The Preservation of Antiquities: A Handbook for Curators. Friedrich Rathgen
sulphates, and especially if it contains free sulphuric acid, only much corroded iron is likely to be found. Moreover the physical condition of the peat may vary; thus it may be dry or damp or even submerged under water, and this variation will exercise some influence upon the condition of the iron.
Iron objects which are covered with the black, so-called “noble” rust (Edel-rost) usually prove very stable. This, like forge-scale, is a ferroso-ferric compound in which there is a preponderance of ferrous oxide where it is in contact with the metallic iron, and of ferric oxide in the outer layer. “Noble” rust is probably in nearly all instances the result of the action of fire, which may have been used in funeral rites, or may have been accidental; very rarely can it have been produced by the reactions mentioned above, as has been suggested by Stapff.
Iron which has been in contact with the bone ash of burnt corpses has certain characteristics. When entirely surrounded with bone ash objects are well preserved[25], and only covered with a thin layer of oxide. How far the ash has acted as a preservative, I will not hazard an opinion, having seen but few specimens, and these had been already varnished to preserve them.
Under certain conditions the phosphoric acid of the bones forms a thin bluish layer of iron phosphate, corresponding in composition to vivianite (Fe3P2O8.8H2O), as was pointed out by Jacobi in a series of objects in the Saalburg Museum at Homburg. These objects also are quite durable.
In earth so full of sodium chloride as is that of Egypt, objects of iron will be readily corroded, and the explanation given above will account for the paucity of iron remains of Egyptian origin. It is difficult, however, to find a satisfactory explanation for the fact that objects found in sea-water are specially well preserved. It may be that, in spite of the presence of free oxygen in solution in the water their complete insulation from the atmospheric air has resulted in the preservation of the objects, as is the case with those which have lain in a stream of fresh water.
Bronze and Copper.
Copper and its alloys are subject to the same far-reaching changes as iron, but the action is less rapid. Bronzes of widely different composition have to be dealt with to ensure their preservation, and to a less extent, copper also[26]. According to von Fellenberg[27] bronze objects may be classified according to the material in which they have been found, i.e. peat mud, water, or earth.
“(1) Bronzes from peat mud are covered with a black earthy mass, which can be easily removed by water and brushes, the alloy then assumes its metallic lustre and the characteristic colour of bronze. The complete preservation of the pure metallic surface of the bronzes, in the same condition as they were when they were submerged, is easily accounted for by the enclosure of the metal in mud of organic origin under several feet of water which effectually excludes the oxygen of the air.
(2) The bronzes found in water, as for example in the beds of lakes and rivers, are less perfectly preserved. They have usually a thin coating of a calcareous deposit, which however often allows the lustre and colour of the metal to appear in places. When such bronzes have dark or green coloured patches or spots, the layer is very thin and may be removed by treatment with acids, which allows the metallic colour to become visible. Bronzes preserved in water still retain the same definite edges and points which they possessed when they entered the water. If bronzes which are markedly incrusted with verdigris are found in water in all probability they had lain in the ground a considerable time before being covered with water, and oxidation had penetrated deeply into the metal before immersion.
(3) Bronzes found in the earth or in graves appear covered with a fine green crust of verdigris which may be either light or dark in colour and which often has a vitreous lustre. This is generally known as Patina.
This crust varies in thickness from that of writing-paper to several millimetres. If the green crust be filed away, or better, removed by dilute nitric or sulphuric acid, the bronze is found to possess a reddish colour; below the crust of cupric carbonate is found a layer of cuprous oxide, which may be removed by ammonia, thus revealing the metal with its characteristic colour and lustre. This condition is characteristic of the slow oxidation of bronze in moist earth. The layer of cuprous oxide between the pure metal and the external crust of copper carbonate has been shown by the examination made by Dr. Wibel to be a product of the reduction of copper carbonate by the metallic copper of the bronze. Bronzes belonging to this category have often lost their former metallic properties, and if of small diameter have often been completely converted into cuprous oxide, surrounded by a lustrous green or blue crust of carbonates. If a metallic core remains, it is found to be crystalline, brittle, and non-coherent, that is, it flies to pieces under the blow of a hammer. Fine ornamentation and sharpness, whether of edge or of point, have often disappeared. This does not occur with bronzes preserved in water.”
In another volume of the series[28] von Fellenberg states that basic copper chloride occurs as a constituent of patina.
A few lengthier quotations may be conveniently given here, in part verbatim, in part abstracted from literature which is not readily accessible.
Reuss[29] states that it has been hitherto generally assumed that copper is first converted into cuprous oxide which is then converted into a green hydrated oxy-carbonate which is separated from the metal by a thin layer of cuprous oxide. The specimens examined by him, however, showed no such dividing layer, the metal being either directly in contact with the malachite[30], or else separated from it by a black or bluish layer of cupric oxide. He further draws attention to the occurrence of irregular knobs two to three lines in height which consist, in part, of azurite[31]. Neither oxides of tin nor chlorine were found. The alteration of the bronze he explains by the prolonged oxidising action of water containing carbonic acid.
In an exhaustive memoir Wibel[32] describes the various kinds of patina as malachite, copper-oxychloride, and azurite, with admixtures of tin oxide, silver, iron oxide, lead chloride and copper chloride. He discusses also the occurrence of the cuprous oxide layer which is said to have been described by Sage as early as 1779. After detailing the observations of Davy, Hünefeld, and Picht, that the metallic copper exists partly in alloy and partly free as crystals in the layer of cuprous oxide, he continues as follows[33]:
“The process of decomposition in bronzes has been regarded as a slow oxidation, in which cuprous oxide marks the first and incomplete stage, while the carbonates represent the later completed phase. The formation of both these substances was assumed to be due to moist oxidation, on bronzes as well as in those superpositions of copper, cuprite, and malachite, so frequently found in minerals. Indeed, no other process of formation of the carbonates is conceivable; moreover cupric oxide, if really present, would be naturally regarded as a product of oxidation. The other substances, such as tin oxide, which are occasionally found, would be produced in part by similar simple processes, in part by the simultaneous action of particular salts, the chlorine compounds, for instance, by the presence of water containing sodium chloride. Similarly the production of cuprous oxide was usually attributed to an incomplete oxidation of the copper, although it might very well be the result of an inverse process, viz. the reduction of pre-existing cupric oxide.”
From the following considerations Wibel thinks that he is justified in his assumption that the layer of cuprous oxide is the result of reduction. Firstly, by no means all bronzes which have been dug up, even though from the same excavation, show the layer of cuprous oxide. Secondly, the cuprous oxide layer is in the crystallized state. Thirdly, ‘all the facts of chemistry show that the formation of cuprous oxide can only take place by reduction, given the ordinary conditions of temperature and pressure.’ Finally, in addition to oxygen and carbonic acid, many salts, those of ammonia for example, occur in the spots where bronzes are found and favour the formation of copper salts. Wibel also quotes in support of his views the experiment of Bucholz[34], that a strip of copper, the upper half of which is immersed in a layer of distilled water, and the lower half in a concentrated neutral solution of copper nitrate carefully poured beneath it, becomes coated with copper and cuprous oxide.
He continues: