can be rendered more compact by cautiously heating it to redness. It will however be advisable to entrust heating and mechanical treatment of objects which are much bent to some skilled silversmith, whose experience may prevent disaster. Silver objects which are largely converted into friable chloride, especially if they are much expanded, or if large portions have broken away in the process of removing the chloride, will hardly bear any other treatment than that of impregnation with gum-dammar solution or with paraffin wax. As silver chloride is easily fused such articles should not be subjected to heat.
Earthy matter can often be removed with a neutral soap and warm water, while calcareous accretions can be dissolved by a 2% solution of hydrochloric acid. Silver which has been blackened by silver sulphide may be laid in a warm 2% solution of potassium cyanide. All objects should be subsequently well washed with warm water.