The deterioration of silver alloys and some aspects of their conservation
Virginia Costa
Abstract
The present article aims to offer a comprehensive approach to the conservation of silver objects. It
is not intended to suggest or recommend any specific recipe or treatment for a given case. Instead,
the paper presents a range of possibilities to assist the conservator in defining the state of
conservation and the possible need for treatment and protection of an object. Besides giving the
background to the structural properties of alloys used in silver artefacts, the possible forms of silver
deterioration are presented, with special emphasis on tarnishing and mineralization. Less frequent
cases, such as selective, localized and stress corrosion are also discussed. A descriptive review of
materials and methods used for cleaning, consolidating and protecting silver artefacts follows,
accompanied by an extensive bibliography.
Introduction
Whether an object should undergo treatment can only be answered after assessing aesthetic,
technical and historical parameters and also taking into account how the object will be stored or
used after the intervention. Perhaps one of the most important considerations is the nature of the
object: from what material and with which techniques it was made, what has subsequently
happened to it and how it has been altered. These factors limit what conservation treatments may be
applicable, from a material and technical point of view. Additional aspects of the object's nature
may be revealed during the course of treatment, and may lead to a change in procedure; the
conservator must be alert to alternative options throughout the course of treatment.
After reviewing some properties of the alloys used in silver artefacts, and the causes and
mechanisms of the typical forms of silver deterioration, this paper will describe the materials and
methods used in the cleaning, consolidation and protection of silver objects. It is not the intention of
this paper to endorse specific treatment methods or to preclude others. Rather, the intent is to
acquaint the conservator with the range of published work available for consultation when he or
she, guided by a thorough examination of the object and its environment, evaluates the state of
conservation of a silver artefact and its possible need for treatment and/or protection.
Silver artefacts and alloys
Among all the metals, silver has physical and chemical characteristics which, combined with the
beauty of its surface appearance, make it an ideal material for producing decorative objects.
Silver was used in two principal forms: as so-called 'solid' silver objects and as a coating. Solid
silver articles actually consist of alloys with other metals, partly to make the alloy harder, and also
to economize on the amount of silver used [1]. These objects, often containing copper as a major
alloying element and lead and gold as impurities, can either be cast directly or cold-worked into the
final form [2, 3] (Fig. 1). Great
Fig. la Cross-section of a silver sheet from a Renaissance silver basin elaborated by repousse,
showing equiaxed grains [after D. Scott, 1991].
Fig. lb Cross-section of a silver-copper alloy sheet fragment: dendritic structure with minimal
distortion, showing that the Sican metalsmiths (Peru, AD 850-1050) have pre-cast the alloy into the
dimensions required for making the tumi, rather than carrying out the expected process of making
the idol from hammered silver-copper sheet [after D.Scott, 1996].
quantities of articles so made are decorated with gold coatings (usually by mercury-gilding or by
'fire-gilding') or with niello, which enhances surface details by contrasting clear and shadowed areas
[4, 5]. In general, the most commonly employed alloys used to produce objects are: so called
'sterling silver' (92.5% Ag, 7.5% Cu); 'coin silver' (90% Ag, 10% Cu); and eutetic or brazing alloy
(71.9% Ag, 28.1% Cu).
Silver was also intensively used as a coating on copper and iron alloys, to yield a rich appearance at
minimal cost [6]. The few and rudimentary early procedures of silver-coating that involved silver
foils attached mechanically or with silver-copper solders [7] were slowly replaced by more efficient
methods, such as mercury-silvering. Some specific applications were incrustations, such as those
found in Merovingian arms [8], metal threads [9| and 'depletion silver', used by native Latin
Americans to enrich the alloy's surface [10, 11]. Since the eighteenth century a plethora of new
techniques, including 'French plating', 'close plating', Sheffield and electroplating have been
introduced [12] (Fig. 2).
Stylistic and typological examination of silver objects can be complemented by laboratory analysis
[13]. The kind of alloy is often determined by elemental analysis [14] and hypotheses about the
provenance of the ore from which the alloy originated have been investigated by isotopic analysis
[15,16] and neutron activation [17]. Besides radiography, which gives information about the
fabrication of the object, other important information about shaping and decorative embellishing
can be obtained through structural examination by metallographic techniques [18, 19]. By means of
scanning electron microscopy (SEM) it is possible, not only to differentiate a chased from an
engraved object, but also to identify the kind of tool used for setting filigrees and granules [20-22].
Attention must be paid when choosing the sampling site where analysis is to be conducted: in
addition to the fact that ancient objects are structurally heterogeneous, deterioration often occurs in
a selective way [23].
Since most 'silver' objects are made from silver-copper alloys, it is important to know the structural
possibilities presented by this kind of alloy. The structure of a material defines not only its
properties, but can also explain the way it has deteriorated. The silver-copper phase diagram
provides insight into the structure produced for each alloy of these two metals (Fig. 3). The diagram
presents the phases existing as a function of the alloy composition and of the temperature
(horizontal and vertical lines respectively). The extreme left ordinate represents pure copper
(melting point 1083 °C), and the extreme right, pure silver (melting point 960 °C). Between both
extremities there is one liquid phase (L) and two different solid phases, from which one is copperrich (alpha) and the other silver-rich (beta). Depending on the composition of the alloy and
temperature it is possible to obtain a great variety of structures in the cold metal.
Fig. 2 Different forms of silver-coating: (a) Section through the rim of a forged denarius showing
how the silver foil has been wrapped round the core [from S. La Niece, 1993]; (b) Cross-section of
a silvered copper nose-ornament from Ecuador: the coating is a silver-copper alloy presenting a cast
structure [from D. Scott, 1996]; (c) Cross-section of a saxophone showing a silver electrodeposit on
brass [from C. Degrigny, 1993].
Fig. 3 Phase diagram for silver-copper alloys: (a) eutectic alloy: fine lamellae of phases alpha and
beta, disposed alternatively; (b) silver-richer alloy: primary beta phase within a eutectic matrix; (c)
even more silver-rich alloy: predominance of the beta phase.
To obtain insight into the solidification process, three cases of different alloy compositions will be
examined. The alloy of lowest melting point, called 'eutectic', is commonly used for solders and
contains approximately 70% silver and 30% copper (case a, Fig. 3). As this alloy cools from the
liquid state, solidification is instantaneous when a temperature of 780 °C is attained. The resulting
eutectic structure consists of very fine lamellae of both phases, alpha and beta, disposed
alternatively.
When the silver content is greater than in the eutectic alloy, not only does the proportion of beta
phase increase, but also the distribution of both is different (case b, Fig. 3). This is because, in this
case, the solidification does not occur instantaneously. The time period necessary to complete the
passage from liquid to solid allows the 'excessive' silver content (relative to the eutectic
composition) to solidify first, in the form of a primary beta phase. Only later, when cooled to a
temperature of 780 °C, does the eutectic solidification take place.
A third possibility, most frequently found in 'silver' objects, corresponds to copper contents up to 5
or 10%. In this case the proportion of the silver-rich (beta) phase is even greater, which reduces the
possibility of eutectic formation and limits the existence of the alpha phase to a few locations
between grains of the predominant beta phase (case c, Fig. 3).
In several alloys that are heated and then cooled very quickly it is possible to get an apparent
structural homogeneity, as only a single phase would exist at low temperature. However, as both
phases are immiscible at room temperature, this is not a stable condition and they have a tendency
to separate into two different phases with the passage of time. Once separated, the minor phase
diffuses to reach a more favourable environment, locating itself at the joints of grains in isolation
and causing weakness in the strength of the alloy [24, 25] (Fig. 4).
Fig. 4 Section of a silver earring from Tell Farah, Jordan (first half of the second millennium BC):
The 'wiggly' grain boundaries result from the precipitation of the copper-rich phase [from D. Scott,
1991].
Forms of deterioration of silver
Corrosion results from the interaction between the metal and its environment and can assume very
different forms as a function of the way in which this interaction takes place. It is a natural process
in which the metallic form obtained by metallurgical methods, tends to return to its original,
energetically more stable, state.
Examples of the typical forms of silver corrosion are given below and their possible causes and
mechanisms discussed briefly.
Tarnishing
Silver objects exposed to the atmosphere become covered by a dark film, which originates from the
interaction with air pollutants under certain conditions. The film growth is generally rapid at the
onset of the process, and tends to slow with time if relatively stable surface products are formed. As
the tarnish film thickens, the colour and reflection of the surface changes from light interference
tones, to a brown and, finally, black film [26, 27]. Excluding the case of very aggressive
environments, there is rarely great danger for the survival of the object. However, tarnish is
aesthetically displeasing and disturbs the legibility of many objects, from ancient works [28] to
daguerreotype images [29].
In most cases, the principal component of the tarnish film has been identified as acanthite (alpha
Ag2S) [30]. The associated species in the atmosphere are reduced sulfur gases, such as H2S, and
organic sulfur compounds, such as carbonyl sulfide (OCS) [31-36] and dimethyldisulfide (Me2S2)
[33]. Sources of these species are oil refineries, paper treatment plants, kitchens, rubber, paints, etc.
In several cases chlorargyrite (AgCl) has also been identified as a major component [37-39]. Its
origin could be explained by the deposition of chloride-containing airborne particles from
combustion processes, dispersion of marine salts or even from the purification treatment of water in
urban areas [38, 40].
The mechanism by which tarnish occurs is explained schematically in Figure 5. Although silver is
considered a noble metal, silver surfaces are very reactive. The non-equilibrated atoms at the
surface easily capture oxygen from the air [41,
Fig. 5 Successive stages of formation of a tarnish layer: the clean Ag surface (a) reacts with
oxygen in the air to form an oxide (b), which is transformed into a more stable specie, Ag2S, by
dissociation of a reduced sulfur molecule in the presence of humidity (c).
42], forming a thin and irregular oxide film (Fig. 5b). In this state, if a molecule of reduced sulfur is
available in the proximity, another reaction will take place, so that more stable surface bonding
forms. The resulting compound is silver sulfide (Ag,S), and a water molecule (Fig. 5c). Inevitably,
the whole process will be accelerated by the presence of humidity.
From these three elements responsible for silver tarnishing -oxidant, pollutant and relative humidity
(RH) - the most investigated is H2S, whose typical indoor concentration is reported from 50-100
parts per trillion (ppt) [36] to 300 ppt [35]. Without exception, its harmful effects are confirmed in
the literature [31, 32, 35, 37, 39, 43-45]. Some authors attributed to carbonyl sulfide (OCS) a central
role in the tarnish process, because it is present in the atmosphere in great quantities, recorded as
from 300-500 ppt [36] to 600 ppt [35] and, in the presence of water, decomposes to yield H2S and
CO2 [31, 32, 46]. Although these pollutants are normally present in very small quantities in the
atmosphere, indoor
environments can include material from which noxious gases evolve, that allow for greater than
expected corrosion rates [44]. Under other circumstances, different rules may apply. For example,
the chloride ion, Cl-, contributes to the deterioration by forming AgCl, and in this way accelerates
sulfuration in the presence of H2S [35, 39, 40].
Oxidants play an essential role in initiating the tarnishing process, confirming that there is no direct
pathway for the reaction between the silver surface and H2S [35, 41-43, 47-50]. The presence of
NO2, either as an initial reactant [48, 51], or indirectly as a catalyst for H,S dissociation [39],
increases the sulfuration. Ozone, O3, in the presence of H2S [40] and ultraviolet (UV) radiation [52]
also has this effect.
The increase in the rate of sulfuration with RH was also demonstrated by several authors, regardless
of the test conditions [31-33, 40, 43, 53]. Condensation of humidity on the surface (50% RH
corresponds to more than one adsorbed water monolayer) forms an electrolyte that can quickly
dissolve gaseous pollutants and accelerate ionic mobility, both factors important for the reactions
that conduce to tarnishing. However, if oxidants are present in great excess, this acceleration effect
can be masked [40].
It is important to remember, when evaluating the harmfulness of an atmosphere, that most
components of 'silver' objects -particularly solders - are actually silver alloys. Although some
pollutants that exist in large quantity, like SO2, are said not to be dangerous for silver [32, 54], they
could be for its alloys, especially those with copper. In this case the less noble part of the alloy will
react preferentially and considerable quantities of Cu2S can be produced [55].
Mineralization
Considerably less investigated than tarnishing, the mineralization process occurs with objects that
have remained under the earth or sea for a long time. Both are chloride-rich environments and the
process goes on continuously, albeit slowly, due to the large ionic mobility of silver. This condition
preserves the marks left by tools, deformation and decoration on the object. In some published cases
the metal is only partially mineralized, for example, in the case of a sieve [56], coins [57] and a
canopic vessel [58]. In other cases objects are completely transformed into AgCl, as in the case of a
lyre [59, 60], a silver bowl from Ur [61] and other objects cited by Rathgen [62].
These reports show that very little was done to characterize the mineralized material and understand
what has happened as a result of the conditions to which the objects have been subjected. Most of
the analyses were conducted to obtain insight into ancient techniques, some involving
metallographic examination of the transverse section [45, 59, 61] and radiography [58]. Current
investigations are providing a more complete characterization, with computer topography, trace
element and isotopic analysis [63].
The defining characteristic of this kind of deterioration is the long duration of exposure to adverse
conditions. Independent of the harmful ion present in the soil, it is possible to say that its corrosivity
increases with the oxygen and water concentration. Particularly for acidic soil, there is a preferential
dissolution of the copper-rich phase, producing cracks, and, in the presence of salts, a large amount
of silver chloride (horn silver) is formed [64]. Air-deficient soils that are pH neutral or basic will,
on the other hand, favour biodegradation, adding factors to the initial corrosion process in the form
of organic decomposition products (ammonia, nitrates, acetic acid, H2S) [45, 65]. In visceral vessels
the decomposition of tissues as well as mummification salts accelerated the corrosion reactions
[58]. In some cases, conversion to silver sulfide might have been due to sulfide-reducing bacteria
[61,66]. A significant amount of silver bromide was also found in corrosion layers on buried
ancient silver [67].
The effects of the fundamental role played by time in this deterioration process can be seen in the
stratified structure of the corrosion products [34]. During the mineralization process there is
migration of material both inwards and outwards. Silver and other elements go outwards, whereas
ions like chloride and impurities from the environment diffuse inwards and replace the original
silver. A redistribution takes place and silver, now in the form of silver chloride, occupies a volume
approximately double the original size. The result is that part of the artefact remains roughly in
place while another part extends beyond the original surface (Fig. 6).
Localized corrosion
This kind of corrosion is not usual in silver objects, and has only been observed in cases where the
RH was very high and sulfur was present [68]. The corrosion products can look like very narrow
filaments (filiform corrosion) or like dendrites ('whiskers'), which project beyond the impermeable
film that covers the metal. Although many instances have been reported in fundamental research
papers [69] and in electronic industry reports [70], only a few examples from museums are related
in detail [71, 72].
It is evident that tiny holes in the lacquer covering the silver surface are very suitable sites for
humidity to condense and to initiate the dissolution of the metal. Silver ions have a high ability to
migrate and move quickly to the surface, where they can accumulate in the form of dendrites or
filaments [33]. The presence of sulfur gases will accelerate these reactions and dissolved SO2 will
even increase the silver solubility from 20 to 3000 mg/L [68]. If the coating has good adhesion to
the rest of the surface, in spite of any holes, the preferred growth direction is outward and whiskers
must form. In the case of bad adhesion, the migration of silver takes place beneath the coating, in a
filiform manner (Fig. 7).
Fig. 6 Mineralization process: migration of material occurs both inwards and outwards; the
original surface is often preserved.
Fig. 7 Localized corrosion takes place when the surface coating is defective: corrosion evolves by
migration beneath the coating or develops outwards.
Selective corrosion
This kind of corrosion occurs in alloys or structurally heterogeneous materials that, like the name,
suggests, deteriorate in preferred areas. As most 'silver' ob]ects are really alloys, usually with
copper, this phenomenon is often observed, even in the cases of light corrosion described in the
literature. Selective corrosion has been observed as a green surface layer, probably of copper
carbonate [51, 56], or as a dark red deposit, due to formation of copper oxides [73]. This means that
the less noble (copper-rich) phase reacts preferentially with pollutants at the surface, forming
corrosion products [55, 74]. They can be cleaned off and the 'new' surface will be rougher and
richer in the more noble element, silver. This phenomenon was known by native South Americans,
who used it superficially to enrich their alloys [6, 10].
Alloys often present chemical or structural peculiarities dependent, in particular, upon which phases
are present, their size and distribution. When exposed to a harmful environment, a difference in
electrochemical potential will be created between the different phases and, as in a battery, electrical
charges will separate, defining anodic and cathodic regions. The less noble phase (copper rich) will
behave as an anode and dissolve preferentially. The surface underneath will be rougher and
enriched with the more noble phase (Fig. 8a).
Over time, and as function of the corrosiveness of the environment, the phenomenon initially
observed only at the
Fig. 8 Schematic structure of a silver-copper alloy: grey grains represent a dispersion of the minor
phase (alpha, copper-rich) within the beta matrix (silver-rich). As a consequence of the preferential
dissolution of the less noble phase, its corrosion products cover the surface (dark grey layer),
leaving the underlying surface richer in silver (white grains), (a). If the copper-rich phase is
accumulated at the grain boundaries, the corrosion process will be localized there, leading to
intergranular embrittlement (b).
surface propagates into the body of the material. The net result is that a macroscopic specimen
disintegrates along its grain boundaries. [75, 76] (Fig. 8b).
Stress-induced corrosion
In the case of stress-induced corrosion, the remarkable brittleness of the artefact can be attributed to
previous intensive cold work to shape and/or decorate it. Examples demonstrated in the literature
include a ewer and a rhyton shaped by hammering [77] and two paired Sarmatian plaques [78],
More complete investigations were undertaken in the case of an Egyptian vase [79].
Intensive cold working leaves residual stresses and induces structural modifications. One symptom
is the precipitation of a less soluble phase, previously maintained in a metastable form in the matrix.
Another is the appearance of strain lines, preferentially concentrated in the overworked regions.
These slip lines act as a barrier for the migration of impurities and minor phases, which accumulate
there, forming a region mechanically weakened and susceptible to corrosion (Fig. 9a).
Fig. 9 Structural modification introduced by cold working: precipitation of a phase and appearance
of strain lines (a). Annealing leads to recrystallization, the new grains forming twins (b).
This effect could be reduced by annealing severely cold-worked objects, but the new recrystallized
grains always maintain traces of the previous work, in the form of twinned grains, which also
concentrate impurities. Furthermore, repeated heat treatment can lead to excessive grain growth,
which also contributes to the brittleness of the material (Fig. 9b).
Materials and methods of restoration
The crucial factor guiding decisions about cleaning and/or consolidation of an object is its stability.
Priority should be given to stabilization and only later can questions about appearance and
legibility, which are generally more subjective, be addressed. In some cases, the preservation of
details such as evidence of manufacturing techniques may guide the selection of methods of
treatment.
In making such an evaluation, it is important to identify the materials that constitute the object and
also its corrosion products: considerable time may be necessary for examination and analysis.
Different approaches may apply, for example, in the treatment of a solid silver object and of what is
merely a thin coating that can be removed completely if care is not taken. Before any treatment
commences, certain technical and aesthetic decisions must be made, because even a minimal
intervention removes a little metallic silver and the repetition of cleaning procedures inevitably
causes progressive wear to the object, corresponding in several cases to a mean loss of 1 to 3 % of
the artefact's weight in a period of 200 to 300 years [64]. Moreover, it should be kept in mind that
corrosion products existing at the surface can act as a barrier for further reaction with the
environment, and that their removal therefore makes the object more reactive [80].
Review of earlier work shows that the historic evolution of cleaning procedures has involved not
only the development of new methods, but moreover their adaptation to each individual object, with
increasing care to neutralize and rinse away the materials used [22, 50, 63, 81]. Bearing this in
mind, no suggestion will be made here about the suitability of a specific product or technique. The
materials and methods most frequently used and reported in the literature will be presented below in
a systematic form, followed by examples, with the aim of showing how each works, the reason for
its choice and, less frequently reported, the consequences of its use.
Removal of surface grime
Before addressing removal of corrosion, it is important to emphasize the first step of most silver
treatments: the removal of accumulated dust and dirt on the object that hinder not only a correct
evaluation of its condition, but also the access of products used for cleaning.
In many cases a simple dusting [64, 82] followed by a rinse with water and neutral detergent [65,
83, 84] and then wiping with a soft cloth are sufficient [64, 85, 86]. All manipulations should be
carried out wearing clean cotton gloves, which prevent subsequent stains from residues of chlorides,
grease and organic acids from the hands [82-84, 87, 88]. Light cleaning can be done with an anionic
detergent [83, 87]. Commercial detergent formulations should be avoided, because they often
contain phosphates and bleaching agents, which can react with silver to produce dark spots [49, 84].
There are very few reports in the literature concerning previous surface coatings. Normally,
solvents must be used to remove wax and/or lacquer [89]. Steam and electrolytic cleaning have also
be employed to remove old lacquer from silver objects [90,91]
Treatment of corrosion products
To simplify, the techniques employed to treat or remove corrosion products are classified in three
principal groups, mechanical, chemical and electrochemical, which are described below, together
with examples of their application. A comparative table of certain characteristics of these methods
is presented at the end of the section (Table 4).
Table 1 Abrasive compounds cited in the literature
Mechanical cleaning
This consists in the removal of the undesired corrosion product through picking off with a tool or
rubbing with other material. Skill is needed to use scalpels or micro-rotary tools. The abrasive
systems most often employed for mechanical cleaning consist of three basic components: an
abrasive, a fluid to maintain it in suspension (usually water or alcohol) and a support to carry the
mixture to the object (soft cloth). In addition to these, commercial products often have detergents,
inhibitors, perfumes, etc. [50, 87, 88] and typically offer fabric or cotton impregnated with
abrasives and additives [82, 83, 92].
Some investigations have been carried out to evaluate and compare commercial products [82, 92]
and also simple abrasive materials of well-defined chemical composition [93]. Table 1 provides
examples of the use of abrasive compounds; their properties may be found in the tables of review
articles, which provide a good basis from which to choose in each particular case [50, 87]. In
practice, in order to avoid scratching the surface, two aspects should be considered: the
homogeneity of the particles size and their hardness. Softer particles require longer polishing time
but avoid scratches.
Chemical cleaning
This procedure consists in bringing the object into contact with a solution that dissolves the
alteration products from the surface and that should be removed carefully after the treatment. The
more suitable solution will be that which removes the maximum of corrosion products while
dissolving a minimum of silver metal. Again, it should not be forgotten that most objects really
consists of alloys with copper, forming phases that could be attacked by several products to which
silver alone is resistant. Niello decorations are in fact silver sulfide films and are indiscriminately
dissolved by all the chemical cleaning solutions commonly in use. Contact must be as quick as
possible, because there is always a danger of corroding the object, with an increase in surface
roughness and retention of cleaning products.
The cleaning solution may be applied by immersion if the object has simple forms, without crevices
or other applied materials. Otherwise, it may be applied locally with a swab or thickened with
gelatine.
An efficient cleaning solution should not only remove the undesirable alteration products from the
surface, but also ensure that they stay in solution, so that largely they can be eliminated.
Very few one-component solutions have this property. Cyanide solutions are an example; however,
in this case the toxicity is so high that its use is not recommended [98].
The presentation here of all recipes cited in the literature is not possible, because there is a huge
number, especially if small variations are considered. On account of this, only some of their
components are presented individually in Table 2, together
Table 2 Products used in chemical cleaning
with remarks on specific applications and the corresponding reference. It should be stressed once
more that this table aims at presenting the variety of chemicals that have been used for cleaning,
which should not be applied indiscriminately.
Care must be taken because of the toxicity of some chemicals: work must be done with an
extraction hood and safety data sheets for the materials used should be followed and kept readily
available.
Electrochemical cleaning
Ideally this technique should reverse the oxidation process, in which the metal had been
progressively transformed into an ionic form, losing its electrons. To allow it to come back into the
metallic state - to 'reduce' its chemical oxidative state - it is necessary to supply electrons by one of
three basic methods. They are named chemical, galvanic (or electrochemical) and electrolytic
reduction, and are distinguished from one another by the type of electron source.
In the first case, a solution containing a reducing agent, such as sodium dithionite (Na2S2O4) is used
[110]. This product oxidizes easily in alkaline medium and in this way supplies the necessary
electrons to the object. As reducing agents also react with oxygen from the air, precautions must be
taken to eliminate the surrounding air with its oxygen.
In the case of galvanic reduction, a less noble metal, such as aluminium or zinc, is brought in
contact with the silver artefact in a conductive solution. A difference in electrochemical potential
will be established between both metals, the less noble being the anode, or the pole where oxidation
takes place. As it corrodes, electrons are liberated to the surface of the artefact where they can
reduce the silver to a metallic state in a direct way, or indirectly, through hydrogen atoms existing
in an intermediary state, which are attracted to the surface. [111].
In the third method, electrolytic cleaning, the electrons are supplied to the object by an external
source. When using a current generator, such as a battery, the object is connected to the negative
pole (cathode) and an inert counter electrode, such as platinum, stainless steel or graphite, is
connected to the positive pole (anode). A conducting solution (electrolyte) is also necessary to close
the circuit. Given a small current applied between both electrodes, the reduction reactions take place
on the object at the cathode. This method, called galvanostatic cleaning, enables the silver ions to
reduce to metallic silver, but, along with any other reducible ionic species at the cathode,
indiscriminately.
In potentiostatic cleaning a potentiostat provides a more selective way to control what happens at
the object's surface than a current generator. The arrangement requires a third, reference electrode,
enabling one to apply and maintain a well-defined potential to the cathode (object). A preliminary
scanning of a wide range of negative electrode potentials, while controlling the current variations,
gives a so-called 'polarization plot'. This indicates the relative reduction potentials of the different
corrosion products for a particular solution employed (Fig. 10) [39, 112]. This solution should be
chosen so that the desired reaction - the reduction of silver ions - takes place preferentially (more
positive potentials) to other, concurrent reactions. Once these parameters have been defined, it is
possible to set the object's potential to a value corresponding to the desired reaction, and electrons
will be supplied only at the corresponding level. In this manner other reactions that
Fig. 10 Potentiostatic polarization plots of different silver coupons in a NaNO3 0.1M solution: (a)
for a chlorinated plate a reduction peak is observed at c. 0 mV NHE; (b) for a sulfurated coupon the
reduction peak occurs at c. -1000 mV NHE. The area beneath each peak is related to the thickness
of the reduced layer. Scan rate: 2 mV/s.
could disturb the main processes, such as hydrogen evolution, can be prevented, and monitoring the
current gives information about the completion of the reduction of a specific corrosion product.
Due to its fineness, this method has been used for the cleaning of thin coatings on metals, such as
silver on cuprous alloys or gold on silver [105, 113]. Moreover, the treatment can even be used on
small, localized areas if an 'electrolytic pencil' is used [114].
Table 3 presents a summary of electrolytic cleaning methods reported in the literature, together with
the solutions used. In 2001, a comparative study of the effect of several cleaning methods on silver
chloride removal was reported [115].
What really happens during the electrochemical reduction is the transformation of the corrosion
product, which changes from resistant and insoluble, into a finely divided metallic form having no
cohesion. It can then easily either be removed in cleaning or 'glued together' by consolidation
treatment.
Comparison of the cleaning methods
To conclude, Table 4 compares important parameters from each cleaning method discussed above,
including process characteristics, removal of material (both corrosion products and metal), residues
left and appearance after treatment.
Besides these methods, the plasma method was successfully used for cleaning thin tarnish layers on
well-preserved silver artefacts and daguerreotypes [121]. In the case of heavily corroded objects,
where the corrosion layers extend over several tenths of millimetres in depth, there are additional
difficulties with this technique [122].
Table 3 Materials and methods of electrochemical cleaning
Consolidation
Consolidation is widely used in conservation as a means of imparting structural strength to an
object that has deteriorated and is in danger of disintegrating. In a material requiring consolidation,
many of the bonds that hold it together have been broken, to the point where the structure can no
longer support its own weight and retain its shape.
Contrary to cleaning, which is an intervention at a surface level, the consolidation goes deep into
the object, sometimes even through it. Its purposes could be several, from structural to cosmetic:
reconstitution of the object to allow its legibility, to restore its malleability or solidity, to 'restore' it
to become closer to its original form.
If the object has fragmented, gluing is often preferable to soldering for re-assembly of parts [76,
123]. In the case of mineralized objects that are porous and should be joined together for subsequent
mechanical cleaning of accretions, impregnation is commonly used. Usually, adhesives such as
epoxy resin [81, 94, 99], polyester resin [25, 124J, Paraloid B72 (Rohm and Haas) [99], Paraloid
B48 (Rohm and Haas) [77] or cellulose nitrate lacquer [99] are used, sometimes with a tissue
support [25, 81, 95, 124].
In the past, in the case of a compact but nonetheless fragile object, malleability used to be restored
through heat treatment. The examples in the literature are various, from treatment immediately
following excavation using fire and alcohol in a hole in the ground [149], to controlled heating
operations, in some cases in combination with chemical and/or mechanical treatments for reshaping
[34, 61, 65, 16, 81, 100, 125-7].
Currently, however, it has been recognized that heating the object can present certain dangers, not
only because the melting points of the existing compounds are quite different and there is a danger
of melting one of them, but also because heating can obliterate the signs of former work [78]. Even
a simple soldering operation [128] can lead to undesirable grain growth, if the object had been
severely cold-worked, or even artificial ageing and its consequences for the stability of the artefact
Table 4 Comparison of the principal characteristics of cleaning methods
[24]. In addition, heat treatment may damage mercury-gilded silver by causing blisters [129].
Moreover, in cases of severe corrosion where there is no longer coherence between grains because
of the amount of corrosion products, annealing is useless, even in a reductive atmosphere [125]. As
noted in Table 5, electrolytic treatment has also been carried out in several instances.
It is important to remember that corrosion reactions are not truly reversible, because metal has
moved from its original site during corrosion. Furthermore, after a long time in burial conditions,
some corroded parts of the object have already washed away or are in a swollen state, such that the
whole object no longer can be regenerated.
A great contraction can take place during the reduction, and dimensional changes must occur due to
considerable differences between the densities of the materials (pAg = 10.5 g/cm3; pAgCl = 5.6
g/cm3). Although the corrosion products can be transformed into a metallic form, they stay,
however, in a powdery condition and for this reason, in most cases, a further treatment of
consolidation or sintering is carried out in order to hold the particles together.
Conservation
The basic rule, independent of the ambient conditions and the type of protection applied, is that the
object should be kept clean, by preventing accumulation of dust and dirt on the surface. Both, even
if they are not particularly harmful in themselves, act as nuclei that capture other particles and
condense humidity [35].
Protection is the prevention of undesired interactions between the object and its environment and
the consequent repeated cleaning procedures. For protection, it is preferable to reduce the
aggressiveness of the environment by eliminating pollutants and adjusting its RH. Where this is not
possible, a barrier must be created on the object's surface with lacquers, waxes or inhibitors.
Environmental protection
To influence the environment it is essential to know its nature. The three elements recognized as
harmful to silver - reduced sulfur gases, oxidants and RH - should each be monitored and their
synergetic effect considered. The scarce information about the composition of museum air shows
that the internal sources of certain elements could be more significant than the external [36, 54,
132]. Because of this, the presence of materials that produce reduced sulfur gases and their
influence on the degradation of materials leading to corrosive off-gassing must be taken into
account [54, 87, 132, 133]. To identify and quantify chemical pollutants in museums a European
study is in progress at time of writing [134]. The effect of light on the tarnishing of silver has also
been investigated [48] and the results show that relatively short wavelengths (2537 A, ultraviolet)
could be more harmful than longer ones.
The effect of specific materials in contact with silver objects, in display or storage, can be evaluated
through simple tests that can quickly show what materials are most reactive, although such tests
may not indicate reactions in the long term. The usual method of testing the material to be
investigated was to keep it in a closed vessel at 60°C and 100% RH for a period of time, together
with a freshly polished silver coupon [135]. Later, other variants were proposed and tested [136,
137]. More recently, electrochemical methods have been proposed as a more quantitative and rapid
alternative: they are based on polarization resistance tests in an aqueous extract of the investigated
material [138] and tarnish stripping on sensitized coupons [117, 139].
Regarding packing, the objects can be put in sealed polyethylene bags or wrapped in paper or tissue
[83, 87,140]. In this case, silver objects should first be wrapped in several layers of pH-neutral
tissue paper that contains no chemicals, such as corrosion inhibitors or buffers, and no sulfur. Only
then should they be protected by an outer wrapper of special cloth and/or paper, acting by one of
two means. These either contain metallic oxides or salts or fine silver particles, which
Table 5 Materials and methods of consolidative reduction
react preferentially with the sulfur-containing gases, or alternatively they contain activated carbon
to adsorb the gas or compounds such as zinc carbonate or oxide that react with the gases to form
metal sulfides [64,141], Logically, these sulfur-absorbing materials should never be used in direct
contact with silver [84].
During display, the use of adsorbents is recommended: silica gel to reduce the humidity, and
activated carbon [83]. For this, impregnated tissues are preferred to papers [87], which lose their
effectiveness more quickly [84].
Surface coatings
To create a barrier between the object and its environment, the coatings most commonly used are
lacquers, waxes and inhibitors. The degree of protection depends on the uniformity and thickness of
the applied layer. Although other suggestions are found in the literature, such as the use of
passivating films (chromates, beryllium, etc.), that chemically increase the resistance to tarnish [42,
65, 142-144], almost no reference has been made to their use with museum objects [127].
Research to compare the performance of surface coatings has evaluated different properties such as
resistance to tarnishing, resistance to abrasion, facility of application and acceptability of
appearance [145], along with permeability to H2S, H2O and water vapour, elasticity, adhesion and
hardness [146]. Also, tests have been carried out in chambers with specific aggressive environments
- sulfurous, carbonic, saline, thermal shock, UV radiation [115, 147-149].
From all the possible types of organic coatings, only two are currently employed by conservators:
cellulose nitrate and acrylics. Although a lot of research has been undertaken in the industrial field,
few articles concern museum objects, where important aspects, like surface preparation and
removability, dictate a totally different approach [150]. Most authors agree that applying two layers
is advisable, to ensure complete covering [86]. In museums, the two principal problems of the use
of lacquers must be considered: alteration of the object's appearance and imperfect covering of the
surface, after the evaporation of the solvent leaves behind a solid coating.
Cellulose nitrate lacquer is only used indoors, because it decomposes rapidly when directly exposed
to UV radiation [150]. In some cases application by spray results in better film formation than by
brush [149]. When examining examples of successful long-term use of this material, the facts that
support the choice are a reduced (5- to 10-fold) frequency of treatment in comparison to non-treated
museum objects, the ease of application, and improved appearance when compared with other
lacquers [52, 84, 151, 152].
The other type of resin used in metals conservation is acrylic. It is a co-polymer, a mixture of a hard
and rigid component with another, soft and elastic. The various molecular weights of the
components make it possible for them to tolerate weather outdoors relatively well. There are
numerous examples of application in the literature [98, 99,104,109], where it is cited that they do
not degrade to the extent of cellulose nitrate lacquers, and consequently have to be replaced less
frequently [64]. It is also noted that a plasma method was tried to apply polymers, but adhesion
problems were found, attributed to silver diffusion through the organic film [153].
Although their application is easier than that of lacquers [85], waxes are generally considered less
protective, because there is less cohesion between wax and metal. Nevertheless, they are intensively
used by conservators [154] and the preferred forms are microcrystalline [83, 155] and wax from the
carnauba tree [87]. A few comparative tests show a good performance in showcases [117] and even
in some aggressive conditions [147, 148]. However, the ease of scratching or locally rubbing off the
wax coatings, and their tendency to collect dust can be a disadvantage if they are much handled or if
levels of dust are not kept to a minimum.
Corrosion inhibitors are usually organic compounds: mercaptans or thiols. They are long-chain
molecules, which stick to the silver surface at one extremity (sulfur group) and make the surface
hydrophobic by means of a methyl termination at the other extremity. The exact mechanism is
complex and often very specific to a particular metal-inhibitor-environment. Generally, the degree
of protection given depends on the extent of the surface covering, which is a function of the type of
molecule and its affinity for silver. A poor covering is worse than none, because, where small areas
are left exposed, the corrosion reaction will occur much faster than elsewhere.
Different inhibitors may be applied in various ways: as coatings by impregnation or by dipping after
cleaning, or directly in storage and exhibition enclosures by delivery in the vapour phase. The
properties of inhibitors have been evaluated through standard methods, but almost all tests were on
clean surfaces, and their effects on museum objects, especially archaeological objects, are less well
understood [33,156,157].
Among the few references in the literature, benzotriazole (BTA), known to protect cuprous alloys,
is mentioned [70, 145, 158]. It may be adapted with caution to silver objects, and in the case of
silver-plated brass, it might protect the brass in detriment to the silver. Reference to 2mercaptobenzothiazole has been made, but its use has not yet been evaluated [117]. As an
alternative to BTA, PMTA (l-phenyl-5-mercaptotetrazole) has been suggested, for its superior
resistance to UV radiation and better surface covering power [48, 51].
Regarding the numerous possibilities raised several years ago [65], such as morpholine, chlorophyl,
pyridine, cysteamine [159] and other organic inhibitors, no research has yet been undertaken or
published.
Volatile inhibitors (VPI) have been tested in a more systematic way [65,117,160], although
investigations have only examined commercial products. As the effectiveness of all of these has
been demonstrated, their suitability must be determined for each case [160], They can be used in a
confined space where an equilibrium can be reached, with the vapours present in sufficient
concentration to give protection. The protection is only effective while the vapour supply is
sufficient and attention must be paid to the possible harmful consequences to other objects or
people.
Conclusion
This review article was written in the belief that is possible to learn from the experience of others
who, before us, involved themselves in the conservation of the objects that we have inherited.
It has been shown that 'not everything that glitters is silver'. This assessment actually concerns
silver alloyed with different elements, especially copper, and of course each alloy reacts in a
particular way with its environment. Depending on the conditions, a different form of deterioration
(or even a mixture of several forms) will be active. It is therefore fundamental to gather all possible
facts from the literature and by direct examination and analysis of the object, to reach a better
understanding of the particular case under study.
As for the materials and methods employed for the restoration of silver objects, an evolution can be
observed over the years. Nowadays, new chemicals and recipes are used with more care, and
treatments that were often employed in the past, such as heat-treating or cyanide cleaning, are no
longer used, in view of their harmful effect, either for future study of the object or for the restorer.
On the other hand, the fact that 'the more it is cleaned, the more is worn away' from the artefact
itself has been well understood and increasing importance has been given to preventive
conservation, with considerable research about environmental pollutants and possible ways of
minimizing their effects.
Although the evolution in this area has been positive, an important point remains to be developed
further. It concerns co-operative research between restorers and scientists, whose main goal would
not be the discovery of a new procedure or formula, but a real understanding of the consequences of
using given products or applying certain techniques. This step is a fundamental one towards valid
preventive conservation.
Acknowledgements
Thanks to Christopher Augerson for the fruitful discussions and to Pau Gorostiza for the handsome
drawings presenting the deterioration cases. Thanks also to Gilberte Dewanckel and Annick Texier,
who made possible my direct participation in case studies of ancient silver artefacts. I would like to
acknowledge Lyndsie Selwyn, Robert Organ and Francois Schweizer for their careful reading of the
text and very valuable comments.
Thanks to the team from the Gerald Ford Conservation Centre, Omaha, and all participants of the
workshop 'Recent advances in silver conservation" for a very agreeable and profitable week. I am
also grateful to all those that contribute sending an important number of references that constitute
this paper and to those authors who allowed reproduction of their illustrations.
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Author
Virginia Costa qualified as an Engineer in metallurgy. She was appointed Professor by the Federal
University in Porto Alegre (Brazil), specializing in corrosion and protection of metals. She
presented her Ph.D. on surface electrochemistry in 1993 at the Fritz Haber Institute of the Max
Planck Society (Berlin-Germany). Since then she has been applying her scientific and technical
background to give lectures on metals conservation and has been acting as consultant for several
museums in Brazil. At the Laboratoire de Recherche des Monuments Historiques in France she
presently carries out studies on the use of stainless alloys for reinforcement of outdoor sculptures.
She also collaborates with the Centre de Recherche et Restauration des Musees de France (Paris)
and with the Institut Royal du Patrimoine Artistique (Brussels) concerning silver conservation.