Chapter IV: Part II: The Preservation of Antiquities (1)

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. Thus the 125 blocks from the Grave-chamber of Meten were each separately photographed. It is only in certain cases that this rule is not observed, as for instance in the case of the numerous Egyptian ostraca, i.e. fragments of earthenware showing inscriptions which had been previously copied.

I. Preservation of Objects composed of Inorganic Substances.

(_a_) Limestone.

The method formerly employed for the preservation of decaying and crumbling limestones was that of simple impregnation, and this is still followed in some cases which will be subsequently described. But as the active agents of destruction are not removed by this method the result is not always satisfactory, and an attempt is now made where possible to remove those salts which are soluble in water, especially the sodium chloride, by the simple process of steeping in water.

If the presence of salt in a limestone is evidenced by a crumbling surface, or by the taste when touched with the tip of the tongue, the question will arise whether it will bear steeping, or whether the destruction is so far advanced that, on being placed in water, the limestone will fall to pieces.

If fractures or cracks can be actually seen in the stone, steeping is contra-indicated, but if the condition is less manifest, a preliminary test should be applied.

A large drop of water, e.g. about 25 cubic centimetre in volume, should be placed on the surface of the stone, and any changes which take place should be carefully noted. If the drop is not absorbed by the stone, it may be due to a layer of dust or to previous saturation with solutions of resin or varnish. Dust may be removed with a moderately hard brush or by rubbing with the finger, but if a limestone has been previously saturated with a varnish solution it will not absorb the water, and is therefore hardly suitable for this treatment. If the drop is absorbed, an iron or a steel point, such as the thick end of a medium-sized needle, should be used to ascertain whether the limestone at the moistened spot shows the same degree of hardness as elsewhere. If this is found to be the case, especially if the pieces are of a large size, the test should be repeated at other spots, including the back of the stone, for a hardened layer on the front aspect may be the result of former treatment. If the result of this examination is satisfactory and no soluble colouring is observed on the limestone, the process of steeping may be applied. If on the other hand the moistened area has become softer, or has become to any extent swollen, or if any colours which may be present show signs of disappearance or fading, treatment with water must be abandoned.

The difference of behaviour is easily explained, for limestones do not always consist of lime only, or, more correctly, carbonate of lime (CaCO_{3}), but often contain sand or clay, and the greater the amount of clay the more readily the stone softens or swells. Even when a limestone has borne this preliminary test satisfactorily it should be carefully watched for an hour or two after immersion and should be at once removed from the water should any further changes appear.

Steeping. The procedure to be observed is as follows. The rapidity with which the salts may be removed varies directly with the quantity of water used in steeping. The treatment of objects of small size presents no difficulties; any vessel of glass, porcelain, or earthenware will serve the purpose. Towards the end of the treatment distilled water should be used, or in default of this, clean rain water should be used in preference to that from a well. For larger objects (as, for example, the large limestone blocks of the Meten Chamber mentioned above, some of which were 1 metre in length and 1/2 metre or more in breadth and thickness), it is convenient to use wooden tubs fitted with a tap in front to draw off the water, and so tilted by means of stones placed underneath that the tap may be at the lowest point. The objects should not touch the bottom of the vessel. Smaller pieces may be suspended or may be made to rest on glass rings or supports of glass rods; while large objects should be laid on blocks of wood so placed as to allow the tub to be cleaned when necessary without removal of the blocks, the weight of which would otherwise entail much labour. The blocks should be as near to the surface of the water as possible, leaving a considerable depth of water beneath, for the heavier salt-laden water sinks to the bottom, thus bringing into contact with the limestone water with a smaller salt-content. The length of the steeping must depend upon the size and porosity of the limestone.

Under certain circumstances phenomena make their appearance which must not be neglected. Thus if the treatment extends over a considerable length of time, the wooden tubs should be provided with lids to prevent the access of light. This was found indispensable in the treatment of the blocks from the Meten Chamber when Berlin tap-water[83] was used, for when the tubs were open a large quantity of brown hydrated ferric oxide appeared on the limestone, the roughness of which rendered its removal an impossibility even with brushes. This oxide is produced by various forms of algae and bacteria which developed in such numbers that the sides of the tubs were frequently covered with a layer of slime, which under the microscope appeared as a confused web of transparent threads[84]. This was brushed off with soft brushes at least once a fortnight, for the slimy covering impeded the access of the water to the limestone. That in this case the ferric oxide was the result of the action of light was proved by the fact that only those blocks which were placed near the windows were discoloured, and that the discolouration was proportionate to the amount of light which fell upon them[85]. Again, after the treatment in the covered tubs some of the blocks became so black that they resembled blocks of coal rather than limestone. After exposure to light for a day or two, especially when the water had been drawn off, the discolouration disappeared without leaving any traces. The colour was doubtless due to a minute quantity of iron in the form of sulphide which, after oxidation in the air and light, became invisible upon the light yellow limestone. Under these circumstances the presence of sulphuretted hydrogen in the water, possibly produced by bacterial action upon the sulphates, was attested by the characteristic smell.

The enormous number of bacteria which develop in the water constitute a great hindrance to the process of steeping, and as to boil such a quantity of water as is required for these large objects is out of the question, frequent changes of water and frequent cleaning of the stone, wooden blocks, and tubs are the only remedies.

Examination of the Progress of the Steeping. The water should be changed at first daily, then by degrees every two, three, or four days, later on weekly only, until finally once a fortnight is sufficient. To ascertain the progress and completion of the elimination the quantity of chlorine in the wash-water may be determined by a simple method of titration[86].

The following short explanation may be of use, and the method is easily learnt. If a solution of silver nitrate is poured into a solution of common salt (sodium chloride), a white curdy precipitate is produced, a process which the following equation will explain:

NaCl + AgNO_{3} = AgCl + NaNO_{3}.

The white precipitate is the silver chloride, whilst the sodium nitrate which is produced at the same time remains in solution and is therefore not visible. As always definite proportions of the two substances, silver nitrate and sodium chloride, react upon one another, by the use of a solution containing a known amount of silver nitrate we can determine the amount of salt, and hence of chlorine present. By cautiously inclining a burette (Fig. 13) divided into tenths of cubic centimetres[87], the silver solution should be dropped into a beaker containing a definite volume of the solution to be examined for chlorine; the level of the silver solution in the burette should be read off before and after pouring out, and the number of cubic centimetres of the silver solution required to precipitate the chlorine will thus be known.

The process when carried out in this manner has one defect, for it is necessary to allow the precipitate to settle in order to see clearly whether an additional drop of the silver solution will produce further precipitation, or whether it will merely cloud the fluid; this defect can, however, be remedied by means of a so-called "indicator." A few drops of a concentrated solution of neutral yellow potassium chromate should be added to the fluid to be examined, which is thereby coloured yellow; the solution is then shaken or stirred with a glass rod, while the silver nitrate is dropped in. Every drop of the silver solution will cause a red precipitate, the colour of which however disappears on stirring so long as there is any chlorine present; only when the silver solution has precipitated all the chlorine does the red colour become permanent, and thus the change of colour of the whole fluid from yellow to red shows with exactness the complete precipitation of the chlorine. For practical purposes all that is required is the so-called decinormal silver solution, and from the number of cubic centimetres of this solution which are required to precipitate all the chlorine the total amount of chlorine present can be readily calculated.

In steeping smaller objects before examination the whole of the water should be well stirred with a glass rod or poured two or three times from one vessel into another: 100, 50 or 25 cubic centimetres are then poured into a graduated glass or drawn up into a pipette. The water should be drawn up by suction slightly above the level of the mark upon the stem of the pipette, the upper end of which is immediately closed with the thumb. By slightly raising the thumb the water is allowed to run off until its upper surface is exactly level with the mark. The amount taken is then placed into a beaker (for 100 cubic centimetres a beaker of 400 c.c. capacity should be used), and, after the addition of a few drops of a solution of potassium chromate, is examined by titration. In the treatment of large objects, for which tubs are required, the necessary quantity of water may be drawn by means of a long pipette from the bottom of the tub, where the quantity of salt is always greatest, or through the tap at the bottom of the tub (as was done with the blocks from the Meten Chamber, in which case about 1 litre was drawn off into a glass out of which 100 c.c. were taken for titration). To obtain results which are comparable, care must be taken that the object is always as nearly as possible in the same quantity of water. After placing the larger blocks in the water, one examination should be made during the first few days, when the titration may require 20 c.c. or more of silver solution. There is no need to examine for chlorine while the water is being frequently changed: indeed, in order to economise the silver solution, this need not be begun until the second month, when the water is changed every fortnight.

As has been stated above, it is only necessary to read off the number of cubic centimetres of the solution used in the titration, for the decrease in these figures is a sufficient indication of the progress of the operation, while the diminution of the chlorine-content may be taken as an indication of the simultaneous removal of the sulphates[88]. In the treatment of small objects in distilled water, the process may be regarded as complete if the red colour is obtained on the addition of from one to two drops (i.e. about 1/10 to 1/5 cubic centimetre) of the silver solution. If, when tap-water is used, and is being changed at intervals of a fortnight or a month, the estimations give a constant result between 0·6 and 1·0, the treatment need not be carried further.

The accompanying table shows the figures obtained from three large blocks from the Meten Chamber. They represent the number of cubic centimetres of decinormal silver solution used for 100 c.c. of the water, which was changed every fortnight. The first column on the left shows the dates upon which the stones were placed in the tubs:

+-------+-------+-------+-------+-------+-------+-------+-------+-------+
|3 Feb. | 7 Apr.| 4 May | 2 June|25 Aug.|22 Sep.| 4 Nov.| 1 Dec.|12 Jan.|
| 1890 | | | | | | | | 1891 |
+-------+-------+-------+-------+-------+-------+-------+-------+-------+
| | 3·0 | 2·5 | 2·3 | 1·4 | 1·0 | 0·9 | 0·8 | 0·8 |
+-------+-------+-------+-------+-------+-------+-------+-------+-------+
|25 Apr.| 9 May |18 July| 1 Aug.|25 Nov.|23 Dec.|16 Feb.|29 Aug.|12 Sep.|
| 1893 | | | | | | 1894 | | |
+-------+-------+-------+-------+-------+-------+-------+-------+-------+
| | 6·0 | 5·6 | 4·9 | 2·0 | 1·7 | 1·5 | 0·8 | 0·8 |
+-------+-------+-------+-------+-------+-------+-------+-------+-------+
|29 July|28 Oct.|25 Nov.|23 Dec.|20 Jan.|16 Feb.| 1 Aug.|29 Aug.|12 Sep.|
| 1893 | | | | 1894 | | | | |
+-------+-------+-------+-------+-------+-------+-------+-------+-------+
| | 3·0 | 2·0 | 1·5 | 1·3 | 1·1 | 0·7 | 0·8 | 0·7 |
+-------+-------+-------+-------+-------+-------+-------+-------+-------+

When repeated examinations gave a fairly constant result of 0·7-0·8 cubic centimetre the process was regarded as complete, for Berlin tap-water itself contains small quantities of chlorine compounds, 100 c.c. requiring from 0·4 to 0·6 c.c. decinormal silver solution. Before using the water from a well or from waterworks, it should be examined to ascertain the number of cubic centimetres of silver solution required to produce the red colouration. As the amount of chlorine compounds in the water may vary it is advisable to repeat the examination[89].

The following table shows the rapidity with which salts can be completely extracted from small pieces of limestone. The limestones were placed in tap-water in three glass cylinders, each containing 2 litres; the amount of silver solution required for the water was 0·45 c.c. per 100 c.c.

+-----------------------+--------+---+---+---+---+---+------------------+
|No. of days of soaking | | | | | | | |
| in water | 1 | 1 | 1 | 2 | 2 | 4 | |
+-----------------------+--------+---+---+---+---+---+------------------+
|1|Weight in grammes 107|3·4 c.c.|1·4|1·1|0·7|0·5|0·5|cubic centimetres |
| | | | | | | | | of decinormal |
| | | | | | | | | silver solution.|
| | | | | | | | | |
|2|Weight in grammes 66|3·0 c.c.|1·2|0·7|0·5| --|0·5|cubic centimetres |
| | | | | | | | | of decinormal |
| | | | | | | | | silver solution.|
| | | | | | | | | |
|3|Weight in grammes 47|3·6 c.c.|0·8|0·6|0·5| --|0·5|cubic centimetres |
| | | | | | | | | of decinormal |
| | | | | | | | | silver solution.|
+-+---------------------+--------+---+---+---+---+---+------------------+

These figures show the numbers of the c.c. of silver solution used for every 100 c.c. of the water, which was changed after 1, 2, etc. days as shown above. After 9 to 11 days therefore the stones could be declared free from salt.

If the accuracy of the titration method be considered unnecessary, either on account of the small number or size of the objects to be treated, or for reasons of expense (the outlay required is however very small), a solution of unknown strength may be used. A comparison between the degree of turbidity produced on mixing the silver nitrate solution with the tap-water, and that produced with the wash-water, will enable the progress of the operation to be gauged.

Advantages and Disadvantages. Although steeping removes the cause of decay, i.e. the salts contained in the limestone, and although permanence may be considered as certain, there are certainly some disadvantages connected with the process, especially when the pieces, on account of their size, must remain in the water for some length of time. Some large and very thick blocks from the Meten Chamber required to be soaked for more than a year.

The small quantity of carbonic acid which is always found in water dissolves small quantities of calcium carbonate, thus the sharp contours of prominent parts may become somewhat rounded. Limestones which have developed fissures may, on immersion, lose small portions which might otherwise have remained attached, though probably for a while only. In such cases it must be carefully noted from which block, and from which part of it, the fragment has broken off, in order that it may be replaced[90].

Limestones which are much cracked, or which are likely to fall to pieces, should be wrapped round with gauze, or held together with twine, before they are put in the water.

In addition to the permanent preservation of the object some other smaller advantages of this method may be mentioned: for example, the layer of dust which is often present is removed and thus traces of colours may be brought out by the steeping which had been concealed by it. Thus certain remains of colour mentioned by Lepsius[91] as being still visible in his time upon some of the blocks from the Meten Chamber were no longer visible when we took them in hand. Moreover traces of green colouring which were visible after the treatment in the eyes of a few large figures in relief were probably evidence that colours had formerly been present.

Drying. When the steeping is finished the limestone is taken out to be dried. Small objects may be placed upon a glass ring, wooden tripod or some such appliance, which admits air on all sides, and may thus be dried by the air only. A piece of paper laid loosely over them will protect them from dust. In winter a hot stove, or similar source of heat, affords a satisfactory method of drying, but wet stones must not of course be placed directly upon the hot iron stove plate lest spots of rust should be produced upon the stone. Large blocks are preferably dried in drying chambers in which in summer time a strong draught is obtained by opening windows on opposite sides, and which in winter are strongly heated and opened every now and then for a short time. The limestones should be laid upon wooden blocks to allow air to pass beneath them, while they must be guarded from dust both above and at the sides with sheets of paper. Several months are often required to dry large blocks completely.

Impregnation. When limestones have been completely dried, especially if they are soft, it is often advisable to impregnate them with one or other of the impregnation agents. To economize material, large objects may be painted over once or twice with a solution of the material chosen, but smaller objects should be immersed in the solution until air-bubbles are no longer formed. If there is a supply of tap-water with sufficiently good pressure, rapid and complete penetration by the fluid can be ensured by placing the object in a vessel containing the necessary fluid under a bell glass, the air from which is then exhausted by a water air-pump[92]. Figure 14 illustrates the application of such an air-pump fixed to the water-tap by means of an india-rubber tube which is firmly bound with wire. An india-rubber stopper perforated to admit a glass tube is fixed in the top of the bell glass, while the smooth ground edge and the thick ground glass plate upon which it rests are smeared with grease or vaseline. The side tube of the air-pump is connected with the interior of the bell glass by an india-rubber tube which is sufficiently strong to resist the pressure of the outer air, and thus when the tap is opened the pressure of the flow of water carries with it the air from the bell glass with which the pump is connected. If the water-tap is suddenly turned off when the air is exhausted the pressure of the outer air will force the water into the bell and cause it to mix with the solution of resin or varnish. To prevent this, a stop-cock or valve should be inserted, or the water-tap should not be turned off until the stopper of the bell-glass has been cautiously raised. A second glass tube provided with a stop-cock may be passed through the india-rubber cork and connected with a manometer to measure the progressive action of the pump (Figure 15). When air-bubbles cease to come from the object under treatment, the glass tap should be closed and the manometer removed, after which the glass tap should be again opened and the water-tap closed[93].

If the object is of some length but not too thick, the bell-glass may be fixed on a strong glass cylinder of a similar diameter having a ground edge (Fig. 15), into which the object and the impregnating solution are then placed.

The following solutions, amongst others, may be recommended as suitable for impregnation:

(1) Shellac dissolved in alcohol.

(2) Solution of gum-dammar[94].

15 grammes of dammar are dissolved in 130 grammes of benzine,
to which is added a solution of 20 grammes of clarified poppy
seed oil in turpentine. If the solution becomes too thick
it should be diluted with benzine and a small quantity of
turpentine.

(3) Rice water or tapioca water[95].

(4) Dilute size.

(5) Waterglass solution.

(6) Linseed oil dissolved in benzine.

(7) Linseed varnish dissolved in 3 parts of benzine or petroleum
ether.

(8) Solutions of stearine or paraffin wax in benzine.

(9) Collodion (free from acid). Zapon[96].

(10) Kessler's fluate.

It may be added that, as a general rule, solutions for this purpose must be used as dilute as possible, for two immersions in a dilute solution are preferable to a single soaking in a concentrated one, which often scarcely penetrates into the pores.

As the preparation of solutions of shellac, gum-dammar, and of such substances as resin, stearine, and paraffin, necessitates heating, and as the solvents are very inflammable, it is advisable to make use of the solution of linseed varnish in benzine. This solution may be obtained at any time at any degree of concentration without the use of heat. Although it has the advantage that it hardens more rapidly than a simple solution of linseed oil it has also one disadvantage, for it gives a somewhat darker colour to light-coloured limestones. No more of the mixture of varnish and benzine should be prepared than is required for the impregnation, for this solution, on standing, throws down a gelatinous precipitate which is not re-dissolved even by heating. As this alteration is accelerated by the action of light, the mixture should always be kept in a dark place.

Collodion and zapon[96], on account of the expense, should only be used for small objects. After impregnation the objects should be covered with glass jars, cardboard boxes, etc., to prevent the precipitation of moisture upon them, as the result of the rapid evaporation of such volatile substances as benzine and ether upon exposure to the open air.

Rice water, tapioca water, or size (the latter of no greater strength than 2%) are only applicable to specimens which are kept in dry rooms, for in damp rooms they readily become sticky, and are liable to be attacked by moulds. Waterglass solution, probably because it is generally applied in too concentrated a form, instead of penetrating the object has a tendency to form a pellicle, which readily strips off. Even dilute solutions, however, are said to be unsuitable, from the liability to the efflorescence of alkali salts.

In the case of marble objects and antique statues of porous limestone, showing colours which are still bright on excavation, but which would soon fade, Rhousopulos[97] recommends impregnation with a very dilute solution (1 in 1000) of waterglass to preserve the colour. The solution should be as neutral as possible: in any case not alkaline. This is several times sprayed upon the object, which is allowed to completely dry between each spraying.

A material which is suitable for large objects to which the solution can only be applied upon the surface is Kessler's fluate[98], which is soluble in water, and which hardens the limestone without completely closing the pores. It offers the additional advantage that it is applicable to thick limestone blocks, the dryness of which is not certain. The solutions numbered 1-9 must only be used when the limestone is dry throughout its mass. The fluate to be used in any particular instance must be decided from the nature of the case. Those most generally applicable are magnesium and zinc fluates and the so-called "double fluate."

The stones from the Meten Chamber were hardened in the following manner: The limestone blocks were placed upright and the surface dusted by the air-current from a Dechend's spray apparatus[99] which was then used to spray them repeatedly with a solution of "double fluate" of sp. gr. 1·16. Owing, however, to the injurious effect of the fine spray of the fluate upon the nose and lungs the stones were turned to a horizontal position, and a solution of fluate of sp. gr. 1·38 was applied by means of a large brush until the fluid was no longer absorbed. For the treatment of limestones on which there are remains of colours the use of a solution of shellac, gum-dammar, or collodion is recommended. Fluates should not be applied until their suitability for the particular purpose has been tested.

All specimens should be kept after impregnation in rooms which as far as possible are free from dust, for the dust which falls upon the surface will set in the varnish whilst it is hardening.

IMPREGNATION WITHOUT PREVIOUS STEEPING. 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. Aqueous solutions, e.g. size, cannot of course be applied, and as it is necessary to make a preliminary trial of a fluate spray, it is generally found preferable to make use of the varnish-benzine mixture. In spite of this, salts may still make their appearance in the form of a crystalline powdery layer on the surface, which can be wiped off with a wet sponge; any moisture must however be removed with a soft dry linen cloth.

REMOVAL OF INCRUSTATIONS AND DUST. Incrustations of earth, lime, or gypsum should be washed off with water or removed by mechanical means, such as gentle rubbing with the finger. The solvent action of acids upon limestone precludes their use for this purpose. Any dust which adheres can be removed by rubbing with stale bread-crumb.

(_b_) Marble and Alabaster.

It is usually only necessary to clean marble with a soft brush and warm water, with the addition perhaps of some good neutral soap. In rare cases the presence of sulphates may perhaps cause some friability. The crystalline structure of marble renders steeping futile, and accordingly impregnation is resorted to. The use of Kessler's fluates may be recommended. Adherent pitch or resin is best removed by a mixture of alcohol and ether. Alabaster seems to remain permanently sound and may be cleaned in the same way.

(_c_) Earthenware.

STEEPING. The same line of treatment should be followed as in the case of limestones. A preliminary examination should always be made to test the power of resistance in water, which is always satisfactory if the clay has been sufficiently baked.

In the case of coloured terra-cotta care should be taken to ascertain whether the colours are likely to suffer during steeping. There is no danger of injury if the steeping is not too prolonged; in fact, the removal of the dust during the procedure often brings out the colours more clearly. If the Egyptian ostraca (clay fragments with black script) require to be washed they should be carefully watched in order to preserve the script, and therefore should be placed in the bath in such a way that the lettering is visible.

These fragments are usually curved and bear the script upon the convex side, care should therefore be taken that they are completely immersed, and that no large air-bubbles prevent the access of the water to any part of the under-surface. The writing is done with either lamp-black or more rarely some form of iron ink, and is retained mechanically by the porous character of the ostraca. In the latter case the characters may be enhanced by the application of a dilute solution of tannic acid, which sometimes proves useful also for limestone pieces.

If these fragments are sufficiently few in number to allow each to be put into a separate glass vessel, the washing out of the salts is completed so quickly that there need be little danger of obscuring the script. When large numbers were to be washed and when the script was already indistinct I have employed the following method: After examination as to their fitness for immersion the fragments are placed on a wooden grating in a tub, in which they remain for a couple of days, during which the water is renewed once. They are then taken out and allowed to dry. All those which still show the script distinctly are separated and their steeping is completed, but the remainder, having been completely dried, perhaps on the top of a warm stove, are brushed over once or twice with a dilute (1:6) mixture of varnish and benzine in such a way that the surface is only moistened, and when dry shows no gloss. The pieces thus superficially varnished are kept in a dry place for about two months, until the varnish is hardened; the process of washing out the salt is then begun again. The thin coat of varnish fixes the script without interfering with the steeping. The varnish solution must be dilute, for a thick coating will partially peel off from the object in the course of the steeping, or will remain in the pores in the form of opaque particles, and thus render the script illegible.

The same difficulty which arose in the treatment of the Meten limestones was frequently met with in the treatment of these ostraca. Those which were of a dark brown colour especially, and to a less degree also the red and the yellow, were covered with a slimy growth of algae. As the script is easily destroyed no attempt should be made to remove these algae from the side which bears the script even with the softest brush, although they should from time to time during steeping be brushed from the underside. The inconvenience caused by algae is, however, less marked in the treatment of earthenware, the light and porous character of which renders prolonged steeping needless, nor is there the same necessity to continue the steeping for the purpose of chlorine estimation. The following results were obtained in the treatment of 13 fragments, the average thickness of which was 1 cm. [3/8th inch], with an average superficial area of 1/10 sq. metre [4 inches]. The tub in which they were steeped contained 85 litres [18-1/2 gallons] of water.

100 cubic centimetres of the tap-water used were found to require 0·5 c.c. of the silver solution, and on each occasion this quantity of the water was tested.

+------------+-----+-----+-----+-----+-----+-----+-----+-----+----------+
| Water | | | | | | | | | |
changed | | | | | | | | | |
| after | 1 | 1 | 1 | 1 | 2 | 2 | 2 | 4 | 5 days |
+------------+-----+-----+-----+-----+-----+-----+-----+-----+----------+
| 100 c.c. of| | | | | | | | | |
| the water | | | | | | | | | |
| used in | 3·0 | 1·3 | 1·0 | 0·8 | 0·9 | 0·7 | 0·7 | 0·6 | 0·6 c.c. |
| steeping | | | | | | | | |of silver |
| required | | | | | | | | | solution |
+------------+-----+-----+-----+-----+-----+-----+-----+-----+----------+

The water was changed at first daily, then every two days, and so on: the steeping could therefore be regarded as complete at the end of a fortnight.

A small figure of earthenware, which weighed only 28·9 grammes, was steeped in 1-1/2 litres of distilled water, and gave the following result for every 100 c.c. used:

Water changed after 2 days required 3·6 c.c. silver solution.

Water changed after 3 days required 0·4 c.c. silver solution.

Water changed after 4 days required 0·0 c.c. silver solution.

The steeping was, therefore, in reality complete after five days, and, as the steeping water was thoroughly mixed before the withdrawal of the 100 c.c., the total quantity of sodium chloride contained in the figure can be calculated as follows:

For the 15th part (viz. 100 c.c.) of the water 3·6 + 0·4, i.e. 4·0 c.c., of decinormal silver solution were used, which is equivalent to 15 × 4·0, i.e. 60 c.c., of silver solution for the whole quantity. Now 1 c.c. of this decinormal solution corresponds to 0·00584 gramme of sodium chloride; the water therefore contained 60 × 0·00584 gr., or 0·35 gr. sodium chloride. Thus the figure contained altogether 1-1/5% of sodium chloride.

In addition to the chlorine compounds, there was also a considerable quantity of sulphates, the presence and disappearance of which were tested by adding to a few cubic centimetres of the water a dilute solution of barium nitrate or of barium chloride[100]. The soluble barium salts give with sulphates a white precipitate or cloudiness of insoluble barium sulphate. If therefore on the addition of a solution of barium nitrate no cloudiness appears, even after some time, it may be concluded that sulphates are no longer present in the water. When the ostraca have been washed and dried, it is often possible to make the script more distinct by varnishing them over with a varnish-benzine mixture (1:6).

It is advisable to subject friable objects of earthenware to the process of impregnation (cp. the impregnation of unbaked clay, p. 81).

THE REMOVAL OF INCRUSTATIONS. Incrustations of earth or lime can be easily removed if the earthenware has been well baked, but trial must first be made with a drop of dilute hydrochloric acid, whether the earthenware itself is not attacked by the acid. The specimen is then placed upon a glass ring or suspended in water containing 2% of hydrochloric acid[101]. This mixture, which must be renewed every 24 hours, will remove incrustations which it would be difficult to remove by mechanical means, while crystals of gypsum of considerable size, which are often found on clay tablets of Assyrian origin, are easily dissolved in from two to four days.

Figures 16 to 21 represent two Assyrian tablets which have been cleaned by myself in this manner. It will be seen that the cuneiform characters, which before treatment were almost invisible, are now distinctly legible.

After this treatment with acidulated water the acid must itself be removed by careful washing in pure water. Here too a solution of silver nitrate will serve as a test, for, so long as any chlorine, and therefore any hydrochloric acid, is present in the water, a white precipitate or cloudiness is produced.

The method of titration with yellow potassium chromate is not applicable here, for the free acid prevents the appearance of the red precipitate. The steeping must therefore be continued in distilled water until the addition of silver nitrate no longer produces any cloudiness.

Baked earthenware which shows colouring, or which has incised lines filled with substances containing lime, must not be steeped in acidulated water, nor will ostraca bearing inscriptions in iron ink stand this treatment; these are, however, fortunately rare: in fact amongst several thousand fragments few have shown incrustations of lime or gypsum. Should any such be found a cautious attempt should be made to remove the incrustations by some mechanical means. Rhousopulos[102] carries out the cleaning of Lecythoi[103] and clay vases, which are painted in water-colours and which have a thin white incrustation, by dipping them into a 5% solution of pure hydrochloric acid. As soon as the colours show the least sign of running, or if an efflorescence makes its appearance, the vase is immediately removed and allowed to dry. It is then dipped into distilled water and allowed to dry a second time. Impregnation is not necessary.

"If the treatment is otherwise successful, but an earthy layer
remains upon the colour, the spots which are thus affected are
lightly touched with the finger whilst the object is still in the
liquid. Rubbing, or any sort of mechanical attack, is absolutely
out of the question."

This process evidently requires the greatest care and constant attention.

(_d_) Slightly Baked Or Unbaked Clay.

IMPREGNATION. If upon examination it is found that a drop of water softens the clay, the same line of treatment must be followed as in the case of limestones which exhibit a similar condition (see p. 73), i.e. they must be subjected to the process of impregnation[104]. As the colour of the clay objects is yellow-brown or red-brown, the varnish benzine mixture will be the most suitable application for the purpose. A considerable number of sun-dried Assyrian clay tablets treated in this manner have given good results, and have undergone no change during the last five years, in fact they may now even be laid in water without crumbling.

In the case of slightly baked or unbaked Babylonian clay tablets the method formerly employed was merely to remove deposits of lime, clay, gypsum, etc., by lifting or scraping them away with pointed or wedge-shaped tools, for the soft clay would not stand treatment with water, still less with 2% hydrochloric acid. The difficulty in avoiding damage to the clay surface, when removing the deposit, makes this method both tedious and risky. Warming to 200-300°C. in a drying oven, or in an iron box embedded in sand, seldom aids the removal of incrustations; moreover, this treatment has no hardening effect upon the clay, and thus does not facilitate the removal of the injurious salts by soaking. A further expedient therefore remains, that of heating the clay to higher temperature, whereby it is fully baked and rendered capable of resisting subsequent treatment with water or 2% hydrochloric acid. At the Royal Museum this firing is done in muffle furnaces[105], the smaller of which has a capacity of about one cubic foot, and is heated by six and twelve Bunsen burners. The temperature is regulated in the same way as in porcelain manufacture by the use of Seger's cones[105], which are placed in the muffle, where they can be seen through the observation aperture. To avoid cracking the heating must be gradual, the gas-supply being very gradually increased. The firing must at first be adjusted to cone 022 [590°C.; Watkin, No. 1, 1094°F.]; the gas is then turned off and the furnace allowed to cool as slowly as possible. To effect this the damper is closed and all openings into the muffle are made up with fire clay. The clay tablet is removed when quite cold (usually in 18-24 hours), and, as a rule, much of the incrustation can then be removed by means of a soft brush. Should the removal prove difficult, and a preliminary trial have shown that it will bear the treatment, the removal of the deposits will be assisted by soaking for two or three days in water. Should the tablet prove capable of bearing treatment with 2% hydrochloric acid it may remain in the acid for 12 to 18 hours. If necessary the acid may be renewed once; it must then be thoroughly removed by steeping in ordinary water and finally in distilled water, until the wash-water is free from chlorides. After steeping, the tablets will be found somewhat softened and occasionally coated with a slimy growth of algae, care must therefore be used in changing or taking them from the water. The best way to handle them is to place the fingers of the two hands under the tablet.

After thoroughly drying the tablets first in the air, then in the drying oven at a temperature of 212°F., supported on glass rings, it is well to impregnate them. This can be best carried out by placing them, while still warm, into melted paraffin wax, and raising the temperature to about 250°F. [120°C.]. The wax is allowed to cool to about 160°F. [70°C.], when the tablet is removed upon a broad band of gauze, any excess of wax is drained off, and the object is wiped with a soft cloth. The benzine-varnish mixture or zapon may also be used for impregnation. If heating in the muffle to Seger cone 022 is insufficient to allow of the removal of the incrustations, or if the condition of the clay does not warrant soaking in water or acid, the object must be again placed in the muffle and fired to Seger cone 010 [950°C.; Watkin, No. 13, 1742°F.], and, if softening occurs upon the application of water or acid after exposure to this temperature, recourse must be had to a third heating to Seger cone 05 [1050°C.; Watkin, No. 18, 1922°F.]. Higher temperatures than this are not advisable, for the lime, sodium chloride, and other salts found in some Babylonian tablets may partially fuse. During firing therefore the appearance of the object must be carefully watched, and the temperature lowered at once by reducing the gas-supply, if signs of fusion are noticed.

ADDITIONAL METHODS OF IMPREGNATION. If clay objects have a smooth surface, it is, according to the "Merkbuch[106]," advisable to impregnate them with Belmontyl oil[107], for varnish in the course of drying gives a lacquered appearance to the surface. According to the same authority the surface of glazed vessels can be restored by impregnating them several times with a mixture of poppy seed oil and benzine [20 grammes clarified poppy seed oil in 270 gr. benzine, i.e. 1 in 13-1/2], and by subsequently brushing them first with soft, then with harder brushes. There are, however, many other substances used in different collections for impregnation, a few of which are subjoined.

In the Museum at Vienna friable clay objects are laid for two or three minutes in a dilute solution of warm size, and when dry are brushed over with a solution of shellac; size alone, or a solution of shellac alone, is frequently used for impregnation, or to give a coating. In the Museum at Wiesbaden thin specimens are impregnated with a solution of white of egg, brittle objects with dilute fish glue, while for hard objects a solution of shellac or melted shellac is used.

(_e_) Fayence.

I have been able to wash out the sulphates from several Egyptian fayence figures in spite of the glaze, the fissures in which allowed the water to penetrate into the interior. The process of steeping, which was necessarily somewhat prolonged, was tested from time to time by the barium nitrate test (vide p. 77).

(_f_) Objects of Stucco and Nile-mud.

These are rare, and in almost all cases contain salts. As, however, they will not bear steeping, they must be preserved by means of impregnation only. The varnish-benzine mixture should be used for this purpose.

(_g_) Sandstone and Granite.

These scarcely need any special preservative process, but Kessler's fluates are useful for the impregnation of weathered sandstones which are exposed to the open air (see p. 72).

They can be cleaned by washing with warm water, while calcareous incrustations may be removed by hydrochloric acid. A thick coating of oil paint was successfully removed from an Egyptian statue of sandstone by placing it in an alcoholic solution of soda. Oil colours and similar substances may often be removed with ease and completeness from stone, plaster, wood, etc., by placing the objects in air-tight vessels together with a vessel containing alcohol. The alcohol vaporises, even at the ordinary room-temperature, and causes a softening of the paint. The time required for the treatment depends upon its age and hardness.

Appendix.

Cement for Earthenware. Restorations.

To fix together pieces of broken pottery good Cologne glue is useful, but it has the disadvantage that it can only be used when warm. For this reason it is better to use liquid fish-glue [Syndeticon], which may, if necessary, be thinned with a little vinegar. Fire-clay dust in waterglass is used in the Museum at Breslau. A thick ropy solution of shellac[108] may also be mentioned, for the use of which the opposing surfaces must be first moistened with alcohol.

Gum arabic and dextrin should not be used, for objects thus cemented readily fall to pieces unless kept in perfectly dry rooms. This, however, may also be said of earthenware which contains salts, if cemented with glue or fish-glue. Previous steeping would obviate this difficulty.

Chalk, plaster of Paris, brick-dust, or fire-clay dust are often added to the fish-glue, dextrin, etc. Without giving additional strength to the cement, these substances may be of use in filling up small gaps between the fragments to be cemented.

For filling up larger gaps the "Merkbuch[109]" recommends stone cement, for the preparation of which it gives the following prescription:

"Mix 500 grammes of Cologne glue with three sheets of strong white
blotting-paper, or four sheets of white tissue paper, shredded as
small as possible, and boil until it becomes thick, stirring the
whole into a perfectly smooth pulp. Let it boil thoroughly, and
while stirring continually, and working with a stout wooden rod,
add 2-1/2 kilogrammes of very finely sifted dry purified whiting.
After working this mixture thoroughly, add 80 grammes of linseed
oil, which must be also thoroughly worked in. To preserve the glue
add 50 grammes of Venetian turpentine. This stone cement will take
any shade of colour if mixed with lamp-black or coloured earths."

(_h_) Iron.

The various methods for the preservation of iron objects which have been or are still in use may be divided into two groups. To the one group belong those methods in which the objects are preserved with their coating of rust, or with the rust that has penetrated them; to the other group belong those in which the removal of rust precedes preservation. The former methods must be applied when the iron has been completely converted into rust or when the rust has only left a small metallic core. These methods may of course be used also for all iron antiquities.

The methods of the second group can be applied to those objects only which still retain a strong metallic core, in which case the objects regain the more or less grey or white surface of fresh unoxidized iron. These methods are at present little known, and therefore but little used, for owners and the general public are still accustomed to see in the covering of rust the evidence of antiquity with which they are loth to part.

In addition to these methods, there are others which are of an intermediate kind, either special or a combination of methods from both these groups.

(1) Methods of preserving Objects of Iron without removal of the Rust.

IMPREGNATION. The earliest processes, which are to some extent still in use in some collections, are simple impregnation methods, in which the object is either painted once or more with the impregnating medium by means of a brush, or is placed directly in the medium itself. In either case the penetrating power of the solution used is directly proportional to its fluidity.

The following media may be used for the purpose:

(1) Warm size.

(2) Warm isinglass solution.

(3) Solution of waterglass.

(4) Solution of shellac in alcohol.

(5) Rubber solution in carbon bisulphide. The mass after swelling
is dissolved in benzine[110].

(6) Copal varnish diluted with turpentine.

(7) Copal varnish mixed with linseed oil[111].

(8) Linseed oil.

(9) Linseed varnish.

(10) Linseed varnish mixed with an equal quantity of petroleum.

(11) Bees'-wax dissolved in turpentine.

(12) Bees'-wax dissolved in benzine.

(13) Petroleum.

(14) Vaseline.

(15) Melted paraffin.

(16) Oleate of lead: 100 grammes of olive oil, 100 gr. of lead
oxide, and 100 gr. of water are boiled until all the water has
evaporated and the mass has become grey. The mass is extracted
by shaking it with alcohol, and the residue is dissolved in
absolute ether, in the proportion of 100 gr. of ether to 5 gr.
of the substance. Before use it should be diluted with a little
ether[112].

(17) Speerschneider's mixture. This consists of 8 parts of rape
oil, 1 part of bees'-wax, 1 part of pine resin, and 2 parts of
benzene[113].

(18) Collodion, or the mixture used in the Museum at
Donaueschingen, which consists of 30 grammes of collodion, 2 gr.
of camphor, and 1 gr. of oxalic ether.

In addition to these materials, there are other mixtures of resin, varnishes, and bees'-wax, with their appropriate solvents, but they do not possess any special advantages as impregnating solutions.

After treatment with size or isinglass, iron objects may be given when dry a coating of linseed oil, linseed varnish, solution of shellac, etc.

The materials numbered 7 to 10 in the above list should be applied warm to enable the viscid fluids to penetrate the rust, for the more readily the solution enters the object the better is the result obtained. Apart from the fact that they are easily ignited at a high temperature, they must not be heated beyond 230°F. [110°C.], otherwise objects which consist largely of rust will fall to pieces[114].

In the process of impregnation a twofold result is aimed at, viz. to prevent the rust from crumbling, and to exclude air from the specimen. The application of heated linseed oil or linseed varnish is founded upon the supposition that these substances enter into a chemical combination with ferric oxide to form a stable compound; this is, however, disputed by some modern authorities[115]. Neutral substances offer a safer method for the exclusion of air, and of these melted paraffin is undoubtedly the best. The paraffin must be quite pure and free from stearine, as can be ascertained from the melting point; thus pure paraffin melts at 130°-150°F. [55°-65°C.], stearine at 160°F. [70°C.]. Paraffin with a melting-point higher than 65°C. should be looked upon with suspicion.

In many collections the objects are heated before impregnation with media which are insoluble in water, or they are exposed to the air for six to twelve months after excavation. This latter proceeding is, however, certainly inadvisable if the iron contains chlorine, and if this is the case not one of these methods produces satisfactory results.

On the other hand, as almost all the iron antiquities which do not contain chlorine compounds may be treated by the methods of the second group, simple and direct impregnation is passing more and more out of use. Before impregnation all soluble substances, especially chlorine compounds, must be removed by steeping.

(2) Preservation by Steeping and Subsequent Impregnation.

KRAUSE'S METHOD. The water used for steeping should be preferably lukewarm, and should be changed every twenty-four hours. It is even better, at least for the first time, to lay the object in water and then raise it to boiling-point, a measure which will allow the more ready penetration of the water. As in the case of limestones, earthenware, etc. (p. 59), care must be taken to place the objects as near to the surface of the water as is possible. Small objects may be put in glass jars, large ones in wooden troughs, tin vessels, or wooden boxes lined with zinc or lead. Any little excrescences on the iron, which are frequently filled with ferrous chloride, should be punctured to give the water unimpeded and more speedy access. Crumbling objects should be held together by tightly wrapping them in muslin. Curators must decide for themselves how far means such as files, chisels, or small hammers may be used to remove the rust or earthy material conglomerated by rust.

Although much recommended, the method of adding soda or lime water to remove the chlorine as soluble sodium chloride or calcium chloride is, in our opinion, inadvisable. Both these substances precipitate the iron from the ferrous chloride or ferric chloride (which are soluble in water) as insoluble hydroxide of iron, which more or less closes the interstices, and thus impedes the access of water to the interior.