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What is a laser?

Laser stands for Light Amplification by Stimulated Emission of Radiation.

A beam of light is produced by the laser that is:

• a very strong (intense)
• of a single colour (pure)
• focused (a typical laser beam spreads out only a few millimetres after travelling several metres).

The energy in a laser beam can be focused so that it can be used for cleaning purposes. Below we will tell you how this works.

What sort of things do you clean?

Lasers can be used to clean a wide range of materials:

• stone, e.g. marble statues that are exposed to wind, rain and pollution
• pottery, e.g. pots that may have been buried for thousands of years
• metal, e.g. bronze parts of statues or the fittings that hold the statue in place
• natural materials, e.g. ivory and parchment (paper).

Not everything can be cleaned using a laser (you can learn more about these items below). A small part of the item should always be tested before cleaning begins.

Why do items need cleaning?

We often think that materials like marble and bronze as will last forever. However they are easily damaged. There are a number of reasons why items may need cleaning. These include:

• atmospheric pollution. The gases produced by burning coal, gas and oil mix with rainwater. This produces weak acid solutions that dissolve marble and other materials left outdoors. The surface may also become blackened by soot and other chemicals
• salt in the air will also attack outdoor materials
• graffiti is a major problem. Paint and ink take time to remove and can often leave stains

How does the cleaning work?

The cleaning works on the idea that some of the energy in the laser beam will be absorbed by the dirt on the surface of the material.

So, imagine we are cleaning a statue:

• the laser is directed at the statue using a 'pen-like' hand piece
• a very short pulse of light is directed at a certain place on the statue
• the dirt particles on the surface of the statue quickly absorb the energy from the beam
• the dirt particles heat and expand
• this all happens very quickly and the forces produced are strong enough to remove the dirt particle from the surface of the statue

Sometimes water is brushed or sprayed onto stubborn dirt before the laser is used. The water is heated by the laser as well as the dirt, leading to explosive vaporisation. This can help to increase the speed of cleaning.

Why is laser cleaning so good?

• accuracy - very small areas can be cleaned and delicate markings are undamaged
• size - the size of the beam can be changed so it can be used to clean either very small areas or life-sized sculptures
• selectivity - the laser beam reacts more strongly with the dirt than with the object's surface
• no contact - even badly damaged surfaces can be cleaned without causing more damage
• control - as soon as the laser is turned off the cleaning stops. There are no cleaning materials left on or within the object to continue working or hide the surface
• reliability - modern lasers are easy to look after
• safe - no dangerous chemicals are used. The conservator only needs to wear safety goggles and a face mask

Are there things you can't clean with a laser?

Before using a laser the conservator must think about:

• the type of dirt, e.g. soot
• the type of material and its condition, e.g. is it damaged?
• the history of the object

Careless or unneeded laser cleaning can lead to great damage, loss of surface detail, and may even speed-up the decay.

Also, laser cleaning is not ideal for:

• some types of algae and lichen. They are burnt rather than removed
• certain pigments will change colour, even if cleaning is very careful

A laser should only be used if it is the best way to clean an item.

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