“Using a pair of tongs, dip the coins one by one into melted lead until the crackling, which begins at once, has ceased, which occurs in from 3 to 10 seconds. The hand should be protected with a glove from the spluttering molten lead. The coin is then thrown into cold water, cleaned, and placed until the next day in hot milk. It may be necessary to repeat the process when the coin has become cold. By this method an olive colour is imparted to the coin which many antiquaries prefer to dark brown, but personally I prefer Krefting’s method because it renders the inscription and designs far more distinct. A coin which after the treatment with melted lead has remained so covered with cupric oxide as to be still illegible can seldom be improved by a repetition of the treatment, whereas had the zinc treatment been applied in the first instance the result would probably have been satisfactory. This conclusion seems to be justified by the extremely small percentage of coins which, in my experience, have remained illegible after the treatment by electrical methods.”
(C.) Preservation of Bronzes by the Exclusion of Air.
In those cases in which the advanced state of decomposition renders the reduction process either inapplicable or at any rate inadvisable, or in which the decay is not likely to be arrested by impregnation, a further method of preservation remains, viz. the complete exclusion of air and moisture.
If air is completely freed from moisture the oxygen can no longer act in conjunction with the copper chloride upon the still intact metal (see page [29] et seq.), and the condition of the bronze will consequently remain unchanged.
A bronze, for example, which shows much decay should be placed after impregnation under a hermetically sealed bell glass, and beneath or near it should be placed some dehydrating agent, of which anhydrous calcium chloride is the most suitable (see note, p. [123]). To exclude the air completely the bell glass should have a projecting ground edge, which should be smeared with vaseline or grease and pressed firmly upon a thick well polished glass plate. The dehydrating agent may be placed in a glass vessel or dish in such a way as to be unseen, or it may be covered with two or three thicknesses of dark gauze or with black cardboard laid loosely over it. If an object is too large for a bell glass, or if several objects are to be exhibited together, a square plate-glass case with iron framework, made air-tight with putty, may be used as shown in the illustration (Fig. [48]). The lower part, containing calcium chloride, is partitioned off by a perforated plate covered with black gauze[154]. A hygrometer was placed behind the head, the indicator of which has remained at zero since it was first fixed several years ago, and the bronze has not hitherto shown any sign of change, although the inlaid gold is in parts raised from the metal by a light-green oxychloride. The cost of these cases is considerable, but for valuable objects this should not be considered. In the place of calcium chloride, sticks or lumps of caustic soda may be used with advantage, for this substance absorbs both moisture and carbonic acid.
Fig. 48. Method of mounting objects in air-tight cases.
This method of preservation is of course applicable not only to decomposed bronzes but to all valuable antiquities, whatever the material may be.