Chapter VI: Part II: The Preservation of Antiquities (3)

When finally removed from the reducing bath, after the black metallic powder has been thoroughly cleaned off with water and a soft brush, the object should be suspended for a short time in water at the ordinary temperature, or so fixed that there is a good depth of water beneath it; it should then be washed in hot water. When the bronze is first placed in water, whether hot or lukewarm, small bubbles of hydrogen will continue to rise for some time, while at the same time a whitish, or sometimes grey, gelatinous precipitate, consisting of a hydrated oxide of tin[144], will often fall from it. The grey colour is caused by the admixture of small particles of lead or copper.

At first I renew the water two or three times a day, then once in twenty-four hours, and finally at longer intervals, using distilled water throughout for small objects, but for larger specimens for the final washings only. For the earlier washings at any rate I use warm water. Cyanides as well as chlorides give a white precipitate with silver nitrate; this reagent will therefore serve to indicate the progress of the operation. If at the end of a fortnight in the case of small bronzes, or in three to six weeks for large objects, the water shows no cloudiness, or if upon the addition of yellow potassium chromate it instantly assumes a red colour (p. 62), the steeping may be considered complete. Some Egyptian bronzes, especially those which contain a large proportion of lead, after steeping exhibit a whitish crystalline coating of lead carbonate or small hemispherical groups of crystals scattered over the surface of the metal, especially where the pores are large; when dry these can easily be removed.

An extended experience points to the conclusion that bronzes should be dried at once, and as quickly as possible. They should be wiped with soft cloths and then dried in a drying chamber or upon glass or metal rings on a stove. A simple form of drying chamber can be made with copper or iron plate of sufficient thickness, with a loose lid provided with a hole fitted with a cork, through which a thermometer passes. This can be heated over a Bunsen burner, but the temperature should not exceed 230°F. [110°C.]. Small objects may be freed from water by immersion in alcohol for twenty-four hours before drying.

The completion of the process may be gauged by the yellowish or reddish yellow colour which the bronzes should assume when they have been dried and wiped with a cloth or brushed; brushes made of the finest steel wire may be used for this purpose. A bright colour is but rarely seen on bronzes which contain lead. Egyptian bronzes frequently contain as much as 20% of lead, and such bronzes have nearly always a dull-grey or blackish appearance. A similar colour is seen on bronzes which contain no lead, but which are very porous, and are in an advanced state of decomposition. In such cases the finely divided particles of reduced metal are retained upon the rough surface of the bronze, and as all metals, when sufficiently finely divided, form a blackish powder without any metallic lustre, the whole object then appears almost black. It is difficult, and in many cases impossible, to remove this dust, especially that retained in the pores. Metal dust is injurious to the lungs, and if recourse is had to brushing, an efficient extractor for the removal of the dust-filled air is required[145]; but brushing and the use of bellows in addition frequently prove insufficient. Washing the objects with benzine is more effectual, but a trustworthy method of giving the bronze a better appearance is to place it into melted paraffin wax[146] at 250°to 285°F. [120° to 140°C.]. Yet the use of paraffin wax should be avoided if possible, for in spite of the most careful washing blue efflorescences may sometimes appear upon thick bronzes in the course of a year. If this should happen they must be washed out at once, and the bronze can again be submitted to the cyanide-reduction process. If however paraffin wax had been applied an attempt would have to be made to remove it by immersing the bronze in benzine or a mixture of ether and alcohol, or by heating, before the reduction process could be repeated.

.]

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. The bronze must therefore be continually watched whilst it is in the cyanide bath, and if necessary should be taken out even before the reduction is complete. This should be done if large pieces or large quantities of a powdery precipitate fall from the bronze, or if it is found that a needle readily pierces the oxidized layer. A specimen of this kind must be taken from the bath, carefully steeped, dried, and impregnated[148].

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. Only in the interior and in the pores is the reduced metal retained.

In addition to the preservation of articles by the removal of the injurious chlorine compounds (as is also the case with Blell's and with Krefting's method for iron antiquities), the process may result in the discovery of inlaid work, inscriptions or ornamentation, the presence of which was not suspected. The accompanying illustrations (Figs. 39 and 40) show bronzes before and after the preservation process, while the axe-blade shown in Figs. 41-43 illustrates equally clearly the advantages which accrue from the treatment. Not less striking is the result of the treatment in the case of the dagger-sheath shown in Figs. 44 and 45 by which the design was discovered.

Reference may here be made to a case described elsewhere[149], in which reduction proved that what had been thought a single bronze object consisted in reality of two pieces which did not belong to each other, but were fitted together by means of a bottle cork of modern date! In another instance a bronze was found upon reduction to be brazed with a hard solder containing zinc, which was thus quite inconsistent with the age ascribed to the object.

A short digression may be here made in order to discuss the question whether the composition of the bronzes undergoes any alteration. Three analyses[150] of Egyptian bronzes before and after reduction by Finkener's method show that the change in composition is so slight as to be immaterial. It is of course obvious that greater differences will be seen in the results of the analyses before and after reduction of bronzes which are in an advanced state of oxidation, for in this case chlorine, oxygen, water, and carbonic acid constitute an appreciable proportion of the total weight. But even in these cases the analysis made after the reduction shows very slight variation from that of the original metal.

+-------------------------+---------------+----------------+
| Osiris | Osiris | Ibis |
|-------------------------+---------------+----------------|
| Before After | Before After | Before After |
| | Reduction | |
|Tin 2·16 2·27 | 4·30 4·21 | 8·66 8·46 |
|Copper 77·83 77·45 | 79·66 79·74 | 88·53 88·75 |
|Lead 19·23 19·86 | 15·51 15·58 | 1·69 1·95 |
|Iron 0·12 0·14 | 0·28 0·24 | 0·21 0·20 |
|Nickel &} | | |
|Cobalt } 0·29 0·24 | 0·20 0·17 | 0·30 0·29 |
|Arsenic 0·17 0·23 | 0·17 present| 0·32 present|
|Antimony trace trace | trace --- | 0·20 present|
--------------------------+---------------+----------------+

The two latter bronzes were tested qualitatively only for arsenic and antimony, and when the three objects were washed the hydrated tin-oxide described on p. 130 was only found in the case of the Ibis. In this connection it should not be forgotten that slight differences in the quantities may be due to errors in the analysis as well as to a want of homogeneity in the alloy.

KREFTING'S METHOD. This method is similar to that used for the reduction of iron (see page 108). The layer of oxidized material is removed in several places by filing, hammering, or rubbing with emery cloth until the metal is exposed. The object is then wrapped round with strips of zinc, and placed in a 5% solution of caustic soda. The hydrochloric acid produced in the process of reduction acts upon the soda to form sodium chloride. Here too the greatest care must be taken that the steeping is sufficient.

Personally I prefer Finkener's method, for potassium cyanide is more easily washed out than soda, and also, although poisonous, is less caustic.

Krefting's method however has proved of considerable success in some cases, notably in the treatment of some 40-50,000 Roman copper coins at the Berlin Museum. These were, with few exceptions, covered with a crystalline layer resembling green malachite or blue azurite and were quite illegible. Various unsatisfactory attempts were made to clean them with ammonia, with warm and cold acids of different kinds, with acid and iron nails, and by electric current both in an acid solution and in a solution of potassium cyanide. The following method finally proved satisfactory[151]:

Krefting's Method Applied to Oxidized Copper Coins.

"A thin plate of zinc with a bright metallic surface is perforated
with a brad-awl, having a diameter of from 2 to 5 mm., until
there are about 50 or 60 holes in each square metre. This is
placed with the sharp edges of the holes uppermost on a row of
glass rings (or crystallizing dishes will serve the purpose) 20
mm. in height resting upon the bottom of a large glass vessel.
The coins, which in this case were 20 mm. in diameter, were then
placed on the zinc plate, so that 7 or 8 of them occupy a space
of 1 square decimetre. Another similarly perforated plate is laid
upon them, and upon this more coins are arranged in the same way,
and so on until there are six or eight double layers. A perforated
zinc plate is then placed on the top with the sharp edges of the
holes turned downwards, and over this a few zinc plates which have
been previously used. The whole pile is surmounted with weights
or stones resting upon glass rings or inverted glass dishes in
order to press the sharp edges of the holes into the closest
possible contact with the coins. A 5% solution of caustic soda is
then poured over the whole, the immediate result of which is an
evolution of gas. The reduction of the coins is usually complete
in fifteen to eighteen hours, after which they should be well
washed. After several rinsings in cold water they are placed,
about 1000 at a time, in a large vessel fitted with a perforated
false bottom containing hot water, which should be renewed three
or four times every day. After four days the coins are wiped
with a cloth and thoroughly dried on a warm oven plate or in a
drying chamber at a temperature of about 212°F. [100°C.]. They
are then brushed with a bristle brush before a dust extractor, a
procedure rendered necessary by the fine metallic dust from the
coins, which then assume a light or dark brown colour such as is
seen on copper coins which are in actual circulation. The practice
of placing the coins whilst still wet into melted paraffin wax
at 260°F. [120°-130°C.], which gives a dark appearance even to
the brightest, has the disadvantage that the wax prevents the
use of sealing-wax for taking impressions, and is therefore not
recommended.

The reaction is analogous to that which occurs in the reduction
of iron. The copper of the coin forms in the alkaline solution
an electric couple with the zinc, and the hydrogen which forms at
the copper end reduces the copper compounds covering the coins to
metallic copper, and thereby loosens them, while the zinc oxide
which is simultaneously formed is dissolved in the soda solution.
In actual practice a part only of the zinc oxide is dissolved,
while the remainder forms a white coating on the zinc[152].
Experience shows that a 4-5% solution is the most suitable for
this method of reduction, which gives the most favourable results
when these details are followed. If for example the zinc plate is
laid immediately on the bottom of the glass trough, if the coins
are laid too close together on the plate, or if there are more
than 6 to 8 double layers in a trough, the process of reduction
is often incomplete, and it is then necessary to treat the coins
a second time. It is scarcely necessary to mention that larger
coins must be placed at proportionately greater distances from
each other.

The 40-50,000 coins which were thus treated had originally been
tinned, but the tin only remained at a few places. When the coins
were washed immediately after the reduction, this tin could still
be clearly distinguished, but on further washing, drying, and
brushing, it ceased to be visible on account of the dark colour
imparted to it by the finely powdered copper. In one or two cases
lead appeared on the surface of the coin, but was easily removed
by mechanical means."

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.

This method of preservation is of course applicable not only to decomposed bronzes but to all valuable antiquities, whatever the material may be.

Appendix.

Methods of Bringing out Worn Lettering upon Coins.

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."

(_j_) Silver.

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.

(_k_) Lead and Tin.

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].

(_l_) Gold.

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).

(_m_) Glass and Enamel.

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].

II. Preservation of Organic Substances.

(_n_) Bones, Horns, Ivory.

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.

(_o_) Leather.

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].

(_p_) Textile Fabrics, Hair.

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.

(_q_) Feathers.

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.

(_r_) Papyrus.

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.

(_s_) Wood.

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 3-1/2 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.

(_t_) Amber.

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."

The Care of Antiquities after Preservative Treatment.

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.

Conclusion.

The methods of preservation which have been described in the preceding pages may be thus tabulated and summarised:

Methods. Application.

Steeping in water, drying Limestone,
and impregnation. 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 Valuable bronzes in an advanced
hermetically sealed. 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.