Developments in Laser Cleaning
|A Greco-Roman marble head excavated in Shropshire - left, after
laser cleaning with an Nd:YAG laser - and far left, covered
in soil and whitewash. The treatment removed all traces of soil
and whitewash without damaging the marble surface.
is a critical part of the conservation process. It serves not only
to improve the aesthetic appeal of an object or building but also
to reveal its true condition so that appropriate action can be taken
to ensure that it survives for many future generations to enjoy.
recent years there has been increasing concern over some of the more
conventional methods of cleaning used on sculpture and sculptural
decoration on historic buildings. Careless and inappropriate use of
techniques, such as air-abrasive and steam cleaning, can lead to severe
damage of the underlying stone surface. The loss of surface detail
by overthorough cleaning can reduce the visual appeal of a surface
and in extreme cases can even lead to its accelerated decay. Even
if cleaning is carried out very carefully, techniques such as air-abrasive
cleaning will result in some loss of material from a surface, particularly
from a decayed crumbling surface, simply because abrasive particles
cannot discriminate between the soiling and the stone surface. The
removal of black encrustations from limestone sculpture is usually
accompanied by removal of the patina which develops on the surface
over a period of time and within which the original surface relief
is preserved. Chemical-based cleaning techniques also have associated
problems: chemicals often leave residues within the stone which can
cause problems later on and once they have been applied their reaction
cannot be suitably controlled. In Glasgow some sandstone buildings
which were chemically-cleaned a few years ago are turning green at
an alarming rate since ideal conditions for algae growth have been
created on the surface. The development of laser-based techniques
during the past few years has been a significant advance in making
conservation methods less intrusive and more controllable. The fundamental
difference between cleaning with laser radiation and conventional
methods is that particles of light, or photons, can discriminate between
the soiling and substrate. This allows the conservator to control
the level to which the surface is cleaned.
laser is a unique source of light, providing energy in the form of
a very intense, monochromatic (a single colour or wavelength), well-collimated
beam (a typical laser beam spreads out only a few millimetres after
travelling several metres). When a laser beam interacts with a surface,
part of the energy is reflected and the remainder is absorbed (assuming
no transmission). The fraction of energy absorbed depends on the wavelength
of the laser radiation and on the physical and chemical properties
of the surface. A laser beam can have no effect on a surface unless
it is at least partially absorbed.
most common laser used in conservation at the moment is the Q-switched
Nd:YAG laser which provides short pulses (typically 5-10 ns long)
of near infrared radiation at a wavelength of 1.064 mm (or 1.064 x
10-6 m). These are effectively very short pulses of heat. The short
pulse length is important since it prevents heat from being conducted
beneath the soiling into the stone surface. This type of laser is
commonly used since most soiling layers are much more strongly absorbing
than the underlying substrate at 1.064 mm. This means that, provided
cleaning is carried out within safe parameters, once the dirt has
been removed, further pulses will have no effect on the surface as
insufficient energy is absorbed to cause any damage - in other words
the process is self-limiting. The Nd:YAG laser is also extremely reliable,
easy to maintain, relatively compact and robust.
laser cleaning systems have become available during the last three
years and are now being used by conservation studios across Europe.
In a typical system the laser head, power and cooling supplies are
housed in a single portable unit which weighs about 125 kg and runs
off a 13A/240V mains supply. In this case the laser beam is directed
by means of a 7-jointed articulated arm with the beam emerging through
a pen-like handpiece within which a lens is used to produce a diverging
beam. The conservator controls the cleaning effect through adjustments
to the energy in each pulse, the number of pulses fired per second
(repetition rate) and the distance between the tool and the surface
(which controls the intensity or spread of the beam). The maximum
pulse energy and repetition rate varies between systems and a few
systems use an optical fibre rather than an articulated arm to deliver
the beam. Most commercial systems are suitable for work either in
a studio or out onsite.
most important cleaning parameter is the energy density, or fluence,
of the laser beam which is defined as the energy per unit area incident
on the surface (energy per pulse/beam size at the surface) and is
usually measured in joules per square centimetre (J/cm2). When working
the fluence should be high enough to remove the dirt layers but low
enough to ensure that the substrate surface is not damaged. At the
Nd:YAG wavelength there is a safe 'working window' within which this
can be achieved for a wide range of materials. This is the 'self-limiting'
regime of laser cleaning. If the fluence must be raised above the
damage threshold of the substrate in order to remove the soiling then
the process will not be self-limiting and, as is the way with conventional
cleaning methods, the conservator must attempt to stop the process
as soon as the soiling has been removed to prevent any damage.
cleaning occurs by a combination of mechanisms, the relative importance
of each depending on the fluence used and the properties of the soiling.
Since most types of soiling absorb strongly at 1.064 mm, cleaning
can usually be carried out at relatively low fluence (<1 J/cm2)
to minimise any risk of damage to the substrate. Strong absorption
of energy leads to rapid heating and subsequent expansion of a dirt
particle (see figure). Since the pulse length is so short the expansion
happens so quickly that the resultant forces generated are sufficient
to eject the particle from the surface. This is a very selective process.
If the fluence is increased slightly then some material will be heated
to a sufficiently high temperature to cause vaporisation. At higher
fluences still (above approximately 1.5 J/cm2; values depend on the
properties of the soiling) the removal mechanisms become more complex
and involve the formation of a plasma just above the surface and generation
of a shock wave. This mechanism is less selective and can result in
damage to the underlying substrate. Cleaning should therefore be carried
out at the lowest practical fluence so that the more selective mechanisms
representation of dirt removal by
'rapid thermal expansion'. This selective
mechanism appears to dominate
the cleaning process
at low fluence.
can sometimes be used to enhance the cleaning effect. By brushing
or spraying a thin coating of water onto the dirt surface immediately
prior to irradiation, stubborn deposits of dirt can be removed without
having to increase the fluence to unacceptably high levels. Dirt particles
become coated with a thin film of water which is also able to penetrate
into cracks and pores within the dirt layer. Absorption of the laser
beam by the dirt layer occurs as normal and rapid heating at the dirt/water
interface leads to explosive vaporisation of the water molecules,
which exerts forces on and within the dirt layer sufficient to eject
further material from the surface. The addition of water usually increases
the cleaning rate significantly.
main advantages of laser cleaning are:
Provided cleaning is carried out within suitable parameters it is
possible to remove layers of dirt without removing any original
material from the surface of the object. Such control allows the
conservator to select exactly what is removed from a surface and
also allows him or her to go back over an area which has already
been cleaned to remove remnants of dirt without over-cleaning. The
technique is sensitive enough to preserve the surface relief; original
tool markings can be uncovered and delicate patinas left intact.
Since energy is delivered in the form of light there is no mechanical
contact with the surface. This allows extremely fragile surfaces
to be worked on.
The laser cleans only where directed. A single laser can supply
a beam with a diameter variable between a fraction of a millimetre
and one centimetre, allowing the same tool to be used for both extremely
precise and relatively large-scale work.
control and feedback
The cleaning action is instantly halted once the laser is switched
off so the conservator can stop the process whenever he or she decides.
The condition of the surface can be continuously monitored by the
conservator during cleaning, allowing decisions to be made at the
earliest possible stage.
Laser cleaning generates very small quantities of waste material
(of the order 100 g/m2 for a uniform black soiling approximately
0.1 mm thick on outdoor limestone). The only waste generated is
the dirt ejected from the surface which is straightforward to collect
and dispose of using efficient extraction systems. There is no use
of hazardous chemicals or solvents and the only protective clothing
necessary is safety spectacles and a face mask. Laser cleaning is
a clean and quiet technique which causes minimum disruption.
Laser radiation at 1.06 Ám has successfully been used to remove
dirt and other coatings from a wide range of materials including:
marble, limestones, sandstones, terracotta, alabaster, plaster,
aluminium, bone, ivory and vellum. In some cases the availability
of radiation at other wavelengths can increase the flexibility of
the tool, for example in the removal of some types of organic growth.
As lasers have very few moving parts, they are also extremely reliable.
laser described in this article has been designed specifically for
work on sculpture and sculptural detail on buildings. More powerful
laser systems capable of cleaning approximately ten times faster are
available and would be more suitable for larger scale cleaning.
cleaning does not work on everything. The cleaning of polychrome sculpture
poses problems since different pigments absorb different amounts of
radiation, certain types being very sensitive. For example, a single
low-energy pulse will be sufficient to turn vermilion from red to
black. In cases where there is evidence of pigment on a stone surface
cleaning is usually carried out in such a way that the area is not
exposed to laser radiation, unless it is known to be stable at the
fluence being used.
laser cleaning of sculpture is usually much quicker than cleaning
by the more sensitive conventional techniques, the large scale laser-cleaning
of buildings cannot, at the moment, compete in terms of speed with
techniques such as grit-blasting. It does however leave the stone
surface intact. The development of laser systems is so rapid that
it might not be too long before large-scale laser cleaning systems
relatively high initial cost of purchasing a laser system is seen
by some as a disadvantage. This should be set against the low cost
of maintenance and the savings that are made on time taken to complete
a job. Purchasing a laser cleaning system is a long term investment.
In the short term it might make more sense to hire an appropriate
system for a particular job. Training courses are available which
will teach the conservator when and how to use a laser.
the past three years interest in lasers for cleaning has increased
rapidly culminating in LACONA I, the first international conference
on Lasers for the Conservation of Artworks which was held in Crete
in October 1995. The conference brought together scientists, engineers
and conservators to discuss current work (which includes the use of
many different types of laser) and future developments in this rapidly
expanding field. LACONA II will be held in Liverpool in April 1997.
- JF Asmus et al, 'Surface morphology of laser-cleaned
stone', Lithoclastia, vol 1, 1976
- V Verges-Belmin et al, 'Elimination des croutes noires sur
marbre et craie: a quel niveau arreter le nettoyage?', Conservation
of Stone and other Materials, MJ Thiel (ed), Proceedings of
the International RILEM/UNESCO Congress, Paris 29 June - 1 July
- MI Cooper et al, 'Characterisation of laser cleaning
of limestone', Optics and Laser Technology, vol 27, no 1, 1995
- MI Cooper and JH Larson, 'The use of laser cleaning to preserve patina
on marble sculpture', The Conservator, vol 20, 1996
- J Wilson and JFB Hawkes, Optoelectronics: An Introduction, Prentice-Hall, London, 1983
- AC Tam et al, 'Laser-cleaning
techniques for removal of surface particulates', Journal of Applied
Physics, vol 71, no 7, 1992
article is reproduced from The Building Conservation Directory, 1997
COOPER completed a PhD at Loughborough, entitled 'Laser
Cleaning of Stone Sculpture'. This work led to the development
of the laser cleaning system currently being used at the National Museums and Galleries on Merseyside. He has also been employed as a Leverhulme Research Fellow at NMGM, continuing research and development in laser cleaning.
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