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. [7] and [8]), or cracked (Figs. [37] and [38]), 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 for some 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.