Friedrich Rathgen

“ 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 [15] 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. ”

“ 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. ”

“ 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) . ”

“ 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. ”

“ 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 [37] cuprous oxide is found. ”

“ Bronze objects are attacked by waters which contain oxygen, carbonic acid and a greater or less percentage of salts. Such soluble salts as are formed are removed by solution, while the bronzes become covered, according to circumstances, with an insoluble layer either of carbonate or of oxide, whereby the form of the objects is preserved. The water then penetrates by capillary action through the porous coating into the interior, and attacks further portions of the metal, forming a layer of soluble cupric salt ”

“ Although no investigations upon the extent of the injurious action of light have as yet been published, light is not without influence upon the outward appearance, and therefore also upon the material condition of antiquities of organic origin. But even inorganic objects, such as pigments, glass, enamel, amber, etc., are affected by light; it is therefore certainly advisable to protect antiquities of all kinds from light during the time in which they are not exhibited to the public. ”

“ 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. ”

“ It will however be advisable to entrust heating and mechanical treatment of objects which are much bent to some skilled silversmith, whose experience may prevent disaster. Silver objects which are largely converted into friable chloride, especially if they are much expanded, or if large portions have broken away in the process of removing the chloride, will hardly bear any other treatment than that of impregnation with gum-dammar solution or with paraffin wax. ”

“ A sword, the hilt of which is inlaid with copper wire or is plated with silver or gold, or the blade inlaid with inscriptions in gold, silver, or copper, may be totally destroyed by incautious treatment; for the ornamentation, if undermined by rust, may be detached with the rust from the underlying iron. ”

“ If a preliminary examination has shown that specimens of limestone will not bear steeping in water, recourse can be had to impregnation only. The treatment of such specimens must be thorough, for merely to paint the fluid upon the surface with a brush almost invariably proves a failure. Instead of penetrating the stone the impregnating medium forms a firm coating which is liable to be lifted, and in parts broken, by the crystallisation of salts, and thus allows the destructive processes to continue uninterrupted. ”

“ 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. ”

“ THE PRESERVATION OF ANTIQUITIES.
The object with which it is proposed to deal should first be photographed, and from different sides if necessary; for the external appearance is often changed during the process of preservation, and it is advisable that a representation of the specimen in its original condition should be kept in case any injury should befall the object, which however rarely happens if proper caution be observed. For this reason in the Laboratory of the Royal Museums at Berlin all bronzes are photographed before treatment, as also are all limestone blocks. ”

“ It is not to be expected that bronzes which are in an advanced state of decomposition (e.g. Figs. 9-12) can be so transformed by reduction as to appear as they did when they left the artist’s hand. For, although the decomposed oxidized layer is now reduced to metal, this no longer forms a coherent mass, but a loose powder, which, being deprived of its essential constituents, chlorine, oxygen and carbonic acid, no longer retains its coherency, but falls to the bottom. [136] Only in the interior and in the pores is the reduced metal retained. ”

“ The bronze should be washed and placed in a very dilute soda solution or in dilute ammonia, after which it should be again well washed with distilled water. As has been explained in Part I., it is to chlorine compounds that the destruction of bronzes is chiefly due, and these are actually produced by the hydrochloric acid treatment. If the bronzes are not thoroughly washed, and this is no easy matter, sooner or later efflorescences will make their appearance, and the process of preservation must be repeated if the destructive action is to be arrested. ”

“ For objects which are free from salts impregnation with zapon possesses the advantage that it renders them less liable to damage from handling or dust, whilst the appearance is scarcely altered, if at all. This applies also to antiquities of metal, for unless the injurious chlorine compounds are removed by simple steeping, or reduction and subsequent steeping, treatment with zapon is useless. To bronzes, which in spite of mechanical cleaning show a somewhat unpleasant grey non-metallic appearance, zapon often imparts a distinct metallic lustre. ”

“ Several combinations of copper with carbonic acid are known, while lead is readily converted into lead carbonate by oxidation. The part played by the carbon compounds resulting from the decomposition of animal and vegetable substances has hitherto received little attention, but this decomposition of organic material is probably the chief cause of the beautiful blue patina. [32] The action of oxygen will depend upon the composition of the metal, upon the locality, and upon numerous other circumstances, while the colour of the patina will vary accordingly. ”

“ Occasionally the zinc dust becomes firmly cemented by oxide to the surface of the lead, and, if this is the case, great care must be used in removing it. The washing process also requires care. A very efficacious method is to allow a stream of warm distilled water, from which the dissolved air has been driven off by boiling, to flow over the object in a porcelain dish. I now omit any impregnation with paraffin wax, and instead recommend removal of the water by alcohol, drying, and coating with zapon. ”

“ 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 bright surface of metallic iron which results from treatment by Blell’s or by Krefting’s method, especially if there is a thin coating of paraffin, should not be touched at all with the bare hand, but only with a cloth or a glove. Bronzes, whether intact or restored, and iron objects, should never be in direct contact with those which show efflorescences.
The usual custom is to attach labels of painted cardboard or metal by means of thin metal wire. The tendency to rust makes iron wire unsuitable, especially for objects containing salt, which are quickly affected ”

“ The specimen has frequently an intact appearance, as if covered with a thin layer of oxide only, whilst in reality all traces of metal have already disappeared, and slight pressure often suffices to break the bronze into pieces. The nearer the colour of the patina approaches to grey, the less solid is the bronze likely to be, a result which is no doubt caused by the presence of lead carbonate. This type of patina has often a yellowish colour, especially on prominent parts, where, being porous, it has retained in its superficial layers substances which were in suspension in the subsoil water. ”

“ Preservation and Cleaning of Coloured Wooden Objects.
For objects of this kind materials should not be used which, like varnish, tend to darken and so to damage the colours. Gum-dammar solution (page 70) answers the purpose, but colourless collodion is better. Colours which are soluble in water (as is frequently the case with wooden objects from Egypt) cannot of course be cleaned with water, but benzine may be applied by means of soft cloths or brushes. Resinous or pitch-like substances may often be removed from coloured objects by turpentine mixed with benzine or ether. ”

“ There is no doubt that these bright-blue efflorescences are the result of an incomplete reduction, which in many cases can scarcely be remedied, for it is often impossible thoroughly to wash objects of great thickness. Thin bronzes, bronze plate, and copper plate remain free from efflorescences. Moreover, many bronzes, especially Egyptian ones, have a hard, non-metallic core, which in the casting has been partly fused or at least hard-burnt, and resists the effects of the washing. ”

“ It is occasionally found that a bronze cannot stand the process of reduction, either because there is only a thin layer of metal over a stout core, or because the metal is permeated with cuprous oxide, which when tested with a file has a metallic appearance. ”

“ Unlike the blue patina, the dark green variety assists the preservation of bronze.
3. Black patina is probably due to a variety of circumstances. The substances which enter into its composition are cupric oxide, lead oxide, lead peroxide, copper sulphide and lead sulphide. If bronze does not contain lead it is blackened only by the action of sulphur. The rarity of black patina is no doubt due to the rapid oxidation of the copper on the originally rough, unpolished surface, which leads to the formation of a green patina. ”

“ The existence of a sufficiently substantial metallic core can be easily ascertained [99] from the weight, for an object, which consists solely or in great part of the oxide of a metal, is much lighter than one of the same size which is largely metal. The ring also affords a test, for an iron object, of which the greatest part is metallic iron, gives a clearer note when struck than one which is chiefly rust. A still more certain test is the use of a file or a drill (comp. page 107) .
Hartwich’s Method [120] . ”

“ Finally, while gently warming the object over a warm, but not too hot, stove, it is coated over with a mixture of 1 part of bees’-wax and 2 parts of turpentine, or better with paraffin dissolved in benzine. Heavy petroleum, which we have found by experience to be a suitable material, is preferable to varnish in so far as the iron is impregnated by a neutral substance which is practically liquid paraffin, but has the disadvantage of being highly inflammable and of being difficult to obtain at so high a specific gravity. ”

“ This method has the advantage that the alcohol penetrates the rust sooner than does water, and also prevents oxidation, which may be actually produced by the water. It may perhaps be advisable to dilute the alcohol, the usual strength of which is 95% to 96%, with about an equal volume of water, for some salts are not readily soluble in pure alcohol. When dry the object is warmed for a few hours in a mixture of equal parts of good linseed varnish and petroleum. The petroleum serves to dilute the varnish, which can [94] thus more quickly permeate the entire mass of iron and rust. ”

“ Since metallic oxides are scarcely, if at all, soluble in water, washing with water, even when a brush is used, will remove only earth or soil which is loosely attached. Compounds containing oxygen or oxygen and chlorine are, however, more or less soluble in ammonia, and, if they are thin and not too compact, after immersion for some time can be removed with a brush. Thick compact layers are loosened with difficulty.
Immersion in 2-5% hydrochloric acid acts more effectively, while sulphuric acid, nitric acid, and concentrated acetic acid have the same action. ”

“ If the water used for soaking salt-containing limestone, earthenware, etc., gives no precipitate, or only turbidity with the silver solution, the determination of chlorine by titration is not applicable.
[87] Though some other kinds of burette may be easier to use, that here recommended (that of Gay-Lussac) is the most convenient for reasons into which we need not enter. The following precautions should be observed: where the burette is not closed by a cork, let a few drops out first to wash away crystals of silver nitrate which may have formed at the mouth. ”

“ For boring out rust spots which have eaten deeply into the iron a dental drill can be used with success, and a great variety of drills and milling cutters can be obtained. The rinsing, which Blell carries out by moving the object to and fro close under the surface in a vessel full of water, may be sufficient for thin iron objects, such as swords, knives, spear-heads, and similar objects. Larger specimens should be freed from the acid by putting them into a still more dilute solution, and, when necessary, by steeping for a short time in water. ”

“ After boiling out with water, steeping in alcohol, drying, and brushing with a soft brush, the coins may be coated with zapon. Coins treated in this way appear to be less brittle than those reduced by Krefting’s method. More troublesome but less dangerous, because potassium cyanide is not used, is the treatment of silver coins with a fused mixture of potassium and sodium carbonates. In this case the silver chloride is converted into silver carbonate, which is then decomposed with 50% acetic acid. Further treatment by washing, drying, and impregnation is carried out as previously described. ”

“ Amongst these salts the most usual is sodium chloride (common salt) , but this is mostly accompanied by potassium chloride, potassium sulphate, magnesium chloride, and calcium sulphate; in short, by those soluble salts which are found in sea-water. In the fine pores of Egyptian antiquities, especially, such salts occur, and their presence is readily explained by the fact that the land of Egypt was originally a sea-bottom. ”