[Illustration: Fig. 6. Portion of bronze horse-trappings showing blue and green patina.]
Two instances may be here quoted as confirming Wibel's view in reference to the reduction of cupric oxides to cuprous oxides and even to metallic copper (see page 17)[56]. In removing a sandy crust saturated with copper salts from a large Egyptian bronze[57], small crystalline masses of copper were seen here and there, separated from the metal beneath by a layer of cuprous oxide to which the admixture of tin gave a whitish tint. The copper was mostly deposited in slight depressions upon the surface of the metal and could be easily removed. Similarly, upon an Etruscan mirror exhibited in the Berlin Museum[58], reduced copper can still be seen forming red spots upon the lighter coloured surface of the bronze, which has already been freed from cupric oxide. The copper also can be removed with comparative ease, and is observed to be separated from the bronze by a thin whitish layer of tin oxide. A quantitative analysis of a small piece showed 100% of copper.
As has been remarked above, the layers of oxide frequently enclose grains of sand and even fragments of clay, earth, and ferruginous particles, so that the original contours of the bronzes are often indistinct or entirely obliterated (see Figures 41-43). These incrustations may occasionally be removed by a careful use of the hammer, but they are often so firmly united with the bronze, which is itself so oxidized, that removal by mechanical means is no longer possible.
[Illustration: Fig. 7. Head of Osiris, showing advanced condition of warty patina[59].]
These incrustations are however not so injurious as the tuberous and warty patina. Figure 8 shows an Etruscan mirror covered with a patina which generally results in the progressive destruction of the bronze[60].
[Illustration: Fig. 8. Etruscan mirror showing warty patina.]
The following series of quantitative determinations of chlorine obtained from the examination of bronzes in the Berlin Museums, shows conclusively the destructive influence of chlorine in the production of patina:
Dark green "noble" patina (wine pitcher, Ant. Misc. Inv. 7161) 0
Green patina on a layer of cuprous oxide (Etruscan vase, Ant. Fr. 1571) 0