CHAPTER 2
Methods in Conservation
A. ELENA CHAROLA1 AND ROBERT J. KOESTLER2
1 Scientific Advisor, Program Coordinator, World Monuments Fund-Portugal,
Mosteiro dos Jerónimos, Praca do Império, 1400–206 Lisbon, Portugal
2 Director, Museum Conservation Institute, The Smithsonian Institution,
Museum Support Center, 4210 Silver Hill Road, Suitland, MD 20746 2863, USA
1 INTRODUCTION
Conservation is an interdisciplinary field that needs to take into account such
disciplines as the humanities, the arts, the sciences and technology, and crafts.
Although the actual conservation intervention is, or should be, carried out by
trained conservators, the degree of that intervention cannot be decided by the
conservator alone. The reason for this is that conservation is not merely a
technical operation on a cultural object. Rather, it is a cultural operation 
carried out by technical means.
Thus, when a conservation intervention is being planned on an object, it is
important that as much information as possible about that object be available to
the conservator. For example, this includes the anamnesis of the object, i.e. its
history, starting with its provenance, its historical context, including the avail-
able technology and materials available at the time the particular object was
created, its subsequent use, and the conditions under which it was kept there. 
It is clear, therefore, that the contribution of historians and art historians is
critical for conservation. Furthermore, the study of the object, its constituent
materials and any deterioration process that may be affecting it, requires the
contribution of the sciences to effectively identify any changes and decay it
may have suffered. Based upon the scientific information acquired, an appro-
priate conservation method can then be devised.
2 PRELIMINARY EXAMINATION
Any conservation intervention needs to follow a logical procedure. This starts
with the visual assessment and the compilation of all the relevant historical
data available, including its recent history and information on any previous
conservation intervention. Then a diagnosis as to the state of conservation of
the object is required. Is the object sound? Does it suffer from any deteriora-
tion? If so, what are the causes? These questions, as well as the identification
of the constituent materials require the support from various analytical tech-
niques that are detailed below. 
The results from these analyses serve various purposes. In the first place, they
have to correlate with the historic information available. For example, was the
pigment identified on a ceramic or on a painting available at the time and place
the object was supposed to have been manufactured? Was the technology used
in its manufacture consistent with the available information regarding the cul-
ture that supposedly manufactured the object? Second, is the object in question
in a sound condition or does it show some deterioration? If so, is that deteriora-
tion active or is it the remnant of a previous decay process that has now ceased? 
These are fundamental questions that need to be addressed so as to determine
if a conservation treatment is required or not; and should a treatment be neces-
sary, the most appropriate method and material need to be determined. Finally,
and as a result of this preliminary examination and study, a condition report needs
to be prepared including any recommendations regarding subsequent treat-
ment(s) or the condition(s) under which the object should be stored or displayed.
3 ANALYTICAL METHODS
The examination of an object starts first of all with a visual examination aided by
the use of loupes and microscopes. Then, if necessary, other analytical techniques
may be employed to study particular aspects of the object. There is no single ana-
lytical method that will provide all the answers needed. Depending on the object
and the problem to be studied, different techniques need to be used. In many
cases, two or more techniques have to be used to confirm the data obtained. 
It is also important to remember that analytical techniques have varying
degrees of sensitivity when it comes to detecting the presence of an element
or compound. Thus, not finding a particular element may not necessarily
mean that this element is not present in the sample, rather it may be that the
technique used does not have the required sensitivity to detect it at a low 
concentration. The sensitivity of a method depends both on the method and
element or compound to be detected. In many cases, it will also depend on the
“matrix”, i.e. the other compounds with which it is associated within the object.
The most useful techniques for conservation purposes are those that will iden-
tify the presence of a specific element, compound, or class of compounds, thus
providing a qualitative analysis of the sample. In many cases, this analysis could
give an idea of the relative concentrations of the elements or compounds found.
However, as mentioned above, this is only an approximation and unless certain
standards are used, the result can only be considered as semi-quantitative.
14 Chapter 2
With the widespread computerisation of analytical instruments, results are
often expressed as percentages, and, since a computer processes them, they
are in most cases accepted as valid results by operators who do not have a
thorough training in analytical techniques. It is therefore important to high-
light that the interpretation of analytical results requires training and experi-
ence to be performed reliably. In particular, when dealing with archaeological
and/or museum objects, which in general are unique, extensive experience is
extremely important as the results obtained have to be interpreted in view of
the historical information available about the object.
Most analytical techniques are based on the following principles: interac-
tion of radiation with matter (such as Radiography, X-ray Diffraction (XRD),
X-ray Fluorescence (XRF) and Fourier transform infrared spectrometers
(FTIR)), or, interaction of elemental particles with matter (such as scanning
electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS),
Thermoluminescent dating (TL) and Radiocarbon dating). More detailed
information on some of the techniques described below and other special
methods (such as UV and IR photography, gas chromatography and mass
spectrometry) can be found elsewhere (Ferretti, 1993; Parrini, 1986; Sinclair
et al., 1997; Ainsworth, 1982).
3.1 Interaction of Radiation with Matter
Electromagnetic radiation has a broad spectrum, ranging from X-rays at high
energies (short wavelength) to low-energy infrared light (long wavelength).
The high-energy X-rays are capable of penetrating through solid bodies (hence,
the need to be extremely cautious when using this radiation and the import-
ance of having a good shielding system to protect operators). Since different
materials have different densities, they will allow more or less radiation to go
through. This is the principle used in radiography.
The high-energy X-rays when passing through matter are able to excite the
electrons of the inner shells of an atom. When the excited electrons fall back
to their original position they release the energy absorbed. This can be meas-
ured and is the principle used in X-ray fluorescence.
Radiation, when passing through an object that has “openings”, that is, in
the same order of magnitude as its wavelength(s), is subject to the physical
phenomenon of diffraction. This is the result of interference, i.e. that some
wavelengths will be enhanced and other cancelled. When dealing with visible
light, this phenomenon is the one that gives rise to the different colours visi-
ble, for example, when a thin oil film forms above water, the colour of an opal
or some butterfly wings. Since X-rays have much smaller wavelengths, the
spacing required to produce diffraction are found in the crystal lattice of
materials. This principle is used for the characterization of materials in X-ray
diffraction.
Methods in Conservation 15
When dealing with low-energy infrared radiation, the interaction with mat-
ter is limited to the absorption of light by the outer shell electrons, i.e. those
used in forming compounds. Hence, particular bonds will absorb particular
wavelengths. This is the principle used for infrared spectroscopy. There are
equivalent techniques for ultraviolet radiation and visible radiation, but they
are mostly used to provide information about concentration of a given com-
pound, rather than for identification purposes such as XRF or IR techniques.
Radiography. Radiography of the object is based on the capacity of X-rays
to pass through most solid objects and to blacken a photographic film. The
amount of X-rays of a given energy passing through an object will be depend-
ent on the thickness and density of the material of the object. In general, the
X-ray source is placed above the object and this, in turn over a photographic
film. Usually, an X-ray tube serves as the radiation source whose energy can be
controlled by the voltage applied to the tube, to produce the radiation. This will
vary from 5 to 30 kV for drawings and paintings on wood, to between 250 and
1000 kV for large bronze and stone statues. The other variable that can be con-
trolled is exposure time. Optimizing the conditions to obtain a clear radiograph
of an object is essentially an empirical operation (see Figure 1). Radiography
will allow one to understand much of an objects manufacturing technology as
well as allowing one to see any previous structural interventions.
X-Ray Fluorescence. XRF will only provide information about the elemen-
tal composition of a sample (see Figure 2). This will be mostly limited to the
16 Chapter 2
Figure 1 X-ray radiograph of sixteenth century iron shot
surface of the object. The analysis is carried out in an air-path, or sometimes
in a helium-flushed environment. One of the advantages of this technique is
that it can be performed on large objects without sampling. However, the weak-
nesses are that, in an air-path, elemental detection is limited to the elements
silicon and those with higher atomic numbers. Given that several layers can be
present on the object, the depth of penetration of the X-ray beam may cause
elemental information to be collected from different layers. In particular, this
hampers quantitative analysis, the results being at best semi-quantitative if
standard materials of similar composition are available for comparison. The
detection limit for this method can be in the order of tens of ppm (
g/g).
X-Ray Diffraction. When monochromatic X-rays impinge on a crystalline
material in which the crystal lattice dimensions are in the order of the wave-
length of the X-rays, diffraction of the beam occurs. This is the result of the
physical phenomenon of constructive (or destructive) interference. As a
result, a diffraction pattern emerges where some beams are reinforced and
other cancelled (see Figure 3).
XRD is generally carried out on finely powdered materials that are spread
uniformly on an amorphous silica petrographic slide that is inserted into an
isolated unit and then irradiated by monochromatic X-rays (in general the
CuK radiation). The detector is rotated with regards to the impinging X-ray
beam, at a given speed. The diffractogram obtained is a plot of the intensity of
the diffracted X-rays as a function of the angle (2) formed by the detector and
the impinging X-ray beam. The finely powdered material, randomly oriented
on the slide ensures statistical probability of obtaining a correct diffraction pat-
tern of the material. The pattern obtained is compared with those of standard
Methods in Conservation 17
Figure 2 XRF spectrum of heavy metal coating of glass
materials. Sensitivity of the method depends on the nature of the material and
ranges between 1 and 25% by weight, an average value being around 5%.
XRD is used for the identification of crystalline materials such as pigments,
metal powders, organic materials and salts. Non-crystalline materials lacking
a regular crystal lattice, such as glass, do not produce a clear pattern.
Infrared Spectroscopy. Infrared radiation is found between the higher energy
(and shorter wavelength) visible light and the lower energy (and longer wave-
length) of radio waves. When infrared light falls upon a compound it will
interact with the outer shell electrons of a molecule, the ones that bond the
atoms together to form that molecule. Thus, the technique can identify dif-
ferent types of bonding arrangements between atoms. Since bonds are not
fixed in space but vibrate by stretching (distance between the atoms changes)
or bending (the angle formed by the bond between the atoms varies), each type
of movement will absorb a specific energy. The infrared absorption spectra
produced can be divided into two regions, the group frequency region and the
fingerprint region.
The group frequency region falls approximately between 4000 to 1400 cm1,
and the absorption bands in it may be assigned to vibration of pairs of two (or
sometimes three) atoms. The frequency is characteristic of the masses of the
atoms involved and the nature of their bond, ignoring the rest of the molecule.
Therefore, IR spectra are useful for determining the presence of functional
groups in organic compounds: alcohols (OH), ketones (CO), amines
18 Chapter 2
Figure 3 XRD Diffraction pattern of forms of calcium carbonate
(NH2), etc. As a result, correlation charts tabulating the vibrational frequen-
cies or absorption bands of the various functional groups are used.
In the fingerprint region (approximately 400–1400 cm1) the absorption
bands correspond to the vibrations of the molecule as a whole, and are there-
fore characteristic for each molecule, and can serve as unambiguous identifi-
cation by comparison with a standard. However, long polymers of a given
family, such as long chains of fatty acids can give almost identical spectra.
The technology in infrared analysis has been improved significantly by the
use of FTIR, which has increased the sensitivity of this method significantly
(see Figure 4).
3.2 Interaction of Elemental Particles with Matter
Materials are made up of chemical compounds that in turn are formed by atoms.
Atoms have essentially a dense nucleus around which a cloud of electrons
balances the electrical charge of the nucleus. If high-energy electrons are shot
at matter, some will bounce off the surface, some will penetrate into it produ-
cing X-rays, while others will interact with the higher energy electrons (inner
shell electrons) of the atoms. These are the principles used for SEM, EDS and
wavelength dispersive X-ray spectroscopy (WDS).
Scanning Electron Microscopy. A useful tool for investigation of the surface
appearance of materials and any changes after treatment is the SEM. This tool is
widely available in universities and some art museums. The principle of
Methods in Conservation 19
1800.0 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 650.0
0.020
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10
0.11
0.12
0.13
0.14
0.15
0.16
0.17
0.18
0.19
0.20
0.21
0.22
cm-1
A
Fresh Oak
ML3 Oak
1590.14
1502.33
1456.69
1417.54
1325.99
1265.45
1220.66
1119.36
1025.57
1728.99
1634.51
1594.52
1504.28
1456.80
1422.13
1370.79
1326.34
1234.74
1154.39
1032.00
895.77
Figure 4 FTIR spectrum of fresh oak and ancient oak (ML3)
operation is to generate an electron beam, with a range of 1–30 kV, at the top
of a high-vacuum column. An electro-magnetic lens focuses the beam into a
fine-spot size that then impinges onto the surface of the object to be exam-
ined. The unique feature of the SEM is that it has a set of scanning coils that
drives the beam across the surface of the object, left to right, in a repeating
fashion, similar to reading a book. As the beam interacts with the surface of
the object different signals are generated from the sample surface. These sur-
face interactions are collected by appropriate detectors, as the beam scans the
surface, and are amplified and displayed on monitors (for a very readable
introduction to SEM, see Postek et al., 1980).
The main image generated by SEM is a surface image, generated from low-
energy electrons (called secondary electrons), that are knocked out of the sur-
face by the primary, high energy, beam. A black and white, 3-dimensional image
of the surface is generated and presented on a cathode ray tube or computer
monitor. Analysis of the surface morphology can produce much useful informa-
tion (see Figure 5).
Another useful method that the SEM provides is that of viewing the surface
of the sample with backscattered electrons. These are higher energy electrons
from the primary beam that have been “bounced back” or scattered from the
surface of the sample. The amount of back-scattered electrons depends on
the atomic or molecular weight of the sample. The higher the atomic weight
the higher the quantity of back-scattered electrons and the greater the contrast
in image generated among elements of different atomic weight. Information
collected in this mode can generate elemental distribution maps of the sam-
ple surface. Figure 6 compares two SEM photomicrographs of a corroded
20 Chapter 2
Figure 5 SEM photomicrograph of wood surface colonised by bacteria
iron bolt; one image produced by secondary electrons (SEI) and the other by
back-scattered electrons.
Energy Dispersive X-ray Spectroscopy and Wavelength Dispersive Spec-
troscopy. Another useful signal collected from interactions of the primary
beam with the sample surface are the X-rays generated within the sample.
These can be analyzed both as a function of their energy (EDS) or their wave-
length (WDS). EDS collects a range of energy signals, typically in the
0–10 KeV or 0–20 KeV range with a multi-channel SiLi detector. Each elem-
ent in the sample produces a characteristic energy that can be collected and
displayed on a monitor (see Figure 7).
EDS is a fast technique that can produce qualitative elemental informa-
tion within a few minutes. Although the computer will process results in a
Methods in Conservation 21
(a)
(b)
Figure 6 Scanning electron micrograph of a corroded iron bolt (a) SEI and 
(b) back-scattered electron image
quantitative manner, care must be taken in its interpretation, as it may be
indicative but not definitive. Some samples, e.g. polished metals, may yield
quantitative results whilst powdery samples will be indicative of relative 
concentration at best. The sensitivity of this technique is typically in the
0.1–0.5% range, depending upon element and matrix.
WDS information is collected with a specific crystal detector that is rotated
through the range where each specific element’s diffraction wavelength is
found. Different crystals are also used for different wavelength ranges and the
geometry of the primary beam, sample and crystal are important for proper
detection and collection of the information. WDS is much slower than EDS,
but is generally one order of magnitude more sensitive than EDS. As a result,
both techniques can nicely complement each other, as they have different
22 Chapter 2
Figure 7 SEM and EDS of an iron bar
strengths and weaknesses, to produce a more accurate composition of the sam-
ple. Elemental maps may also be generated from EDS or WDS data. Figure 8
compares dot mapping with regular SEM images for inclusions in glass.
3.3 Dating Methods
The dating of an object is critical both from an archaeological point of view
and from a museum point of view. In the first case, it will allow a date to be
attributed to a settlement, or provide a chronological sequence in an excav-
ation. In the second case, it may confirm or refute the assigned provenance of
the object. Unfortunately, there is no single dating technique applicable to all
materials. For example, obsidian objects can be dated by the thickness of the
hydration layer that forms on this volcanic glass once an object has been
fashioned out of it. Wood can often be dated by dendrochronology, which
Methods in Conservation 23
Figure 8 SEM and dot mapping of inclusions in glass
counts the number of rings that trees develop with growth. Apart from the
radiocarbon dating discussed below, there are several radioactive methods for
dating other elements (see Dijkstra and Mosk, 1981). Each of the available
dating techniques is limited to only some types of materials, and all of them are
complex techniques that require much experience to be able to assess the data
produced.
Radiocarbon 14 dating. Radiocarbon dating uses the physical and biologi-
cal phenomenon of incorporation of the radioactive 14C isotope into the bios-
phere (all living plants and animals; see Bowman, 1990). This isotope is
continually formed in the upper atmosphere where it is rapidly converted into
“marked” carbon dioxide 14CO2. This in turn mixes within the atmosphere with
the “normal” carbon dioxide (mostly 12CO2) and in waters and is eventually
utilized by living organisms and incorporated into their tissue. The ratio between
these two isotopes is about 1:1012. The activity associated with the 14C in this
equilibrium is about 15 disintegrations/minute. 
Once an animal or plant dies, it ceases to accumulate CO2 and the level of
14C slowly decays as it disintegrates by ß-emission. It takes approximately
5600 years for the number of 14C atoms to decay by one half; this is called the
half-life of the isotope. This rate of disintegration is directly proportional to 
the number of radioactive atoms present. Since the decay constant for 14C is
known, by measuring the decay rate of the object at the present time (and
knowing the original decay rate) it is possible to calculate the age of the object.
Dateable materials are the organic matters that includes pollen, bone, char-
coal, shell and many animal products and remains. There are a number of
assumptions that go into dating and are best left to experts in the field to take
into account when trying to determine a 14C date for any material.
Thermoluminescent dating (TL). This technique is useful for dating pottery
and ceramics (Wagner et al., 1983). It is based on the cumulative effect of
radiation from disintegrating radioactive isotopes present in minerals of most
rocks, clays and soils. The ionizing radiation may cause electrons to detach
from their parent atom and become trapped in lattice defects of the material.
As the material ages, defects accumulate. When a ceramic object is fired,
trapped electrons are freed and, since radioactive minerals are incorporated in
the object, defects start to accumulate again.
TL uses this same principle to date the object. A small powdered sample of
the object is heated and by measuring the emission of light in excess of the
incandescent glow that is produced the date of the firing can be calculated.
The technique requires calibration of each particular sample, since every clay
particle will accumulate defects at a different rate. Although the operational
principle of the method appears simple, it requires experience to interpret the
data correctly.
24 Chapter 2
4 DIAGNOSIS OF DETERIORATION PROCESSES
During the examination of the object it is important to assess its condition
and determine whether it has deteriorated over time. If so, what were the
causes that led to this change and is it still occurring?
For museum objects, in many cases the deterioration occurred prior to its
acquisition by the collection; however, some deterioration agents may have
entered into the body of the object, become dormant and continue to be a prob-
lem. Such is the case with archaeological ceramics that have been contaminated
with soluble salts, or of wooden objects that bring with them insect infestation.
Although in some cases it may appear that the deterioration is not progressing,
this may re-appear at any moment prompted by a change in environmental con-
ditions (temperature and relative humidity (RH)). The detection of these possi-
ble dormant deterioration factors is critical since it will allow a conservation
treatment to be designed specifically to address them. For example, some
potassium glasses are susceptible to changes in RH, around the 40% level. If
the RH varies here, cracking and crizzling may occur. Some modern materials,
e.g. Masonite®, absorb moisture very rapidly, but release it slowly. This means
that transient spikes in humidity may lead to moisture retention by a material,
with subsequent fungal growth.
5 CONSERVATION TREATMENTS
Conservation treatments can be divided broadly into four main categories:
cleaning, desalination, consolidation and disinfestations. Within each of
these categories there can be several sub-categories for the different cases and
different materials that have to be addressed, such as the de-acidification of
paper. The following is an attempt to give a broad picture of the aim of these
treatments and the problems that need to be considered. Three principles should
guide any conservation intervention:
● Reversibility or retreatability
● Compatibility
● Minimum intervention
The concept of a “reversible” treatment was suggested by the traditional
periodic removal and re-application of varnishes to oil paintings. However,
this cannot really be extended to all objects. For example, an archaeological
object may have an incrustation resulting from its long burial. Removal of this
incrustation will result in the loss of information about the type of incrus-
tation and the mechanism by which it was formed; information that may be
crucial from an archaeological or historical point of view. Furthermore, this
process is not reversible. The decision to remove the incrustation is based upon
Methods in Conservation 25
a judgment of values, e.g. the value of the object itself is more important than
its history; hence, part of the historical value is sacrificed as a function of the
aesthetic value of the object.
A further example is furnished by the case where a fragile object requires
consolidation. How can one expect that this treatment will be “reversible”,
i.e. that it can be completely removed without risk of stressing the object 
even more by this removal? These considerations have led to replacing the
“reversibility” concept with that of “retreatability” (see Teutonico et al., 1997).
In this case, any treatment applied should not preclude or hinder any future
treatment. An example of a totally irreversible treatment is the complete
impregnation of stone statues with in-situ polymerized methyl methacrylate.
The concept of compatibility asserts that any material applied to an object
should be compatible with it, i.e. its properties should be as close as possible to
eliminate future stresses. The problems that appear when different materials are
put together in an object are illustrated by many of the museum objects that pres-
ent conservation problems today. Consider, for example, an object that has a
metal inlay in wood, as compared to stone inlay in stone. The latter has far fewer
problems than the former, precisely because their properties, such as expansion
coefficients upon heating or during changes in relative humidity, are similar.
Finally, the minimum intervention concept is the most important. This is
because, as discussed above, any action taken on an object is irreversible, as time
is irreversible. Any treatment applied changes the object and will interfere or pre-
clude other analyses that one might want to do in the future. This is particularly
important given the fast development of more sophisticated analytical techniques
that can provide data that had not even been envisaged only 50 years ago.
5.1 Cleaning
This is probably the most frequent treatment performed on museum objects.
It can range from removal of loosely-deposited surface dust to that of hard,
adherent concretions. In general, it is considered as a simple and self-evident
task and consequently not much thought is given to it. However, it is not a
simple task and, in many cases, difficult decisions have to be made about how
it has to be performed. If a metal object has a heavy calcareous incrustation
covering most of it, would removal of this incrustation, which may result in
the removal of the original patina the object had, be acceptable? Loss of the
original patina means that part of the information that patina carried (How was
that patina formed? Was it natural? Was it the result of an intentional treatment?)
would be lost, and with it, part of the authenticity of the object.
If the case presented by a metal object is difficult, it is even more difficult
in the case of a stone object, because the concept of “patina” is yet to be gen-
erally accepted for this material. In some cases, for example some porous
26 Chapter 2
limestones, a hardening of the surface will result from their exposure to an
unpolluted environment, referred to as “calcin” by the French. Some ferrugi-
nous sandstones may develop a thin, black surface layer (not to be mistaken
with a black gypsum crust) when exposed outdoors, through migration of
iron and manganese oxides to the surface. These can be considered “natural”
patinas equivalent to those formed by metals such as aluminum and copper.
On the other hand, there are man-applied patinas, such as the “scialbature”
applied to marble sculptures and, in some cases, corresponding with the 
formation of an oxalate surface deposit.
At heart of the matter is to what degree should the object be cleaned? Does
it have to look new? Or should it show its age? The current school of thought
is that it should show its age. But this was not always the case. A good exam-
ple is provided by the Elgin marbles – the marble sculptures acquired by Lord
Elgin from the Parthenon in 1801–1802. These sculptures still had traces of
polychromy when they were acquired; however, at the time, the perceived “clas-
sical” marble sculpture was white. Hence, the sculptures were cleaned down to
the bare marble (Oddy, 2002). This clearly emphasizes the point that conserva-
tion is a cultural operation carried out by technical means. The conservator
has to use his judgment in assessing the degree of cleaning required and this
needs to be confirmed in conjunction with the historians and art historians.
Cleaning techniques may range from a simple dusting with a soft brush, to
careful removal of light deposits with aid of a scalpel, or in some instances
using spit on a cotton swap to effectively remove the deposit, with the enzymes
in the spit providing a cleaning action. For harder deposits, microabrasive tech-
niques can be used, or, in some cases, poultices may be applied.
5.2 Desalination
If the object contains potentially damaging soluble salts, as is the case for
most archaeological ceramics, the approach to their removal will depend on
the amount of salt present in the object. If it contains only a relatively low
amount of them, then brushing off (or vacuuming off) the efflorescence may
be the most effective method. However, if the object contains a large amount
of them, then either poulticing or successive baths in distilled or de-ionized
water may have to be the procedure used. These approaches need extra care,
since successive poulticing may affect the surface finish on the object and, in
the case of desalination by water immersion, the thoroughly water-soaked
material will be far more delicate to handle.
5.3 Consolidation
When an object turns fragile because of its age and the consequent deterior-
ation of the material, as is the case of textiles, papers and many objects from
Methods in Conservation 27
natural history collections; or, because of the deterioration suffered by insect
infestation or the presence of soluble salts, the object needs to be consolidated.
There are many products and procedures for carrying out a consolidation
treatment, but they all involve introducing another material into the object. As
discussed above, the ideal “reversible” treatment does not exist. Furthermore,
once the treatment is introduced it may interfere with future objectives. For
example, previously buried bones in natural history collections can be very
friable and have been consolidated systematically to allow their presentation.
This, however, precludes that they can be dated after treatment (especially if
organic resins have been used), since more carbon has been introduced in the
sample. It may also, preclude analysis of DNA, or other biological molecules.
5.4 Disinfestation
The problem of insect and microbial infestation of art is centuries old.
Techniques to control it have included such things as herbal treatments, fire
smoke and, most recently, chemicals. All have provided some degree of
effectiveness if not complete control of the pests, but too often the treatment,
whilst meant to save the art, has created damage of its own. A recent approach
developed at the Metropolitan Museum of Art, and other institutions, is a
non-chemical treatment using oxygen-free environments. The best choice for
generating an oxygen-free environment is to use argon gas. This gas is heav-
ier than oxygen so that it sinks to the bottom of the enclosure or bag forcing
oxygen out of the object and pushing it to the top of the bag. This approach
has proven very successful for insect control and also kills some species of
fungi, if the residual oxygen level is low enough (but has no effect on spores).
The concept of anoxic treatment is simple, and is the same for any anoxic
gas used. It consists, essentially, of the following three steps:
1. Isolate the object from the oxygen-rich environment;
2. Replace the oxygen-rich air with an anoxic (oxygen-less) air to the
desired residual oxygen level; and
3. Wait until the insects die and then remove the object from its anoxic
environment.
Whilst simple in concept, each step requires an understanding of environ-
mental, physical and biological factors that may affect the procedure. Perhaps
the most important of the three steps is isolation of the objects. Isolating an
object requires construction of a suitable barrier around the object. This
means any enclosure system must successfully maintain such a low level of
oxygen for extended periods of time, ideally with a minimum of intervention
and cost. The enclosure may be hard-walled or soft-walled. The soft-walled,
e.g. heat-sealable oxygen barrier film, gives the flexibility to create any sized
28 Chapter 2
enclosure specific to the need and is very portable – it permits an enclosure to
be constructed on-site, reducing potential damage and costs of shipping.
How low an oxygen environment and for how long must it be held? To
determine how long insect-infested objects must remain in a given argon
environment has been determined by actual measurement of insect respir-
ation before and after treatment by use of a gas cell FTIR system to measure
the CO2 produced by insects or fungi. Using this system it is possible to
detect the presence of one insect in a 10-L bag within 4 h. A treatment length
of 3–4 weeks, at less than 500 ppm (0.05%) oxygen in argon, is the recom-
mended time (depending on insect species, life stage, size of object, and tem-
perature and humidity conditions).
6 PREVENTIVE CONSERVATION
Following the considerations of the problems presented by conservation treat-
ments, the ideal would be that these should not be necessary or are only
required in a minimum of cases. From this consideration the idea of “preventive
conservation”, that is, prevent the need of a conservation treatment, was born.
The concept of preventive conservation is to try and maintain, and monitor,
the most appropriate environmental conditions around the art so as to reduce
stresses on the objects and preserve them with no, or minimum treatment. For
insect control, this is referred to as integrated pest management (IPM), but the
principals of IPM can be extended to control of other problems in storage of art.
IPM is basically good house-keeping: keep moisture away from materials –
whether in the form of liquid water or high humidity; keep food, plant and ani-
mal products isolated from the art; keep an airflow around the objects to reduce
the risk of moisture buildup; keep the environment clean. All objects are not the
same and some may require special storage or display conditions, for example,
a potassium-rich glass may “crizzle” if the humidity oscillates above and below
40% RH, whereas a soda-rich glass may not show any response to the change.
7 CONCLUSIONS
It is clear, from the brief discussion of but a few of the methods used in con-
servation that this is a large, interdisciplinary field. While the contribution of
curators, art historians and scientists is fundamental, the conservator probably
has the most difficult task: actually working on the object. This is an enormous
responsibility and requires that the conservator acknowledges it as such. The
key approach in conservation is respect for the object to be conserved. This
requires that the conservator be a good observer, a talented craftsman and
endowed with the clear discernment required to determine the exact point to
which any procedure should be carried to.
Methods in Conservation 29
What contributes to the complexity of the conservation field is the juxta-
position of the rigor needed in the methods used for analysis to the varying
approaches that conservation requires, depending on time and location. Thus,
while analytical methods rely on the use of standards against which measure-
ments can be compared, conservation definitely does not have such standards.
Furthermore, conservation relies heavily on the crafts component – a point that
is being forgotten as the application of science has gained terrain in conserva-
tion. But science can never, nor should it, replace conservation. The contribu-
tion of science, with regard to conservation, is to help understand deterioration
mechanisms that affect works of art and other cultural objects. This will certainly
aid in diminishing their deterioration rate and assist in devising better preventive
conservation approaches and better conservation methods, but the applications
of these methods will still depend on the sense and sensibility of the conservator.
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Methods in Conservation 31