THE PRESERVATION OF ANTIQUITIES.
The object with which it is proposed to deal should first be photographed,
and from different sides if necessary; for the external appearance is
often changed during the process of preservation, and it is advisable
that a representation of the specimen in its original condition should
be kept in case any injury should befall the object, which however rarely
happens if proper caution be observed. For this reason in the Laboratory
of the Royal Museums at Berlin all bronzes are photographed before
treatment, as also are all limestone blocks. 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.
[Illustration: Fig. 13.
Gay-Lussac's Burette. 1/6 nat. size.]
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].
[Illustration: Fig. 14.
Air-pump fixed to water-tap.]
[Illustration: Fig. 15.
Apparatus for impregnation by extraction of air fitted to manometer.]
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.
[Illustration: Fig. 16.
Fig. 17.
Assyrian clay tablet with incrustations.
Before and after treatment.]
[Illustration: Fig. 18.
Fig. 19.
Fig. 20.
Fig. 21.
Assyrian clay tablet before and after treatment.]
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.
[Illustration: Fig. 22.
Babylonian clay cone before treatment.]
[Illustration: Fig. 23.
Babylonian clay cone after treatment--firing,
treatment with hydrochloric acid and steeping.]
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.
The process of steeping can here again be controlled by the use of the
silver solution (p. 62), for if there no longer appears any or only very
little cloudiness the steeping may be considered complete. The length of
time required for steeping depends upon the thickness of the rust and
the porosity or existence of cracks in it, and if the objects are of
considerable size, it may extend over several weeks.
After steeping the object should either be dried in the open air,
and later on a warm stove, or be placed for a few days in alcohol to
remove the water, after which the rapid evaporation of the alcohol will
quickly dry it. The steeping of iron objects in warm alcohol has been
recommended[116], but if their size is considerable the method is an
expensive one. This method has the advantage that the alcohol penetrates
the rust sooner than does water, and also prevents oxidation, which may
be actually produced by the water. It may perhaps be advisable to dilute
the alcohol, the usual strength of which is 95% to 96%, with about an
equal volume of water, for some salts are not readily soluble in pure
alcohol. When dry the object is warmed for a few hours in a mixture of
equal parts of good linseed varnish and petroleum. The petroleum serves to
dilute the varnish, which can thus more quickly permeate the entire mass
of iron and rust. On account of the inflammable nature of the mixture the
warming should be done over a water-bath. For small objects a cylinder
made of ordinary tin-plate, measuring from 6 to 10 inches [15 to 25
cm.] in diameter and 6 in. [15 cm.] in height, may be used. To increase
stability the lower half should be of a smaller diameter, and fitted
into an iron tripod. The same end is attained by soldering a ring round
the middle of the cylinder, which will rest on the ring of the tripod.
The cover consists of a number of copper rings gradually diminishing in
diameter, which fit closely into one another, thus enabling porcelain
vessels of various sizes to be used. For larger objects, such as swords,
two long rectangular troughs (Fig. 24) of stronger plate should be used.
The following sizes will probably be found useful: one about 40 inches
[100 cm.] long by 4 inches [10 cm.] broad, and 4 inches [10 cm.] deep,
and the other slightly larger. Handles should be fixed at the upper edges.
Three iron bars 1 inch [2-1/2 cm.] thick and 4 inches [10 cm.] in length
are laid across the bottom of the larger trough, on which the smaller is
placed. The space between the two vessels is filled with water to a depth
of 2 inches [6 cm.]. The trough is warmed on a stove, or better, where
gas can be had, by means of a number of Bunsen burners fitted with rose or
ring burners, over which the trough may be supported upon tripods. While
heating care must be taken that the water does not boil over, which can
be easily avoided by regulating the gas supply. As the water evaporates
further quantities should be added as required. After simmering for about
two hours, the objects should be removed and allowed to drain; they should
then be placed on a tripod, or on glass rings, on the warm stove in cold
weather, to accelerate the evaporation of the petroleum and the setting
of the varnish. In summer drying chambers may be used; these are sold by
dealers in physical and chemical apparatus, or can be made at little cost
by a tinsmith.
[Illustration: Fig. 24.
Water-bath. 1/15 nat. size.]
If the objects have been steeped in pure alcohol, or at least towards the
end of the treatment in three changes of alcohol, so that all the water
is replaced by alcohol, they may be dipped directly without drying in the
varnish mixture, for the alcohol evaporates in the varnish bath which is
at a temperature of 194°-203°F. [90°-95°C.]. As the varnish hardens, the
iron thus treated acquires a glazed surface; other means of impregnation
may therefore appear preferable, e.g. a solution of gum-dammar or melted
paraffin. For impregnation with the dammar solution the object must first
be dried, and the air-pump used in the way described on p. 68. On account
of the inflammable nature of the benzine heat must not be applied, nor
indeed is it necessary.
When impregnating with pure paraffin[117], specimens may be lightly
wiped with a cloth, but need not be dried. The paraffin may be heated
to 212°-248°F. [100°-120°C.] without danger, so long as it is kept from
direct contact with the flame. A thermometer should be used, and, as soon
as the paraffin has melted at a temperature of about 60°C., the object
should be placed in it by means of tongs. When the temperature has risen
above 212°F. [100°C.] the water is converted into steam, and causes a
brisk ebullition of the melted paraffin. The quantity of paraffin used
should, therefore, be such that its level remains at the least 2 inches
[5 cm.] below the upper edge of the vessel.
When the bubbles have ceased to rise, thus showing that all the water
is expelled, the paraffin should be allowed to cool to a temperature of
180°-190°F. [80°-90°C.]. The iron should be taken out with tongs, and
the liquid allowed to run off. It should then be wrapped, while still at
80°C., in soft blotting-paper or in a piece of old linen to absorb the
superfluous paraffin. If the surface of the object is very uneven, or if
there are deep cracks or holes in which the paraffin can collect, it will,
when cold, form a white mass, and should therefore, while still warm and
fluid, be soaked up with filter paper, or distributed evenly by means of
suitable brushes. The superfluous paraffin may also be absorbed by putting
the object in dry sawdust; any sawdust which remains attached can be
removed when cold with benzine, or it may be scraped from the spots where
it has collected with a knife or spatula. Any spots where the iron may
have become exposed may be covered with a thin coat of paraffin dissolved
in benzine.
EKHOFF'S METHOD[118]. The objects are laid for two or three months in
water which is changed every two or three days, a small quantity of
quicklime being added[119]. After this steeping, and after some of the
rust has been removed mechanically, the object is lightly dried and put
into heavy petroleum of sp. gr. 0·85 to 0·95, which is then heated up to
220°F. [105°C.]. A thermometer should be used to ensure this temperature.
This temperature being higher than the boiling-point of water, the water
contained in the object evaporates and causes the petroleum to bubble, as
in the method previously described. When all the water has been replaced
by the petroleum the bubbling ceases. After the fluid has somewhat cooled
down, the iron is taken out and is allowed to remain for about an hour in
sawdust, which absorbs the superfluous oil. Finally, while gently warming
the object over a warm, but not too hot, stove, it is coated over with a
mixture of 1 part of bees'-wax and 2 parts of turpentine, or better with
paraffin dissolved in benzine. Heavy petroleum, which we have found by
experience to be a suitable material, is preferable to varnish in so far
as the iron is impregnated by a neutral substance which is practically
liquid paraffin, but has the disadvantage of being highly inflammable and
of being difficult to obtain at so high a specific gravity.
STRABERGER'S METHOD. This method, for the description of which I am
indebted to Herr Straberger, has proved effective in the preservation of a
number of iron antiquities in the Museum at Linz on the Danube. Even iron
objects, which had been in bad condition and had undoubtedly contained
chlorine, have after treatment by this method shown no signs of change,
while the dull black surface has an agreeable appearance.
Straberger places the newly-excavated objects immediately into linseed oil
to prevent the access of air. After remaining in the oil for some time
they are taken out, wrapped in cloths saturated with linseed oil, and
removed packed in sawdust. Upon arrival they are unwrapped and put into
water, to which a small quantity of soda is added to remove the oil more
easily. The water is frequently changed, and the objects are meanwhile
cleaned mechanically with emery paper and hard brushes. Any blisters
are removed by the aid of a small hammer and chisel. After steeping they
are dried and smoked over a candle flame which is allowed to play over
the whole surface. The soot is then rubbed off with a cloth or soft
brush. Objects with a smooth surface may be rubbed with india-rubber.
The preservative action of this proceeding depends upon the fact that
during the smoking, in addition to the soot, oily products of combustion
are deposited from the candle flame, which prevent the access of air and
moisture to the iron.
"Objects which are much decayed or cracked should, when cleaned
and thoroughly dry, be again placed into linseed oil which has
been slightly warmed and should remain therein for a few days
before being smoked. Upon removal from this second oil bath they
should be lightly wiped and dried over a moderately warm stove
or in the sun. Patience is necessary, and nothing further should
be done until the oil has entirely dried in the fine cracks and
crevices and firmly binds the mass. The oil crust on the surface
is then loosened by soaking in a strong soda solution and wiped
off, after which the object is dried, smoked over the candle
flame, and the soot wiped or brushed off with a soft brush. The
smoking and wiping may be repeated if necessary."
Herr Straberger states that his treatment has been successful when
impregnation with isinglass and coating with shellac has failed.
The methods of Hartwich and Jacobi hold an intermediate place between
the above methods and those which will be subsequently explained. With
the former they have this in common that they do not call for the entire
removal of the rust and that they require the use of linseed oil; on
the other hand their application presupposes the existence of a strong
metallic core, otherwise when the rust is removed they will show merely
a skeleton of the original object. The existence of a sufficiently
substantial metallic core can be easily ascertained from the weight, for
an object, which consists solely or in great part of the oxide of a metal,
is much lighter than one of the same size which is largely metal. The ring
also affords a test, for an iron object, of which the greatest part is
metallic iron, gives a clearer note when struck than one which is chiefly
rust. A still more certain test is the use of a file or a drill (comp.
page 107).
HARTWICH'S METHOD[120]. This method is intended for objects of an
especially large size, the hard oxide coating of which does not allow
satisfactory steeping. Hartwich heats the object to redness, allows it to
cool slowly, and then scrapes off the outer layer which has been rendered
friable by this treatment. The subsequent procedure is that of Krause's
method, viz. warming in linseed varnish.
JACOBI'S METHOD. The method of preservation of iron antiquities used in
the Saalburg Museum at Homburg is described by Jacobi as follows: The
object is heated in the fire of a forge, which causes the chief part
of the rust to flake off, while any rust which still adheres is removed
when cold by water and brushing. The object is again held in the flame
with tongs and heated (smaller objects may be placed on an iron plate);
and during the heating is quickly taken out three or four times and each
time brushed over with linseed oil. Most of the linseed oil is thus burnt
and the deposition of carbon gives to the iron a black colour, while the
oil which has been partially burnt or hardened by the heat produces a
slight lustre. This process, as carried out at Homburg by a locksmith,
is that which blacksmiths ordinarily use to blacken iron objects and to
protect them from rust. The preservation has proved permanent, and only
in rare cases has it been found necessary to repeat the process. These
good results are probably due to the fact that the antiquities of iron
preserved in that Museum are for the most part found in good condition,
having very little rust and certainly containing only a very small amount
of chlorine. Iron articles which contain chlorine but which still have
a good metal core, after washing, drying, and a cautious preliminary
application of heat, are ready for treatment by Jacobi's method.
INLAID IRON OBJECTS require especially cautious treatment. Although I have
not had any personal experience in the treatment of objects of this kind,
good results have been obtained in several Museums, especially in that at
Mainz.
The following quotation from the "Merkbuch" (p. 75) describes the method
which is applied at Mainz, where it probably originated:
"Objects of this kind which are likely to have been originally
inlaid with silver, gold, copper or brass, as is frequently the
case with objects of the Merovingian period, are not placed in
alcohol after the steeping, but are warmed and dipped three or
four times into a hot dilute solution of isinglass. The heating
is necessary, otherwise the isinglass will set on the surface and
will not penetrate into the interior. When the object has been
dried and the isinglass has set, the layer of rust which covers
the inlaid ornaments is scraped off with a graving tool, and any
spongy hollow parts are filled up with a paste made of iron rust
and isinglass, before the inlaid work is cleaned. During the
scraping the object is held in the left hand on a little wooden
board covered with plush or thick chamois leather, to which it is
fixed as firmly as is necessary by means of a vice. In scraping
special care must be taken that the graving tool follows the lines
of the designs, for in scraping across the design it may slip
under the flat silver thread and raise it out of its place. When
the ornamentation has been completely laid bare, it is rubbed
with emery cloth and then polished with a brush and fine emery
powder. The piece is then dipped into a solution of gum-dammar,
and, when the surface is dry, emery is again used to remove the
varnish, which gives the silver a slightly yellow colour. The
object is then protected from the influence of air and moisture
by the transparent retouching varnish of Sohnée frères (Paris)."
A modification of Krefting's method (p. 108) has proved eminently
successful in the treatment of iron objects inlaid with silver.
Krause[121] recommends that the article be placed, with the inlaid surface
downwards, for 24 hours in a mixture of
10 grammes of 40% acetic acid,
10 grammes of ammonium chloride,
70 grammes of distilled water,
10 grammes of aluminium powder.
It is then removed from the bath, carefully brushed and washed, and,
if the inlaid work is not yet cleaned, is replaced in the bath. This
is repeated until the inlaid work is completely exposed. Spots of
ferroso-ferric oxide which are difficult to remove may be ground away
by an emery wheel, care being taken that the inlaid surface is held
against the lower side of the wheel (which must be rotated in the reverse
direction) so that it is always in sight.
All the methods of this group, which have been applied to many articles in
various Museums, exhibit one inherent defect, for any rust which remains
after treatment may cause the continued oxidation of the iron. The effects
of this action of rust are, I believe, extremely small, and it must at the
same time be admitted that iron antiquities, even if they have been well
steeped and afterwards impregnated, do not always remain in a permanent
and sound state of preservation. If in such a case the well-known small
watery bubbles should make their appearance, the steeping has undoubtedly
been insufficient. This evil can be remedied by gradually heating the
object to redness to destroy the impregnating material, and by a careful
repetition of the steeping and impregnation.
(3) Preservation of Iron Antiquities by Removal of the Rust.
STEFFENSEN'S METHOD (COPENHAGEN). The objects are carefully heated over
a flame and are then laid in dilute sulphuric acid. The sulphuric acid
dissolves a certain amount of the iron, and it is found by experience that
the chemical action is strongest at those spots where any rust remains,
and that this is detached by the hydrogen which is produced. When the
cleaning is sufficient, the iron is laid in a dilute soda solution to
neutralise the acid, and is afterwards well washed with water and dried in
an oven. When dry the iron is brushed over with a solution of bees'-wax
(or better of paraffin) in benzine, the evaporation of which leaves a
protective coating of bees'-wax or paraffin.
BLELL'S METHOD. The method proposed by Blell and applied by him to many
of the objects in his collection is distinct from that described above,
although in its earlier stages the principle is the same. The following
quotation is taken from the description of his method which the author
read before the Antiquarian Society[122] at Königsberg:
"If a specimen is found to have a sufficiently strong core of
iron it should be heated in the furnace to bright redness and
then dipped into water. The expansion of the iron caused by the
heat and the subsequent contraction caused by the sudden cooling
thoroughly loosens the layer of rust. Large iron objects with a
strong and firmly attached incrustation of rust will require a
repetition of the process. By this means not only is the rust
converted into a red powder which is easily rubbed off, but
the object itself is rendered more suitable for the subsequent
treatment. At the same time the heating process removes any
coating of oil, fat, etc., which may have remained from previous
attempts at preservation, and which would interfere with the
further stages of the process. Smaller or delicate specimens
should be treated in the flame of a spirit-lamp, but special care
must be taken that there is sufficient iron present. Sword blades
and other tools and weapons with sharp edges should be heated
only, for the sudden cooling may cause cracks in the cutting
edges."
To complete the removal of the incrustation of rust which has been
loosened by the heating process, or by the heating and sudden cooling,
the object should be placed
"in a well-stirred mixture composed of one part by weight of
sulphuric acid in nine parts of water. Bubbles of hydrogen will
immediately rise and the rust will begin to separate. In freshly
prepared acid objects which are not very rusty will be freed from
rust after four to six hours, those covered with a deeper layer
of rust in about twelve hours, but several days, or even weeks,
may be necessary. The duration of the process depends upon the
strength of the acid and the character of the rust, viz. whether
it is thick and solid, or thin and porous, and whether the iron
is of a soft, or of a hard character.
When first making use of this method it is advisable to use dilute
acid and to take out the objects several times in the course of
the day and examine them, while during the night they should be
taken out of the acid and placed in soft water[123].
For the acid bath and for rinsing it will be found convenient to
have two pairs of wooden troughs having the following internal
measurements:
(1) An internal length of 10 inches [25 cm.] by 7-1/2
inches [19 cm.] in breadth and 4-3/4 inches [12 cm.]
in depth, which will be useful for the larger number of
objects.
(2) For long narrow objects, e.g. sword-blades, and long
spear-heads, the internal measurements should be 40 inches
[100 cm.] long by 4 inches [10 cm.] broad and 3 inches [8
cm.] deep.
Small fragile objects are most satisfactorily treated in glass
vessels or glazed earthen pots or vases.
The acid must have free access to all parts of the object; if a
sword, for example, lies flat upon the bottom, the under-surface
apparently remains unacted upon by the acid. This should be
remedied by the use of a couple of small wooden supports.
Frequent rubbing with a cloth and forge scale[124] or coarse
sand greatly helps in removing the rust, but gentler treatment
is required for the smaller and more fragile objects. The rust
is often very firmly attached in some portions of the object,
and in this case those areas which have been already freed from
rust should be coated over with lard, which is free from salt, to
protect them from further action of the acid, while the pockets
of rust are alternately treated with acid and graving tools. No
particle of rust should be allowed to remain, for sooner or later
it will begin to spread, whatever precautions may be taken.
The action of the acid becomes less effective if it has been used
for several objects. A little fresh acid should then be added.
The more active the sulphuric acid, the brighter will be the grey
colour of the iron after the rust has been removed. If old acid
has been used the iron will be of a dirty grey colour, and should
then be placed into fresh acid for a short time until it assumes
a clear light grey colour.
The third part of the process begins with the removal of the iron
from the acid bath and has as its object the removal of every
trace of the acid, otherwise the rust will very quickly return
and cover the whole surface. The object is therefore immediately
and repeatedly rinsed in soft water and carefully dried; the
cheapest material for this purpose is cotton waste, but ordinary
linen-cloth must be used for objects with jagged edges, for the
threads will catch in the notches and hinder the drying. This
should be done without delay, or a change of the colour from light
grey to yellow will betoken a new formation of rust. Articles
showing a very complicated construction, which are however rare
from the Iron Age, should be packed in perfectly dry hot pinewood
sawdust, while those which are still more difficult to dry, for
example, coats of chain-mail, after thorough rinsing, should be
immediately put into a pan with melted lard, free from salt, and
boiled until the cessation of bubbling shows that all the water
has been driven off by evaporation.
They are then rubbed dry or are laid in hot sawdust, after
which they are brushed over with melted lard and placed in this
condition for at least half an hour in a moderately hot cupboard
until the fat has penetrated into the finest pores of the iron.
That this has really taken place may be proved by the use of a
file.
When by this means all trace of sulphuric acid has been removed
the fourth stage of the process is reached, viz. the removal of
the grease from the surface and the subsequent application of some
preparation to prevent the access of air and moisture. Most of
the grease is removed by placing the objects in a warm place on
blotting-paper. Any grease still remaining on the surface can be
entirely removed with a cloth or paint-brush by means of benzine.
If no restoration or repair is required nothing more is necessary
than to apply the protecting solution."
A white varnish has much to recommend it from its protective power, but
as it gives to iron an unsatisfactory gloss, it is preferable to use a
solution of bees'-wax in benzine.
Having made use of Blell's method in a number of cases I have a few
suggestions and modifications to offer. The heating should be carried out
carefully and gradually, lest the sudden conversion of the moisture in the
rust into steam should cause small explosions which would scatter pieces
of rust. There is no danger of this if the objects are heated in an oven;
they should not therefore be heated in an open flame. For smaller objects
I use a box six inches [15 centimetres] square, of strong tin-plate
loosely covered with an iron lid, or with a piece of asbestos sheet; but
if the objects are large, e.g. swords, spearheads, etc., I heat them on a
strong piece of tin-plate bent round to form a channel, and covered with
a long piece of asbestos sheet, the edges of which are bent over the edges
of the channel, to retain the heat as much as possible.
It is advisable, in my experience, to use the sulphuric acid well diluted,
e.g. in the proportion of 1 to 20, and to renew it several times if
necessary. In mixing concentrated sulphuric acid with water great caution
is required on account of the evolution of heat. The acid should be poured
in a thin stream into the water, but not vice versâ, and the mixture
should be constantly stirred with a glass rod. If a glass vessel is used
for the mixing, it must not be too thick lest the heat should cause it to
break, but the larger the proportion of water to the sulphuric acid, the
less considerable will be the rise of temperature.
For boring out rust spots which have eaten deeply into the iron a
dental drill can be used with success, and a great variety of drills
and milling cutters can be obtained. The rinsing, which Blell carries
out by moving the object to and fro close under the surface in a vessel
full of water, may be sufficient for thin iron objects, such as swords,
knives, spear-heads, and similar objects. Larger specimens should be
freed from the acid by putting them into a still more dilute solution,
and, when necessary, by steeping for a short time in water. It may also
be advisable to put the objects into dilute soda solution to neutralize
the sulphuric acid, but this does not do away with the necessity for
steeping in water. The brown coating of rust which may possibly follow
the steeping can be removed by the use of steel-wire brushes, which can
now be made of such fine wire that their softness almost equals that of
a moderately soft tooth-brush. Brass-wire brushes should not be used, on
account of the yellow colour which they give to the iron. I always put the
objects directly after steeping into clean fat heated to 250°F. [120°C.],
for brushing over with fat and warming in a stove often caused a slight
tarnish to cover the surface. I have also used paraffin wax instead of
fat.
For the method of restoring iron antiquities and of filling up large gaps,
the reader should refer to Blell's detailed account; it will here suffice
to quote his statement that a mixture of iron filings with tin filings
can be used for this purpose. These are melted and applied by the aid of
a blowpipe.
The accompanying illustrations represent iron antiquities which have been
treated by Blell's method: the sword (Fig. 25) proved, after reduction,
from its two ridges to be a scramasax; on the spear-head (Fig. 26)
treatment revealed a small copper ring at the most constricted part, while
the fibula, which previously had been a mass of rust, now shows the spiral
which had been totally disguised.
[Illustration: Fig. 25.
Iron sword treated by Blell's method.]
[Illustration: Fig. 26.
Iron spear-head treated by Blell's method.]
[Illustration: Fig. 27.
Iron fibula treated by Blell's method.]
KREFTING'S METHOD. The electro-chemical method of Krefting was
originally published in "Aarsberetning fra Foreningen till Norske
Fortidsmindesmaerkers Bevaring," 1892 (p. 51), but in "Finska
Fornminnesföreningens Tidskrift[125]" there is a translation into German
by H. Appelgren, and an additional series of observations and experiments
by him. His remarks are equally applicable to Blell's method, and the
following extracts and quotations from this paper give Krefting's method
of procedure and the circumstances under which it should be applied.
Small fragile objects such as fibulae, thin clasps and bracelets or those
which are much eaten away by rust, are not suitable for this mode of
treatment, thus:
"A knife which is much corroded, and which when taken out of the
earth shows a distinctive form (for example, that of the Early
Iron Age), may lose so much by the application of the electric
current that every distinct sign of its original character is
destroyed. The characteristic edges of a spear-head or of an
axe of the late Iron Age, or the equally characteristic point
of an iron sword, may, if the rust has eaten deeply into them,
be unrecognisable when removed from the electrolytic bath. A
sword, the hilt of which is inlaid with copper wire or is plated
with silver or gold, or the blade inlaid with inscriptions in
gold, silver, or copper, may be totally destroyed by incautious
treatment; for the ornamentation, if undermined by rust, may be
detached with the rust from the underlying iron."
On the other hand, objects of sound metallic iron covered with an
incrustation of rust about 1/25 inch [1 millimètre] in thickness may
be easily cleaned in this manner, but if on using a file the metal does
not appear at all, or only at a depth of 1/8 inch [3 millimètres], great
caution must be used. If there is reason to believe that there is gold
or silver inlaid work undermined by rust, Appelgren recommends that the
object should, as a preliminary, be laid in clean water, which should be
renewed every day. After some time, three weeks at the most, sufficient
rust will have been cleared away by carefully brushing with a steel
brush to lay bare the ornamentation, at least in part, and it can then
be ascertained whether there is any rust underneath which would, if
Krefting's method were used, cause the ornamentation to be detached.
The line of treatment is as follows: The metallic iron core is laid bare
by filing in several places. The specimen is then wrapped with strips
of zinc in such a way that the zinc is in actual contact with the bare
metal (Fig. 28). The whole is then placed into a 5% solution of caustic
soda[126]. Appelgren uses a solution of {3-1/2}-{4-1/2} lbs. [{1-1/2}-{2}
kilogrammes] of caustic soda in 2 gallons [10 litres] of water. The rust
is cleared away by voltaic action; the iron forms the negative pole, the
zinc the positive of a voltaic cell, in which the water is resolved into
its constituents, viz. oxygen and hydrogen. At the negative pole, i.e.
the iron, the hydrogen rises up in small bubbles and acts in part by
mechanically detaching the rust as in Blell's method, in part also by the
chemical conversion of the rust into metallic iron, or into a compound
which contains a smaller quantity of oxygen than does ordinary rust. The
oxygen combines with the zinc to form zinc oxide, which is dissolved in
the soda solution. The process is usually completed in 24 hours[127].
The black powder which is loosely attached to the iron is best rubbed off
with wet sand and fine wire brushes. Any hard pieces of black stable rust
(Edelrost), magnetic oxide of iron, which have not yielded to the electric
current should be removed by means of a small chisel. After rinsing the
object thoroughly in water, it should be placed in melted paraffin at
240°F. [115°C.], which will expel every trace of moisture. On removal the
melted paraffin should be allowed to drain off, and thus leave when cold
a protective covering upon the iron[128].
The following points should be observed in the application of the method.
Vessels of glass or glazed earthenware should be used for the reduction,
while long swords can be put into tall glass cylinders or into wooden
troughs, the interior of which must be coated over with paraffin. The soda
solution must be kept in a closed glass bottle[129]. It should be diluted
with water until the specific gravity, as shown by the hydrometer, is
1·06; the mixture will then contain about 5 per cent. of caustic soda.
During the reduction process the mixture frequently assumes a brownish
colour as the result of the presence of organic matter associated with
the rust. On account of the dissolved zinc which it contains it cannot
be used a second time, unless regenerated by boiling with quicklime. The
solution is, however, so cheap that this is scarcely worth the trouble.
The objects should be handled with metal tongs, and should not be touched
with the hand until they have at least been dipped or rinsed in water,
for the soda solution has an injurious effect upon the skin. A basin
containing vinegar, dilute hydrochloric or sulphuric acid should always
be at hand into which the fingers should be quickly dipped if they have
been in contact with the caustic soda. These materials will serve also
for cleaning the vessels used in the reduction process.
The zinc strips should be 1/4 to 1/3 inch [1/2 cm. to 1 cm.] in breadth,
and should be cut out of a piece of sheet zinc of moderate thickness, but
of sufficient pliability.
Any firmly fixed rust may be removed by mechanical means, such as the
graver, drill, etc., as has been previously mentioned. If in rinsing a
slight layer of oxide appears, although this is rare, it should be brushed
off with a steel-wire brush.
If one portion only of a specimen requires reduction (the other portion
having, for example, remains of wood attached, and therefore being
unsuitable for reduction), that portion only should be wrapped with the
zinc and immersed in the solution.
The results obtained by Krefting's preservation-process are quite as
surprising as those which are afforded by Blell's method. Figure 29, taken
from Appelgren's work, shows the lower portion of a spear-head before
and after treatment, by which it became apparent that the whole socket
was plated with silver, with two engraved and gilded animal figures. Fig.
34 represents a piece of a sword, on which an inscription was brought to
light by the reduction process.
[Illustration: Fig. 28.
Krefting's method. Iron spear-head wrapped with strips of zinc.]
[Illustration: Fig. 29.
Iron spear-head before and after treatment by Krefting's method.]
[Illustration: Fig. 30.
Iron pin from "Danes' Graves," Yorks. [Cp. Yorks. Phil. Soc. Report,
1897.]]
[Illustration: Fig. 31.
The same after treatment by Krefting's method, still showing chalky
accretions.]
[Illustration: Fig. 32.
Iron object from Lamel Hill[130], York. It appears to have been
originally rivetted to wood or leather.]
[Illustration: Fig. 33.
After treatment by Krefting's method.]
[Illustration: Fig. 34.
Piece of iron sword-blade showing inscription, after treatment by
Krefting's method.]
HARTWICH'S REDUCTION METHOD[131]. This method is only applicable to small
objects, because it necessitates the subjection of the objects to red-heat
in a glass tube in a current of hydrogen. By these means the hydrogen
combines with the oxygen of the oxides, which are thus reduced to metallic
iron. Owing to the explosive nature of a mixture of hydrogen and air,
this process should only be carried out by one who is conversant with
chemical methods, for results which are equally good can be obtained at
less expense by Krefting's method. For Hartwich's method a strong core of
metal is essential, for although objects which are entirely oxidized may
be thus reduced, the result will be the formation of a more or less loose
iron powder which is frequently in such a fine state of division that by
union with the oxygen of the air, in consequence of the great amount of
surface presented, it becomes red-hot with the formation of ferric oxide
as a combustion product.
It is advisable to apply a combination of Blell's or Krefting's method
with one of the first group (under certain conditions) to such iron
objects as are found, during the process of preservation, to be penetrated
by black stable rust to such a degree that the complete removal would
only leave a kind of iron skeleton. Fig. 35 represents such an iron
dagger-sheath[132], the dark spots upon it being rust. After heating
and cooling down and a short treatment with acid the removal of the
rust was proceeded with mechanically, but was not completed. The object
was then well steeped, and when dry was warmed in the varnish-petroleum
mixture[133].
[Illustration: Fig. 35.
Iron dagger-sheath after treatment by a combination of Blell's and
Krefting's methods.]
Iron objects, the size of which is inconsiderable, such as arrow heads,
small rings, etc., can be very quickly reduced, if they still have a
well-preserved core, by heating them for a short time in molten potassium
cyanide[134]. The cyanide may be melted in a porcelain crucible supported
by wire gauze on a tripod over a good-sized Bunsen burner, and the object
introduced by the aid of tongs. The reaction is accompanied by vigorous
effervescence and is soon complete. It is then taken out and dropped
into cold water. By repeatedly boiling in fresh quantities of water it
is thoroughly cleansed, then treated with paraffin wax, or the water may
be expelled by alcohol. It is then dried, and finally impregnated with
zapon. If the cyanide treatment is insufficient, any remaining rust may be
removed by drills or other suitable tools. Hitherto this method has only
been applied to a small number of objects, but there is no doubt that its
use may be largely extended. Owing to the poisonous nature of the cyanide
this method should be left to those who possess chemical knowledge.
The disadvantage of the process lies in the difficulty of fusing large
quantities of the potassium cyanide[135].
(4) Preservation of Medieval Iron Objects.
A complete treatise on this subject would be beyond the limits of a
handbook, the following observations, therefore, will be sufficient
for our purpose. The rust spots on objects of this kind are frequently
only superficial and can be removed either mechanically by rubbing with
pumice or emery, etc., or chemically by a concentrated solution of sodium
sulphide[136]. To prepare this, sodium sulphide is dissolved in water, or
flowers of sulphur are boiled in a solution of caustic soda. If the object
is too large for immersion, the solution may be applied with a brush, and
if the layer of rust is thick, the application must be repeated. After
treatment the object must be rinsed in water and dried.
Small articles can be freed from rust by immersion in strong fuming nitric
acid[137], for strong acid dissolves the rust only, while it induces
in the iron the so-called "passive[138]" condition in which it is not
acted upon even by dilute acids, and can be safely washed in water. When
thoroughly cleaned, the most suitable protective is some neutral substance
such as paraffin wax, vaseline, or paraffin dissolved in benzine, but any
of the numerous forms of oil or fat may be used.
(_i_) Bronze and Copper[139].
Well-preserved bronzes with a stable patina, such as the highly esteemed
glossy stable or "edel" patina, or that which, although not glossy, covers
the bronze with a rough and often crystalline coating, should not be
interfered with. Such bronzes as need treatment should be subjected either
to simple cleaning or to some appropriate method of preservation.
THE CLEANING OF BRONZES. Bronzes, the metallic substance of which is more
or less intact, while the surface is hidden under earthy or sandy material
cemented together by copper compounds, may be cleaned either by mechanical
or chemical means. When the materials forming the incrustation are more
firmly cemented together than they are to the material beneath (which
often still retains a polished surface), a small hammer may be used, but
more adherent portions require the use of small chisels, which can be made
to order in different shapes or sizes. I have used with advantage hammers
with striking surfaces like those shown in Fig. 36. The two on the right
are rounded so that they touch the object at one point or on a line only.
The process may be facilitated by the use of Springer's method. A warm
thick solution of glue should be spread upon the incrustation covering
the bronze. As the glue dries and becomes cool it scales off, carrying
with it some portion at least of the crust, thus leaving the metal clean.
That part of the glue which remains can then be readily detached by gentle
strokes with a hammer. The eyes should be protected when using the hammer,
whether on the incrustation or on the glue.
[Illustration: Fig. 36.
Hammer heads, natural size.]
OTHER METHODS. Since metallic oxides are scarcely, if at all, soluble in
water, washing with water, even when a brush is used, will remove only
earth or soil which is loosely attached. Compounds containing oxygen or
oxygen and chlorine are, however, more or less soluble in ammonia, and,
if they are thin and not too compact, after immersion for some time can be
removed with a brush. Thick compact layers are loosened with difficulty.
Immersion in 2-5% hydrochloric acid acts more effectively, while sulphuric
acid, nitric acid, and concentrated acetic acid have the same action. The
frequent use of these reagents is, however, strongly to be deprecated,
for it is impossible to remove the acid by simple washing with water
after the incrustation has been removed. The bronze should be washed and
placed in a very dilute soda solution or in dilute ammonia, after which it
should be again well washed with distilled water. As has been explained
in Part I., it is to chlorine compounds that the destruction of bronzes
is chiefly due, and these are actually produced by the hydrochloric acid
treatment. If the bronzes are not thoroughly washed, and this is no easy
matter, sooner or later efflorescences will make their appearance, and the
process of preservation must be repeated if the destructive action is to
be arrested.
Various attempts have been made to remove the incrustation by raising the
bronze to a red heat. This process is not recommended; for not only does
it give to the bronze an unpleasant appearance, but it detaches any inlaid
metal (gold or silver) or enamel which may be present.
In conclusion, it may be stated that, although the process is slow
and laborious, the best results are obtained by careful removal of
incrustations by mechanical means.
Preservation of Bronze and Copper Objects.
(A.) METHODS OF IMPREGNATION. The impregnation of bronzes, as of the
majority of antiquities, has for some time been carried out by the use
of solutions similar to those already enumerated for iron. These are
applied directly or after the specimen has been either steeped in water
or treated with dilute acids. This latter treatment, as has been already
stated, is to be avoided, and if used all acid must be washed out before
the object is dried. Steeping in water is of little use, because compounds
containing oxygen or chlorine are often insoluble in water, which will
at most only wash off loosely attached dirt or earthy material. The
impregnation process may therefore be applied directly, and this should
be done in all cases in which the surface is much corroded, warty (Figs.
7 and 8), or cracked (Figs. 37 and 38), or in which there is little or no
core of metal. Impregnation is also the only means of preservation when
the formation of oxides has raised inlaid metals or enamel in such a way
that the removal of the oxides would detach them. The "Merkbuch[140]"
recommends poppy seed oil and benzine mixture (p. 70) or the gum-dammar
solution. To obtain thorough impregnation this should be carried out by
extraction of the air, as has been already recommended in the case of
limestone (p. 68). The object must also be perfectly dry, which may be
insured either by exposure to moderate heat or by keeping it for some time
over anhydrous calcium chloride[141]. The object is placed under a glass
bell jar, the edges of which are smeared with vaseline to ensure contact
with the glass plate upon which it rests. The calcium chloride should be
placed in an open glass vessel, beneath the bronze, but care must be taken
that they are not in actual contact.
[Illustration: Fig. 37.
Osiris showing cracking and destructive patina.]
[Illustration: Fig. 38.
Boeotian bridle with cracking patina.]
Immersion of bronzes in paraffin wax at 240°F. [115°-120°C.] gives results
which are as good, if not better, than those obtained by the use of
solutions.
Should efflorescences make their appearances upon bronzes which have been
impregnated, their further spread may often be successfully prevented
by smearing fish-glue on the parts affected. Fish-glue, however, has not
proved a satisfactory material for the complete impregnation or coating
of bronzes which are in the last stages of decay.
(B.) PRESERVATION BY REDUCTION. It has been previously explained (pp.
28 _et seq._) that the efflorescences upon bronze known as creeping or
malignant patina which may in time cause the complete destruction of
the metal are due to the action of sodium chloride. It is found upon all
Egyptian bronzes and upon those from some other localities.
The metal, especially the copper, is converted into the so-called basic
chloride. In the reduction processes an attempt is made to reduce these
compounds again to metal, while the chlorine thus liberated forms chemical
compounds, which may be subsequently washed out with water. There are two
methods which effect this reduction, viz., that of Finkener (Berlin) and
that of Krefting. The principle of both is electrolytic, and both bring
about the complete removal of the patina and the restoration of a clean
metallic surface.
To complete this portion of the subject a third method may be mentioned,
viz., reduction by heat in a stream of hydrogen. This method[142] is,
however, only applicable to small objects.
FINKENER'S METHOD. Care must be taken when examining the bronze that the
metallic-looking mixture of cuprous oxide with other copper compounds is
not mistaken for metallic copper. When it has been ascertained that the
bronze still has a good metallic core, and that any inlaid metals which
may be present rest on the metal itself and not upon a crust of oxide, a
platinum wire should be tightly wound round it. This should be connected
by an insulated copper wire to the zinc or negative pole of the first of
3 or 4 Daniell cells, or, better, of two accumulators arranged in series.
The object should then be immersed in a 2% aqueous solution of potassium
cyanide. In the same solution, as near as possible to the bronze without
actual contact, should be placed a piece of platinum foil connected first
by an emerging platinum wire, and then by an insulated copper wire to
the positive pole. The potassium cyanide completes the electric circuit
and electrolysis takes place, whereby the water is split up into its
constituents. The oxygen appears in small bubbles upon the platinum foil,
but the hydrogen does not immediately make its appearance at the other
pole, for, by combination with the chlorine and oxygen contained in the
bronzes, free hydrochloric acid and water are formed. The hydrochloric
acid in turn acts upon the potassium cyanide to form potassium chloride
and hydrocyanic acid, both of which substances are dissolved in the water
of the bath. The hydrocyanic acid can often be recognised in the room by
its characteristic smell of bitter almonds. The process may be expressed
by the following equations (neglecting the water produced by the oxygen
of the oxide, which is of no importance in the process):
CuCl_{2} + 2H = Cu + 2HCl,
HCl + KCN = KCl + HCN.
Although the chief portion of the potassium chloride and hydrocyanic acid
are dissolved in the bath, the remaining traces of these substances must
be removed by very carefully washing the bronze in water, after which it
should be dried, and if necessary finally subjected to impregnation.
Some further observations may be made in connection with the practical
application of this process.
Of course, other primary batteries may be used instead of the Daniell
cells, but these latter may be specially recommended for the ease with
which they can be procured and for the steadiness of their action.
Information concerning the method of filling and using them may be
obtained at any shop where they are sold. The copper wire and platinum
wire should not be too thin, but must be at least from 1 to 2 mm. in
thickness: they should be fastened together by binding-screws, and care
must be taken that both the wire ends and the screws have clean surfaces.
Glass vessels or glass cylinders are most suitable because the process
of reduction can be watched, but large objects will of course require
glazed earthenware baths. If wooden boxes are used they must be coated
inside with paraffin wax. The strength of the cyanide solution should be
2%. Having a large number of reductions to carry out, I keep a 20% stock
solution in a large bottle, one part of which is diluted with nine parts
of water when required for use. Potassium cyanide is, as is well known,
a strong poison, and care should therefore be taken to prevent access to
any sore or cut on the hands; this can be done by the use of india-rubber
finger stalls or gloves.
If the bronze object is neither too large nor too heavy it may be
suspended in the bath by looping the platinum wire over the edge of
the vessel. It is a convenient plan to use different coloured wires to
distinguish the negative and positive poles of the battery, but should
any doubt arise as to which wire should be connected with the bronze
or which with the platinum, the following test will readily decide the
question. Moisten a small piece of white filter paper with a drop of a
solution of potassium iodide[143], and touch the two conducting wires
with it simultaneously: a brown spot will be seen on the paper at the
point of contact with one of the wires; this is the positive wire, and
must therefore be connected with the platinum. If the current is passing
through the cyanide bath and the bronze, bubbles of gas will appear upon
the platinum foil, or the products of the decomposition of the potassium
cyanide may change the colour of the bath near the platinum to yellow or
brown, while at the same time cloudy streaks under the bronze will show
where the potassium chloride and hydrocyanic acid, resulting from the
reduction of the copper compounds, are meeting with the cyanide of the
bath. If the platinum wire is not firmly fixed round the bronze, hydrogen
may be formed upon it, and should this occur the wire should be drawn
tighter.
Whilst the reduction is going on it is advisable to renew the potassium
cyanide at least once, or even several times, if large and greatly
oxidized bronzes are under treatment, for otherwise all the potassium
cyanide may be consumed by the changes in progress; this can be
ascertained with certainty by a smell of chlorine. When the bath requires
renewal the bronze may be taken out with a pair of metal tongs, or if too
large, two strong copper wires should be passed underneath it, the ends
of which are wound round a strong glass rod or wooden stick. The bronze
should then be well rinsed or brushed with a soft brush before it is put
into the fresh bath.
Bronzes are frequently met with which are much deformed by an earthy or
sandy layer cemented by oxide. These incrustations can be partly removed
by a preliminary treatment with dilute hydrochloric acid, but the bronze
must be afterwards carefully rinsed with water or even steeped to prevent
unnecessary decomposition of the cyanide by the acid. Before reduction it
is useful to secure thorough penetration by placing the vessel containing
the solution and the bronze under a bell glass attached to an air pump,
as has been previously explained (p. 68).
During the process of reduction small whitish-green crystalline needles
often collect on the platinum foil, but although in large numbers they
are so minute that it has not been possible hitherto to determine their
composition; they seem to contain copper and cyanogen. After some time
the platinum becomes covered with a whitish-green or brownish deposit,
which should be removed by rinsing in water and brushing; if this should
not succeed the platinum must be dipped in hydrochloric acid, rinsed with
water, and rubbed with fine sand. The glass vessel may be cleaned in the
same way.
The reduction is complete when all the chlorine, previously combined with
the metal, has combined with the hydrogen produced by the electrolysis
of the water. There being no further chlorine with which the hydrogen
produced by the continued action of the current may unite, the completion
of the process is marked by the appearance of bubbles of that gas upon the
surface of the bronze. The bubbles which rise from beneath often mark out
the outlines of the object upon the surface of the bath.
Before the bronze is washed it should be placed in a fresh cyanide bath,
but of 1% strength only. For large and especially for thick objects, this
bath must be renewed several times, so as to allow the washing process to
begin in the bath itself whilst the current is still passing through it.
Care should also be taken that every side of the object in turn faces the
platinum foil for some time, for if one side remains turned toward the
platinum throughout the process, it will sometimes assume the red tint of
copper, while the rest of the bronze retains a somewhat dark colour.
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.
[Illustration: Fig. 39.
Bronze bull showing warty patina.]
[Illustration: Fig. 40.
The same after reduction by Finkener's method[147].]
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.
[Illustration: Fig. 41.
Bronze axe-blade before treatment by Finkener's method (Aeg. 13203).]
[Illustration: Fig. 42.
The same side after treatment.]
[Illustration: Fig. 43.
Reverse side of axe-blade after treatment.]
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.
[Illustration: Fig. 44.
Fig. 45.
Dagger sheath before and after treatment by Finkener's method.]
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."
[Illustration: Fig. 46.
Roman coins before treatment.]
[Illustration: Fig. 47.
Roman coins after treatment by Krefting's method.]
Cleaning Copper Coins by Melted Lead.
Although the results obtained by this method are less satisfactory than
those produced by the preceding, it has the advantage of simplicity[153].
"Using a pair of tongs, dip the coins one by one into melted
lead until the crackling, which begins at once, has ceased, which
occurs in from 3 to 10 seconds. The hand should be protected with
a glove from the spluttering molten lead. The coin is then thrown
into cold water, cleaned, and placed until the next day in hot
milk. It may be necessary to repeat the process when the coin has
become cold. By this method an olive colour is imparted to the
coin which many antiquaries prefer to dark brown, but personally
I prefer Krefting's method because it renders the inscription
and designs far more distinct. A coin which after the treatment
with melted lead has remained so covered with cupric oxide as to
be still illegible can seldom be improved by a repetition of the
treatment, whereas had the zinc treatment been applied in the
first instance the result would probably have been satisfactory.
This conclusion seems to be justified by the extremely small
percentage of coins which, in my experience, have remained
illegible after the treatment by electrical methods."
(C.) Preservation of Bronzes by the Exclusion of Air.
In those cases in which the advanced state of decomposition renders the
reduction process either inapplicable or at any rate inadvisable, or in
which the decay is not likely to be arrested by impregnation, a further
method of preservation remains, viz. the complete exclusion of air and
moisture.
If air is completely freed from moisture the oxygen can no longer act in
conjunction with the copper chloride upon the still intact metal (see page
29 _et seq._), and the condition of the bronze will consequently remain
unchanged.
A bronze, for example, which shows much decay should be placed after
impregnation under a hermetically sealed bell glass, and beneath or near
it should be placed some dehydrating agent, of which anhydrous calcium
chloride is the most suitable (see note, p. 123). To exclude the air
completely the bell glass should have a projecting ground edge, which
should be smeared with vaseline or grease and pressed firmly upon a
thick well polished glass plate. The dehydrating agent may be placed in a
glass vessel or dish in such a way as to be unseen, or it may be covered
with two or three thicknesses of dark gauze or with black cardboard
laid loosely over it. If an object is too large for a bell glass, or if
several objects are to be exhibited together, a square plate-glass case
with iron framework, made air-tight with putty, may be used as shown in
the illustration (Fig. 48). The lower part, containing calcium chloride,
is partitioned off by a perforated plate covered with black gauze[154]. A
hygrometer was placed behind the head, the indicator of which has remained
at zero since it was first fixed several years ago, and the bronze has
not hitherto shown any sign of change, although the inlaid gold is in
parts raised from the metal by a light-green oxychloride. The cost of
these cases is considerable, but for valuable objects this should not be
considered. In the place of calcium chloride, sticks or lumps of caustic
soda may be used with advantage, for this substance absorbs both moisture
and carbonic acid.
[Illustration: Fig. 48.
Method of mounting objects in air-tight cases.]
This method of preservation is of course applicable not only to decomposed
bronzes but to all valuable antiquities, whatever the material may be.
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.