The first group comprises almost all the bronzes which are found in peat, which show, with rare exceptions, a metallic, often somewhat darkened, surface. Their state of preservation depends upon the nature of the peat in which they are found, but the metal surface has, in the majority of cases, become somewhat rough and etched, although all the details are clearly distinguishable. More rarely one side retains the original polished surface while the other side is much corroded. If a much corroded bronze is found, the peat in which it has lain has probably contained free sulphuric acid (see also p. [13]). All bronzes found in water must be included also in this group. The second group will then comprise all bronzes with an oxidized patina.
The classification given by Villenoisy seems entirely unsuitable, for it does not by any means exhaust all the kinds of patina which may occur. Thus no mention is made by him of the frequent occurrence of a patina which contains chlorine. If we separate the dark brown and the blackish patina, in so far as these two colours are pure, from those of a green colour, the first two varieties cannot be regarded as groups, because the tones of colour differ too much, and because, as Villenoisy himself observes, widely different patinas often occur on one and the same bronze. The durability of a patina upon a bronze cannot be judged either by the outer appearance or by the chemical composition alone. The fact that there has been no alteration in the outward appearance for many years offers no guarantee against further changes taking place. Thus a Minotaur[ [53] in the Berlin Museum, which for many years had shown no sign of change, was eventually found to be completely covered with numerous bright green spots over its entire surface. My own opinion is that the only patina which is really stable is that which consists of combinations of oxygen, hydrogen and carbonic acid with the metal, somewhat similar to those analysed by Schuler (see page [24]), and by Arche and Hassack (see page [27]). The presence of sulphides, and even of sulphates, does not seem to be injurious.
If a patina is to deserve the name of a good, sound, or, as it is termed, a “noble” patina (Edel-patina), the original contours of the bronze with all its markings must be distinctly visible. For this the patina must not be too thick, must be of moderate hardness, and above all must have an enamel-like surface. Apart from chemical influences, such a patina can only have been formed in those cases in which the alloy has been homogeneous, fine-grained, dense and not porous, and when its surface has been so smooth that oxidation has taken place very slowly. Under these conditions the colour of the patina may vary greatly, for it may be bright green, blue, or of darker shades from yellowish to brown, or even black. These latter tints often denote patina layers of very slight thickness. My own observations confirm Villenoisy's view that the brown and the black patina are for the most part due to the presence of lead in the bronze. Rein[54] holds the same opinion in regard to Japanese bronzes.
Certain forms of patina are not necessarily prejudicial to the preservation of bronzes, i.e. the green and blue varieties which have the composition of malachite (CuCO3, Cu(OH)2) and azurite (2CuCO3, Cu(OH)2), both of which are very often found on the same bronze. This variety of patina shows a crystalline structure. The simultaneous formation of both varieties, which is due to the greater exposure of one part of the bronze than another to the action of moisture, is well shown by a specimen in the Berlin Museum[55] (Fig. [6]). This consists of the frontal portion of a Boeotian bridle, over parts of which leather straps had probably been tightly fixed. Those parts which had been thus somewhat protected from moisture were covered with blue azurite, which contains a smaller quantity of water. But the crystalline structure of these kinds of patina has often the disadvantage that the surface of the bronze is no longer clear, and consequently engraved markings and even stamped impressions are not visible. On page [142] may be seen illustrations of Roman coins, some parts of which are totally illegible. More frequently met with than these varieties or than the so-called “noble” patina, is that in which the bronze presents a more or less rough and pitted surface, light or dark green, or even grey in colour if there is a large proportion of lead present. More rarely the tint is blue or brown. The behaviour of such kinds of patina varies greatly, but durability is for the most part assured if, under the layer of green oxide, a reddish layer of cuprous oxide is found. This rule is perhaps not invariable, for I have often found cuprous oxide present under the so-called spreading patina, but absent beneath one which is undoubtedly durable.
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.