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 process of 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 one or two cases lead appeared on the surface of the coin, but was easily removed by mechanical means.”
Fig. 46. Roman coins before treatment.
Fig. 47. Roman coins after treatment by Krefting’s method.
Cleaning Copper Coins by Melted Lead.
Although the results obtained by this method are less satisfactory than those produced by the preceding, it has the advantage of simplicity[153].