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].
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:
| Percentage of chlorine | ||
| 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 | |
| Dark blue “noble” patina (Etruscan wine pitcher, Ant. Fr. 608) | 0 | |
| Bright blue “noble” patina (Etruscan mirror, Ant. Misc. Inv. 7275) | 0 | |
| Bright blue “noble” patina (lid of vessel, Ant. Misc. Inv. 6322, 292 a) | 0 | |
| Hard greenish-yellow exfoliating patina upon a bright green, softer patina (Roman saucer, Ant. Fr. 1601 a) | 0 | |
| Bright green fairly firm patina, the colour rubbing off somewhat in parts (handle of vessel, Ant. Fr. 1440) | 0 | |
| A firm smooth green layer upon a brighter soft patina (mirror, Ant. Fr. 136) | 0 | |
| Blue crystalline patina (harness from Boeotia, Ant. Misc. Inv. 8579) | 0 | |
| Rough dark green patina (situla, Ant. Misc. Inv. 8509) | 0 | |
| Greenish “noble” patina (sword, Ant. Fr. 1144) | trace | |
| Rough green softer patina, with admixture of earth (funnel, Ant. Misc. Inv. 8582) | trace | |
| Dark green, compact warty patina (mirror, Ant. Fr. 32) | trace | |
| Green warty patina, with translucent cuprous oxide (mirror, Ant. Misc. Inv. 3312) | trace | |
| Green and blue crystalline patina (Buto, Aeg. 13135) | 1·7 | |
| Bright green cracked and warty patina (muzzle of the harness from Boeotia, Ant. Misc. Inv. 8579) (see Fig. [38]) | 1·7 | |
| Green firm warty patina (Etruscan mirror, Ant. Fr. 53) | 2·1 | |
| Completely oxidized Cyprian bronze fragment (Ant.) | 2·2 | |
| Green cracked patina upon a thick layer of cuprous oxide (bronze fragment from Troy) | 4·0 | |
| Completely oxidized Cyprian bronze fragment (Ant.) | 4·2 | |
| Bright green efflorescent patches upon dark tuberous patina (bronze fragment, Ant.) | 5·9 | |
| Bright green powdery patina in the hollows of a darker smoother patina (Horus, Aeg. 11010) | 6·7 | |
| Bright blue powdery moist patina (Aeg. 12663) | 7·4 | |
| Green and blue patina mixed with grains of sand (Buto, Aeg. 13132) | 8·3 | |
| Bright green cracked patina (bronze fragment from Troy) | 9·3 | |
| Bright green powdery patches, dark green rough patina (cup, Ant. Fr. 1654) | 10·2 | |
| Thick greenish black tuberous patina (Besa, Aeg. 9716) | 10·8 | |
| Green firm patina, with brighter patches (Buto, Aeg. 13787) | 11·3 | |
| Bright green powdery patina (Isis with Horus, Aeg. 14078) (copper) | 12·5 | |
| Green tuberous and cracked patina (Horus in the lotus flower, Aeg. 2409) | 13·1 | |
| Bright green powdery excrescences (Buto, Aeg. 13787) | 13·9 | |
| Bright green soft patina, with a dark and somewhat firmer surface (door hinge from Babylon, Aeg. V.A. 2185) | 15·1 |
A due consideration of these figures must lead to the conclusion that as a rule a malignant patina is one which contains chlorine. That traces of chlorine are found in many cases of benign patina need cause no surprise, for frequent handling alone may suffice to bring about such a condition. Nor is this rule invalidated by the fact that a patina which is proved to contain chlorine (e.g. that of the mirror[61] depicted on page [40]), has remained unchanged for years under certain conditions, for the formation of patina depends upon various causes, and it often happens that a bronze carries a patina which outwardly seems to have stood the test of years, yet internally oxidation has continued and becomes outwardly visible only when some mechanical injury to the patina allows variations of temperature to exert a greater influence. A specimen is often regarded as bronze, whereas in reality it does not even contain a metallic core, but consists merely of cuprous oxide, copper oxychloride, tin oxide, etc.[62], and is therefore incapable of further change. On the other hand it is not surprising to find a patina, which, although containing no chlorine, affords but a poor protection to the bronze, for in this case the cause may lie in the non-homogeneous and porous nature of the alloy.