20 Apr Laser Stone Cleaning, An overview
Spencer Hall ACR
PAYE Conservation have recently completed laser cleaning trials with the assistance of Dr Martin Cooper of Lynton Lasers to two differing projects with great success, these being St Georges Cathedral, Southwark and The National Gallery, Trafalgar Square.
What does it do? How can we use it?
Laser can be used to clean carbonation/sulphation from a variety of substrates (limestone, sandstone, marble, terracotta etc) in a very controlled manner. It is most suited to working on delicate material in a workshop environment, but can be used successfully on site. There are however limitations, due to scale of operation and in turn cost effective nature. Trials should always be encouraged to establish suitability and are influenced by nature of substrate, the level of soiling and other environmental factors.
Hand held units now make it far more portable (see images below). All units however run off a 240V supply (110V version is not currently available, but special dispensation can normally be gained for use as a ‘specialist’ piece of equipment – RCD breaker must of course be used etc).
Safety issues are paramount. Laser can damage retinas a considerable distance away, so light shielding around the scaffold is essential. All adjacent areas surrounding MUST also be well signed to avoid people unexpectedly chancing across it in use. Goggles must be worn – these are specific to the wave length of laser being used and so guidance must be sought. All dispersed material should be collected via vacuum fitted with HEPA filter at source.
You can hire an Nd:YAG machine for approximately £700 a week without operative or purchase a unit for in the region of about £20K. The equipment generally requires 2 people to manage it and operation is intense and so ideally needs to be shared. Training is essential to understand the both potentials and limitations.
The word L.A.S.E.R. is an acronym for Light Amplification by Stimulated Emission of Radiation. The term LASER is a generic term, which describes the way in which laser radiation is produced. There are many different types of laser, each producing a different form of laser radiation. Lasers differ in terms of the lasing medium, which may be a solid, liquid or gas, and in the way in which energy is introduced into the lasing medium to produce excitation. Laser radiation may be emitted as a continuous beam or as a short burst of energy in pulses of varying duration depending on the type of laser, at wavelengths ranging from the near UV to the far IR parts of the electromagnetic spectrum. Light from a laser typically has very low divergence. It can travel over great distances or can be focused to a very small spot with a brightness level which exceeds that of the sun.
How does it work?
In layman’s terms, the laser is effective and absorbed via dark material and the energy vibrates said material at rapid rate so it expands and is dispersed. The laser does not have the same effect on lighter base material and so it naturally stops short of impacting on the stone beneath. This is not to say that damage cannot occur, so again good training and practise are essential. The basic idea behind laser cleaning is that the dirt layer absorbs radiation very efficiently at the wavelength being used, whereas the object reflects most of the energy away from its surface. This means that once the dirt layer has been removed any further laser pulses hitting the clean surface (at the same distance) are not sufficiently intense to cause major damage. The energy absorbed by the dirt layer is almost instantaneously converted into heat; this extremely fast heating generates forces sufficiently strong to eject the dirt particles away from the surface (which should be collected at source with the previously mentioned vacuum).
Variety of types of laser are available. Common type is short pulse infra-red typically at a wavelength of 1064 nm. (as trialled at St Georges and The National Gallery). Energy value is controllable, as is the number of bursts of energy per second (between x1 and x30 – normally used however around x15,). Working distance from object approximately 200mm – laser converged by lens at approximately 100mm, but cleaning ‘sweet spot’ is beyond that. Too close and you can damage the surface of an object, too far away and it is ineffective. Experience informs the operator where they should position themselves – although each substrate material behaves/performs differently.
Overriding statement is that no blanket rules can be assumed and that trials are essential in every case. Polychromy can be affected/damaged by laser exposure, so an understanding of any pigments that may be present is a must in advance.
The control offered by laser cleaning enables the conservator to remove unwanted layers without over-cleaning the valuable surface of the artwork; patina, fine surface detail, tool markings and important surface coatings can be preserved. Laser cleaning systems provide the conservator with an extremely gentle method of cleaning which can be used to remove dirt from very fragile surfaces.
As an example, the stone inscription at St Georges was so friable it literally crumbled at the touch of a finger. Once cleaned however, (with no loss to the original fabric) it could be consolidated and the cleaned stone saved.
The laser in operation – note focal point with approx. 8mm diameter
Using the vacuum at close quarter in conjunction with the laser
The 240V supplied power unit
Signage and shielding the area fully from the public
Requisite PPE including laser specific safety goggles
Completed trial. Area cleaned (LHS) and area soiled (RHS)