Читать книгу The Preservation of Antiquities: A Handbook for Curators - Friedrich Rathgen - Страница 8

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If in some cases it may be uncertain whether the destruction of antiquities of limestone or earthenware has been due to mechanical or to chemical influences, this uncertainty is excluded in the case of metallic objects, of which those of bronze and iron chiefly come under the notice of the antiquary.

From the first piece of metallic iron which he possessed man must have soon become acquainted with its untoward property of rusting, but even at the present day opinions differ as to the origin of rust, and the cause of its rapid spreading. It has long been known with certainty that iron containing but little carbon (wrought iron) rusts with greater ease than iron which is rich in carbon (cast iron or steel), and that the rust is a compound of iron with hydrogen and oxygen (hydroxide). That rust is of variable composition may be inferred from the variations of shade from yellow to dark brown which are met with.

Widely different views are held on the question of the production of rust. Some[9] maintain that iron rusts only in the presence of water containing free oxygen and carbonic acid (CO₂) in solution, a ferrous bicarbonate being first formed; the bicarbonate is then converted into ferrous carbonate, which finally yields the hydrate with evolution of carbonic acid. This carbonic acid continues to attack further areas of metallic iron. Others[10] maintain that, while the formation of rust may proceed as described, carbonic acid is not necessary, and that free oxygen alone causes rusting when atmospheric moisture is condensed upon the surface of iron. That iron remains free from rust when in a solution of caustic potash or soda is said to be due to the absence of free oxygen and not to the removal of carbonic acid. Spennrath holds, in opposition to the opinion of Axel Krefting[11], that rust once formed cannot act as an oxidising agent, except by virtue of its power of condensing water and retaining it in its pores. Similarly E. Simon finds the chief cause of the corroding action of rust in the property of absorption, that is surface-condensation of gases. This condition is comparable to that of liquefaction, and produces rapid chemical action. Under certain circumstances ferrous hydrate is formed instead of ferric hydrate, particularly when iron is subjected to vibrations, as Tolomei[12] has observed in iron rails etc. Stapff[13] believes that mixtures of ferric hydrate with ferroso-ferric oxide, which possess a similar composition to forge scale, are formed under the influence of thermal waters. According to Irvine[14] rusting proceeds rapidly when two kinds of iron, such as cast and wrought, are in contact, since their electro-chemical relations may result in a voltaic couple. The electric current brings about the decomposition of the water, and the evolved hydrogen, being in the nascent state, combines with the nitrogen dissolved in the water to form ammonia, as had been previously observed by Akermann[15]. Similarly, electric currents are said to be caused by the contact of ferroso-ferric oxide with metallic iron, thus causing a further oxidation of the iron[16].

The presence of certain neutral salts, especially sodium chloride (common salt), has a very marked influence on the destruction of iron[17].

When iron filings are exposed to air and moisture, oxidation takes place; the action is, however, according to Krefting, far more intense in the presence of an alkaline chloride. A mixture of iron filings and sodium chloride exposed to moisture is converted in a few days into a black powder which has the following composition:—11·4% FeO, 80·0% Fe₂O₃, 8·6% H₂O, thus resembling the “iron-black” of Lemery; on extraction with water the filtrate is found to be alkaline and to possess a tallow-like smell[18]. Without entering further into Krefting’s researches, we will quote the hypothesis with which he concludes:

“The iron probably combines with small quantities of chlorine from the sodium chloride, causing alternate reduction and oxidation, and this, owing to the ease with which iron salts pass from one stage of oxidation to another, very soon gives a visible result in the formation of rust:

Fe + 2NaCl = FeCl₂ + 2Na 2Na + 2H₂O = 2NaOH + H₂.”

If these results be compared with observations made upon the condition of iron objects which have been excavated, it is evident that these are in many cases exposed to the action of the air to a lesser extent while buried, and that their decomposition will advance more rapidly when they have been withdrawn from their protective covering of earth. The condition of the objects differs according to the kind of iron, the length of time during which they have been buried, and the character of the soil in which they are found. In one place objects are found covered with a slight layer of rust only, in another with a thicker layer, in another there remains but a small core of metal, or even none at all, or the layer of rust may be intermingled with particles of earth or clay. The rust may be uniform in colour and hardness in one case, and in another soft areas, generally light in colour, may alternate with darker, harder patches, while frequently the harder layer is found below the lighter and softer, etc.—conditions which depend on the occurrence of the various iron compounds. The behaviour of all, however, when placed in collections, even in the driest of rooms, is the same; all sooner or later undergo change, and portions of rust become detached, until in the course of time every trace of the original metallic core is oxidised. A closer inspection generally shows in these cases small brownish, glistening bubbles[19] which prove, when touched, to be drops consisting of chlorine compounds of iron surrounded and permeated with oxides. Krefting[20] gives as the average of a series of analyses of the rust on northern antiquities the following composition:

Ferric oxide	7·05
Ferrous oxide	12·7
Carbonic acid	3·9
Calcium oxide	0·58
Magnesium oxide	0·09
Ferrous chloride	0·260
Calcium chloride	0·280
Magnesium chloride	0·023		0·61% Soluble salts.
Potassium chloride	0·018
Sodium chloride	0·027
Water chemically combined	8·0
Moisture	1·50
Organic matter	0·97

Thus the chief part in this rapid decomposition is played by the chlorine compounds, as indeed was previously determined[21] by the experimental proofs already given. If ferrous chloride is present the further decompositions can be explained by such equations as those given by Olshausen[22].

6FeCl₂ + 3O = Fe₂O₃ + 2Fe₂Cl₆; 2Fe₂Cl₆ + 2Fe = 6FeCl₂.

The equations do not claim to give a complete statement of the reactions, for other reactions take place at the same time; thus ferric hydrates and carbonates and perhaps also intermediate compounds of oxygen and chlorine occur; they show however that in the oxidation of ferrous chloride, oxides and ferric chloride are produced, so that new and hitherto intact particles of the metal continually react with the ferric chloride.

In many cases the action of the chlorine is not only seen in objects placed in a collection, but also in freshly excavated objects. Not infrequently iron objects are found which are covered with large hard blisters, and are thus more or less deformed. The interior of these blisters consists of a mixture of ferrous chloride with oxides, but the shell has become so hard by complete oxidation that it can only be removed with hammer and chisel.

Iron objects found in peat differ from these chlorine-containing specimens which are found in soil, and although sometimes much corroded, many are well preserved. Blell[23] is of the opinion that if peat is free from tannic acid, the finds will be well preserved, while the theory advanced in the Merkbuch[24] is that tannic acid acts as a preservative. The latter view is probably the more correct, for although ordinary tannic acid seldom occurs in peat, yet peat contains a series of compounds which are tanning agents, such as ulmic, humic, and crenic acids. These form iron compounds which, being insoluble in water, protect the metallic iron beneath from further action. If, however, the peat contains 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.