Fig. 46. Roman coins before treatment.
Fig. 47. Roman coins after treatment by Krefting’s method.
Cleaning Copper Coins by Melted Lead.
Although the results obtained by this method are less satisfactory than those produced by the preceding, it has the advantage of simplicity[153].
“Using a pair of tongs, dip the coins one by one into melted lead until the crackling, which begins at once, has ceased, which occurs in from 3 to 10 seconds. The hand should be protected with a glove from the spluttering molten lead. The coin is then thrown into cold water, cleaned, and placed until the next day in hot milk. It may be necessary to repeat the process when the coin has become cold. By this method an olive colour is imparted to the coin which many antiquaries prefer to dark brown, but personally I prefer Krefting’s method because it renders the inscription and designs far more distinct. A coin which after the treatment with melted lead has remained so covered with cupric oxide as to be still illegible can seldom be improved by a repetition of the treatment, whereas had the zinc treatment been applied in the first instance the result would probably have been satisfactory. This conclusion seems to be justified by the extremely small percentage of coins which, in my experience, have remained illegible after the treatment by electrical methods.”
(C.) Preservation of Bronzes by the Exclusion of Air.
In those cases in which the advanced state of decomposition renders the reduction process either inapplicable or at any rate inadvisable, or in which the decay is not likely to be arrested by impregnation, a further method of preservation remains, viz. the complete exclusion of air and moisture.
If air is completely freed from moisture the oxygen can no longer act in conjunction with the copper chloride upon the still intact metal (see page 29 et seq.), and the condition of the bronze will consequently remain unchanged.
A bronze, for example, which shows much decay should be placed after impregnation under a hermetically sealed bell glass, and beneath or near it should be placed some dehydrating agent, of which anhydrous calcium chloride is the most suitable (see note, p. 123). To exclude the air completely the bell glass should have a projecting ground edge, which should be smeared with vaseline or grease and pressed firmly upon a thick well polished glass plate. The dehydrating agent may be placed in a glass vessel or dish in such a way as to be unseen, or it may be covered with two or three thicknesses of dark gauze or with black cardboard laid loosely over it. If an object is too large for a bell glass, or if several objects are to be exhibited together, a square plate-glass case with iron framework, made air-tight with putty, may be used as shown in the illustration (Fig. 48). The lower part, containing calcium chloride, is partitioned off by a perforated plate covered with black gauze[154]. A hygrometer was placed behind the head, the indicator of which has remained at zero since it was first fixed several years ago, and the bronze has not hitherto shown any sign of change, although the inlaid gold is in parts raised from the metal by a light-green oxychloride. The cost of these cases is considerable, but for valuable objects this should not be considered. In the place of calcium chloride, sticks or lumps of caustic soda may be used with advantage, for this substance absorbs both moisture and carbonic acid.
Fig. 48. Method of mounting objects in air-tight cases.
This method of preservation is of course applicable not only to decomposed bronzes but to all valuable antiquities, whatever the material may be.
These methods are founded upon the fact that the sunken areas of the coin are, by the pressure of the die in stamping, rendered denser than the raised portions, such as the inscription. The earliest method is that published by Brewster, reported by Süpke[155]. The coins when cleaned are placed upon red-hot iron, which causes the oxidation of the entire surface of the coin. The thin film of oxides varies in colour according to the duration and the intensity of the heat. The oxidation of the letters of the inscription differs from that of the surrounding parts, and is recognisable by a difference in colour. Drude [156], treating more especially of silver coins, remarks that the inscription is rendered legible by heating them to redness over a Bunsen-burner. It then, according to “Prometheus[157],” when viewed in a dark room, appears dark on a bright ground, especially if the coin has been previously polished and then roughened again by slightly etching it with acid. In conclusion, the method of Roux[158] may be quoted:
“The smooth-worn and polished coin is placed in a solution of copper sulphate or of some other metallic salt, and suspended between the electrodes of one or more cells of a battery (any other form of continuous current will serve the purpose). If the current is weak, the electrodes must be near to the coin. The stronger the current the more rapidly the impression appears. On the side which faces the anode or positive plate the impression is metallic; on the other side, after gently wiping off the less firmly adherent part of the oxide, the impression appears in grey lines. These markings can be fixed by varnishing them with a thin alcoholic solution of shellac. To render the impression legible on both sides, the coin should be placed upon the four upturned feet of an insulating stand. The larger the coin the deeper must be the layer of solution above and below the coin. The depth below should be equal to the radius of the coin.
This can perhaps be most conveniently carried out by placing that electrode in immediate contact with the coin which upon immersion in the solution becomes tarnished with the metal, i.e. the cathode or negative pole. Other portions which it is not intended to treat should be first covered with varnish.
The striking success of this method is due to the fact that that portion of the metal which has been compressed by the stamp is a better electrical conductor than the rest; no success could therefore be expected from the use of this process for the restoration of such objects as worn engraved copper-plates, etc.”
Preservative treatment of silver is scarcely necessary (cp. pp. 49-52), except in those cases in which the silver is alloyed with a large percentage of copper, and which show efflorescences similar to those which appear upon bronzes containing chlorine. Electrolytic reduction will be found to be the most suitable method of treatment in such cases. To treat silver coins they should be placed in contact with iron nails in lemon juice. Instead of the citric acid, which is the active principle in this process, other diluted acids and other metals, e.g. zinc, may be employed. Flinders Petrie[159] has shown that the reduction can also be effected by a weak solution of common salt. Silver chloride is soluble in ammonia, and thin layers may be removed by the application of ammonia by means of a soft brush. Thorough rinsing with pure water, drying with soft cloths, and cautious warming are always essential.
An excellent reducing agent for single coins, the characters of which are rendered illegible by a layer of silver chloride, is molten potassium cyanide, or a mixture of this substance with sodium or potassium carbonate. In a short time the silver chloride is decomposed and removed from the smooth surface of the coin. 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.
Silver which has become friable (p. 51) can be rendered more compact by cautiously heating it to redness. 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. As silver chloride is easily fused such articles should not be subjected to heat.
Earthy matter can often be removed with a neutral soap and warm water, while calcareous accretions can be dissolved by a 2% solution of hydrochloric acid. Silver which has been blackened by silver sulphide may be laid in a warm 2% solution of potassium cyanide. All objects should be subsequently well washed with warm water.
Objects of pure lead and pure tin are rare. If much oxidized they should be washed with warm water, dried, and impregnated with a gum-dammar solution or with paraffin wax (pp. 70 and 91). If in a good state of preservation they may be freed from any earthy or calcareous coating or from lead carbonate by the cautious use of very dilute nitric acid followed by steeping in water.
Ceresole [160] cleans oxidized leaden seals with 10% acetic acid, neutralises the acid with ammonia, and after five minutes in alcohol coats them thinly with wax. The seals are preserved between glass dishes (Petri dishes), the space between the dishes being filled with cement. I employ Krefting’s method for leaden medals, using either zinc and very dilute sulphuric acid, or zinc dust and caustic soda. 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. To preserve the specimens after treatment, more especially from the injurious action of perspiration from the hands, they are placed between dishes of glass or of celluloid[161].
Objects of pure gold usually need only be cleaned with soap and water and a soft brush; lime may be removed by the application of a 2% solution of hydrochloric acid. A coating of silver chloride occurring on gold which contains a large percentage of silver may be removed by ammonia, or, in certain cases, by the alternate use of dilute hydrochloric acid and ammonia.
A layer of red ferric oxide (see p. 53) is of frequent occurrence upon gold objects, and may be removed by warming the object in a stronger solution of hydrochloric acid, but soft brushes will often serve the same purpose. Pure gold being very soft, only the softest so-called “silver brushes” should be used, and all pressure or bending should be avoided. If friable the object should be carefully impregnated with a solution of gum-dammar (p. 70).
If covered with a film of dirt, or if when in a collection objects of glass or enamel undergo any alteration, they should be washed or steeped in lukewarm water. When dry they should be treated with pure olive oil or poppy-seed oil, which may be diluted with benzine. The oil helps to restore the lustre to the glass and to bring out the colour of the enamel. When thus treated the objects should be carefully protected from dust.
A decomposition of ancient glass when deposited in a museum has been hitherto only rarely observed, but allusion may be here made to the so-called ‘sweating’ of glass which is a question of considerable importance in Industrial-Art collections. In this case preservation is insured by washing with distilled water, drying, and coating with zapon. Further particulars may be obtained from the paper by Pazaurek[162].
Many curators dry carefully and impregnate them with a gum-dammar solution or shellac; isinglass or glue are however preferable, for these aqueous solutions may be used for the treatment of damp objects, which could scarcely be dried without cracking. In order to permeate the object these solutions must be very dilute, and are most advantageously applied at a temperature of about 120°F. [50°C.]. The impregnation may also be effected in rarefied air under a bell glass (p. 68). Friable bones and similar objects which might fall to pieces in the solution during impregnation should be bound with strips of gauze or with string before immersion; they are easily removed when cold. To prevent the formation of mould a small quantity of dissolved corrosive sublimate[163] is added to the glue, or when dry after impregnation the objects may be covered with a solution of shellac or resin. Impregnation is of very general application, and is frequently used for the preservation of fossil and pleistocene bones.
At Copenhagen the method used to render leather soft and pliable is to place it in train oil for an hour and then dry it with filter-paper. Lanoline may also be used with success [164]. Poppy-seed oil in benzine (p. 86) is said to produce good results, but the “Merkbuch” recommends the preservation of leather in this condition in alcohol[165].
Earth and soil may be removed by mechanical means, and, occasionally, careful washing may be possible. The objects should be dried and impregnated with a gum-dammar solution (p. 70), poppy-seed oil (p. 86), or a solution of india-rubber (p. 90), or they may be preserved in alcohol (p. 159). Some textile fabrics in the Copenhagen Museum owe their excellent state of preservation to Steffensen’s treatment, i.e. impregnation with a solution of india-rubber in turpentine with the addition of bees’-wax.
The following account of the treatment of textile fabrics from the Lake-Dwellings is due to Herr Heierli, of Zürich:
“The pieces as they were taken up were laid on the ground and thus slowly allowed to dry in the air. They were then placed between glass plates, the edges of which were pasted over with paper. Old pieces which had been dry for a long time, and which had become tender and friable, were laid on the ground and watered from time to time until they were soaked through; they were then treated in the manner already described.”
Egyptian textile fabrics preserved between glass plates often deposit a thin layer of salt on the glass, but this is easily wiped off (see p. 155). It must first be ascertained by a previous trial in each case whether the salt can be removed by steeping in water or in alcohol and water.
Hair found in peat has always a dark-brown colour from impregnation with peaty matter. The method proposed by Bille Gram [166] for restoring the natural colour consists of repeated and alternate treatment with very dilute alkali solution and acid at about 120°F. [50°C.]. When the liquid ceases to show coloration the natural colour of the hair is restored.
These do not require any treatment beyond protection against insects, which is attained by immersion in an alcoholic solution of corrosive sublimate, or by spraying with corrosive sublimate in either alcoholic or aqueous solution. Of course the poisonous nature of corrosive sublimate necessitates caution in its use and it should be always labelled as such.
The use of naphthalene is not always successful, and white scales of naphthalene are apt to make their appearance; nor does finely powdered pepper sprinkled on the feathers, either alone or mixed with finely powdered alum, give satisfactory results.
The method of cleaning and preserving papyrus in use in the Egyptian department of the Royal Museums at Berlin is as follows: Those pieces which are folded together or rolled are carefully straightened, and, if very friable, they are first placed between damp filter paper to render them uniformly pliable. Dust and dirt are removed with soft paint-brushes, crystals of salt which are often found[167] are picked off with forceps. Any growths of fungus are carefully scraped off with a knife. The papyrus thus prepared is then placed between two thick polished glass plates, the two opposing surfaces of which are covered with a very thin layer of vaseline. Air is frequently admitted to dry the papyrus, while the pressure of the glass plates tends to smooth it out, and after it has been so treated it is mounted between thin glass plates, the edges of which are pasted over with paper covered with an oil paint.
A papyrus preserved between glass plates often shows round the edges a whitish border about two millimetres in breadth, and on separation the glass plates show a slight film of the same white material on the surface which had been in contact with the papyrus. The formation of this film, which consists chiefly of common salt and is easily wiped off, may be prevented by previously washing the papyrus in distilled water, a proceeding which experience has shown to be harmless. As the papyrus will swim on the surface it should first be immersed in alcohol until soaked through; the process of steeping is then quite simple. The thinness of papyrus enables the steeping to be completed after 24 to 48 hours by two changes of the water, and care must be taken lest a too prolonged steeping should obliterate the lettering. The water assumes a yellowish or brown tint and the papyrus becomes somewhat lighter in colour on drying. Papyrus may also be preserved by zapon (see Appendix), but this method has no advantage over that of mounting between glass plates.
To preserve adequately articles of moist wood (and they are generally in this condition when first excavated), preliminary measures to prevent their drying in the air must be taken immediately after they are dug out of the earth. If found in water, as for instance articles from pile-dwellings, they should be conveyed in water; moist objects should be wrapped in several thicknesses of moist cloth, and the whole wrapped in gutta-percha membrane, or in a layer of moist moss. The cracks which arise in wooden objects which have become dried may frequently be closed up by laying them in lukewarm water.
As the earliest attempts at preservation were probably made upon wooden objects there is scarcely a collection in which a number of methods are not employed. One exception only is known to me, and here, after a plaster of Paris cast has been taken, the object is simply allowed to shrink. The methods proposed and carried out are so different and so numerous, especially as regards the liquid used for impregnation, and in such variety, that it is only necessary to deal with the most important. These may be divided into two classes, viz. dry and wet.
(1) Dry Preservation of Wood.
Moist or wet objects are placed in thin size or in a solution of isinglass till they are impregnated, after which they are dried gradually in a shady place. A solution of shellac, or varnish diluted with petroleum or benzine, is then put on with a brush.
Sometimes the objects are placed directly into a mixture of varnish and petroleum, or they are impregnated with melted paraffin. The former is preferable as a means of impregnation if there are cracks or holes, for the superfluous solution readily drips from the wood when it is taken out, while paraffin sets too soon to drain out of the cracks, and thus imparts an unnatural white appearance to the wood. Owing to the large size of the vessels which would be otherwise required, paraffin is only useful for small or medium-sized objects, but when making use of varnish one end of a large object [168] may be placed in the mixture while the solution is repeatedly poured over the object. After two or three days the opposite end should be placed in the solution. By repeating this process every part of the object will soon be thoroughly impregnated.
Objects of still greater size, such as a Viking’s ship, can only be preserved by painting the surface. In such cases it is advisable to begin with dilute varnish so as to allow the impregnating solution to penetrate as deeply as possible into the material, instead of merely forming a skin.
A solution of waterglass has in one instance been used for the preservation of a large boat, but the result is not satisfactory.
Leiner’s Method[169]. The wooden articles are laid in glycerine mixed with a small percentage of carbolic acid. The length of time during which they remain in the glycerine depends upon their size. When taken out they are lightly wiped and preserved without further treatment. If a growth of mould should occur it may be washed off.
Objects thus treated retain their moist condition and should therefore be very carefully protected from dust.
Speerschneider’s Method[170] (cp. p. 91). Small specimens are heated for two hours in a mixture of
8 parts of rape-seed oil,
1 part of bees’-wax,
1 part of pine resin, and
2 parts of benzene.
Larger objects require a proportionately longer heating, but the mixture must not be allowed to actually boil. The moisture rises as steam and causes the solution to bubble. The bubbling however continues after the moisture has been driven off; great care must therefore be taken that the heating is not so prolonged as to cause the object to shrink. The highly inflammable nature of the mixture renders great caution necessary, and should it ignite, a lid, which should always be in readiness, should be put on the vessel. After impregnation the objects are wrapped in blotting-paper and laid in ashes for four days to prevent the access of air. The aim is doubtless to insure thorough absorption of the superfluous liquid which remains upon the object, which exposure to air would prevent by causing the mixture to set too rapidly. The same mixture can be used repeatedly, but each time two-thirds of the original quantity of benzene must be added.
Herbst’s Method[171]. The moist objects are boiled in a saturated solution of alum for two hours (hot water dissolves about 31⁄2 times its weight of alum), but if they are of some thickness the time must be proportionately longer. They are then taken out, and when the alum in crystallizing has made them more or less firm, the crystals adhering to the surface are washed off with warm water.
When thoroughly dry the wood is brushed over with hot linseed oil, which operation is repeated until no more oil is absorbed. A final thin coating of varnish or shellac is then given. According to Steffensen, the method followed at Copenhagen is to lay the objects in warm thin size for a quarter of an hour after impregnation with alum. This alum-method is there used for objects of oak, although the “Merkbuch” (p. 60) states that only the varnish-petroleum mixture should be used for impregnating this class of object.
(2) Preservation of Wooden Objects in Liquids.
The expense entailed by this method renders it applicable only to articles of small size.
The preservation of small objects in a flat vessel, the bottom of which is covered with glycerine, has the disadvantage that glycerine extracts organic substances and thus assumes a brown colour. If glycerine is used the object should undergo a thorough preliminary steeping, and the glycerine should be renewed until it remains colourless. Closed cylinders filled with glycerine or a mixture of glycerine and water are not convenient because wood nearly always floats in the liquid. This may be remedied however by the addition of alcohol.
Jenner’s Method[172]. When the objects have been thoroughly cleaned with water, pure alcohol, diluted with water until the specific gravity at 54°F. [12·5°C.] reaches 0·96, is poured over them. After six or eight weeks the alcohol is poured off and replaced by fresh alcohol of the same specific gravity. This alcohol is examined in a year’s time, and should always show a specific gravity of 0·96. The alcohol which has been poured off may be filtered, and if necessary decolourized by animal charcoal; when the specific gravity has been again raised to 0·96, by the addition of fresh alcohol, it may be used again.
The same process is applicable to textile fabrics, yarn, and leather.
Protection against Wood-worms, etc.
All the methods mentioned above will destroy insects and their larvae.
In cases in which it is either impossible or undesirable to use immersion or external application, as for instance in the treatment of objects of dry wood, the larvae may be destroyed by dropping petroleum, an aqueous solution of potassium arsenite, or corrosive sublimate, into the various small openings. This will also help to prevent further attacks.
If solutions are not applied insects may be destroyed by the vapour of carbon bi-sulphide or of crude benzene. These liquids, which are sufficiently volatile at the ordinary temperature, should be placed, together with the objects to be treated, in a closed box.
I have used a similar method for the destruction of wood-worms in Egyptian coffins. The coffin is placed in a large wooden box lined with tin plate. The lid, also lined with tin, is provided with projecting edges, to which strips of felt are glued. The weight of the lid by compressing the felt is sufficient to render the box air-tight. Six or eight glass vessels containing crude benzene are placed at the bottom of the chest and of the coffin itself. It need scarcely be added that the box must not be opened near a fire or light, as the vapour forms an explosive mixture with air; it is in fact advisable to have no light or fire in the room.
Insects can also be killed by naphthalene vapour, but as naphthalene is insufficiently volatile at ordinary temperatures, the method above described is more convenient[173].
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.
A method of cleaning gilded or brightly coloured ecclesiastical figures which is used in the Breslau Museum is the application of a mixture of copaiba balsam and ammonia. This method is similar to that used to clean paintings[174], the action of the solution being that of a mild soap.
Antiquities which were originally uncoloured, but which have been subsequently painted, may be cleared of paint by means of a solution of caustic soda in water or alcohol.
After the mechanical removal of any adherent earth and dust, the specimen should be rubbed carefully backwards and forwards between the fingers covered with a soft woollen glove. Particles of soil should be picked out of any holes and indentations by using a strong horse hair[175]. It is then preserved by impregnation with a solution of shellac, poppy-seed oil, or isinglass (pp. 70 and 86).
The following particulars of the method used in Messrs Stantien and Becker’s collection of amber have been supplied by Prof. Klebs:
“Amber is preserved best in distilled water: I add a very small quantity of glycerine and a still smaller amount of alcohol. A proportion of alcohol greater than 1% is injurious to the amber. A thick layer of gelatine containing glycerine is an excellent medium for the preservation of large objects if they are kept free from dust. This layer should be washed off and renewed every few years.”
In addition to the protection from dust afforded by closed glass cases, it is also important to protect objects from the action of direct sunlight, especially during the summer months. There is, for instance, no doubt that the decay of bronzes, even of those with a patina which is apparently sound, is hastened by the great variations of temperature, caused by the rays of the sun falling directly upon them. Similarly objects which have been preserved by the application of solutions of resin or varnish should be protected from the direct access of sunlight, for the sudden warming may easily cause cracks. Nor should antiquities be kept near the heating apparatus. There is another precaution, to which too little attention has been paid, viz. the protection of objects as far as possible from even diffused daylight. 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.
The public is effectually prevented from fingering antiquities which are enclosed in glass cases, but it may be well to remind those who have to handle them in the course of their duties that contact with the bare hand can only be harmful, even though fingering is understood to be beneficial to modern bronzes by inducing the formation of patina. 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; thus light coloured earthenware may soon be covered with spots of rust. Copper wire and nickel wire are liable to be similarly attacked. Many years ago it was noticed[176] in the Ethnological Museum at Berlin that nickel wire when in contact with silver objects which were covered with silver chloride was destroyed by the formation of a deliquescent green nickel salt. Silver or platinum wire forms the most suitable means of attachment, but if the expense of these is too great, copper or nickel wire may be used, except in the cases mentioned above.
Small objects of any kind, which one still frequently finds kept in open cases, are better preserved in upright glass cylinders with glass stoppers, or in cheaper glass tubes, one end of which is fused and the other closed with a cork.
The methods of preservation which have been described in the preceding pages may be thus tabulated and summarised:
| Methods. | Application. |
| Steeping in water, drying and impregnation. | Limestone, Earthenware, Iron, much corroded. |
| Direct impregnation. |
(1) Unbaked earthenware, etc., (2) Bronze objects with little or no metallic core, or showing a cracked or warty surface, (3) Objects of wood and of other organic substances. |
| Removal of compounds of oxygen or chlorine | |
| (a) by chemical process, | Iron objects in a good metallic condition, |
| (b) by electrolytic process. |
(1) Iron objects with a sound metallic core, (2) Bronze objects with a sound metallic core. |
| Mounted thoroughly dry and hermetically sealed. | Valuable bronzes in an advanced state of decomposition. |
There will be no difficulty in the choice of methods for limestone or earthenware, whether kiln-dried or sun-dried, for a simple experiment will prove whether steeping is likely to cause injury or disintegration.
The methods are themselves simple and inexpensive. For organic substances the chief question is the choice of the most suitable medium for impregnation.
Iron and bronze present some difficulty, although the use of a file will readily show whether reduction is feasible.
The simplicity of the apparatus required for Krefting’s method gives it an advantage over other methods, at any rate for iron objects. Objection has been taken to the methods of reduction, because they give to the objects thus treated an appearance to which the public are not accustomed. It may be safely asserted however that this appearance more truly represents the object when in actual use, than the oxidized and rust-covered specimens to which we are accustomed in antiquarian collections. To those who value an antique object for the crust that covers it, all methods of restoration must be objectionable. Such persons ought to object to the removal of the incrustations which hide the cuneiform inscriptions on clay tablets. On the other hand, those who regard these methods with approval should go a step further and confide their collections to experienced hands for some form of treatment which may bring to light inscriptions and inlaid work which will greatly enhance their value.
To spread the knowledge of these methods and to invite the co-operation of others is the aim of this book. As to the best method to be used in each particular case it is unnecessary to lay down any hard and fast rule, for this can only be learned by observation and experience.
For this purpose a proper brush is required with strong bristles, closely set as in a scrubbing brush; the brush should have a firmly fixed handle, preferably slightly curving upwards to save the knuckles from being bruised upon the stone. A so-called “silver brush” will serve the purpose. The paper should be stout and stiff enough to resist the blows of the brush without tearing. An admirable paper, which possesses these qualities, is specially prepared for the purpose by the O.W. Company, 100, Great Russell Street, London, W. As a substitute for the specially prepared paper stout packing paper may be used with satisfactory results.
The stone should be tilted if possible at an angle of about 45°, and the surface bearing the inscription should be well washed or carefully scraped free of dirt and foreign matter and should be rendered thoroughly wet. A piece of the special paper of suitable size should be soaked in water for a minute or more. It should then be carefully applied to the surface of the stone in such a way as to prevent air-bubbles. This may be assisted by gently smoothing it with the hand or back of the brush. When close adhesion has been secured, and all air-bubbles removed (this can sometimes be done by pricking through the paper with a pin), the paper should be sharply beaten with the brush, the blows being delivered from the wrist and not from the shoulder until it begins to show a fluffy appearance. It should then be peeled off and allowed to dry, after which it may be rolled or folded without danger of injury to the embossed inscription.
Should the paper tear, another piece soaked as before may be placed on the top and beaten until it becomes incorporated with the first. If the letters are large and deep, or if the surface is much cracked, two or more sheets superimposed should be used. In the case of large inscriptions it is advisable to take impressions by sections, care being taken that each sheet slightly overlaps the preceding one to prevent the possible omission of some of the letters.
It is also useful to take at the same time a pen or pencil copy of the inscription, for a comparison of the copy and the squeeze will often prevent errors in deciphering. The squeezes can be very well deciphered by artificial light, while doubtful letters may sometimes become clear on holding up the sheet to the light. The reverse side of the squeeze, upon which the inscription stands in relief, may afford great assistance when read by the aid of a mirror. A photograph of the squeeze will often reveal more than a photograph of the inscription itself.
The method is described by S. Reinach in his “Traité d’Épigraphie Grecque” (Introduction, p. XX. ), where he also refers to Hübner, “Ueber mechanische Copien von Inschriften,” 1871.
Further particulars may be given of the new preparation known as Zapon. This substance is now made on a large scale, and can be obtained from the British Xylonite Co., Brantham Works, Manningtree (Xylonite lacquer F. 6631). The following excerpt is from a short communication in “Prometheus” (XV. 1904, pp. 485 and 499), which deals with the preservation of wax seals and of glass.
Zapon, the invention of Crane, of Shorthills, U.S.A., has been used for 20 years past for the protection of metals from oxidation and the action of sulphuretted hydrogen. Although the products of the various manufacturing firms differ in composition, zapon is essentially a solution of nitro-cellulose in various solvents. The nitrated cellulose, i.e. gun-cotton (pyroxyline), is generally, with the addition of camphor, dissolved in a mixture of amyl acetate (hence the peardrop-like smell) to which distillation products of petroleum, etc., are added. It comes into the market as a faintly yellow, slightly oily liquid. Its use as a preservative depends upon the fact that the evaporation of the solvent leaves behind it a fine transparent coating of gun-cotton (pyroxyline). Zapon for preservative purposes must have a neutral reaction, and must not under any circumstances redden litmus paper. Its use in this connection is due to Schill, who also recognised its suitability for other materials, as, for example, for plaster casts, the treatment of which is eminently simple, for it consists in dipping small casts, or in painting larger ones with a soft brush. It is advisable to begin at the top and apply it from above downwards, using a clean dry cloth to wipe off any excess of the fluid which collects in the deeper parts of the cast. If zapon containing about 4% of gun-cotton is used, the coating left on drying is scarcely visible; with a 5% solution a certain degree of polish results. Casts treated with zapon are less easily damaged by dust than those untreated, and may be cleaned with soap and water without injury to their surface, provided that a soft brush is used, but brushes which are stiff enough to injure the zapon coating will damage the contours of the statue. It should only be used for objects kept under cover, for rain and wide variations of temperature will attack them almost as readily as untreated casts. It can be used with equal success for antiquities of stone, clay, baked or unbaked, or for plaster after the soluble salts have been thoroughly removed by steeping, for if this has not been done the salts will soon crystallize out and loosen the protective coating. 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. To enhance this lustre by a second vigorous application is not recommended, for this gives the impression of a varnish. To protect articles of silver from the blackening influence of sulphuretted hydrogen, zapon is very useful, but does not afford absolute protection unless it has been thickly applied. In collections of armour much use may be found for this material. The objects are dipped and then placed in a drying oven at 105°F. [40°C.] to secure rapid drying and uniform distribution. The amyl acetate or other solvent is best conducted away, as it evaporates, into a flue or into the open, although the vapours can hardly be considered dangerous to health.
The following references will afford some information on the use of zapon in the preservation of Archives:
E. Schill, “Anleitung zur Erhaltung und Ausbesserung von Handschriften durch Zapon-Imprägnierung,” Dresden, 1899.
O. Posse, “Handschriften Konservirung,” Dresden, 1899.
G. Sello, Das Zapon in der Archivpraxis (“Korrespondenzblatt des Gesamtvereins der deutschen Geschichts- und Alterthumsvereine,” L., 1902, p. 195).
Schoengen, Over hat Zapon (“Nederl. Archivenblad,” 1902, 1903, Nos. 1 and 3).
J. Perl, Das Archiv-Zapon (“Korrespondenzblatt,” LII. , 1904, pp. 119 and 435).
G. Sello, Die bei der Zaponverwendung in der Archivpraxis gemachten Erfahrungen (“Korrespondenzblatt,” LII. , 1904, p. 439).
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