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Archaeology conservation - laser cleaning

Read another fascinating blog by Helen Cunningham-Jones who is currently undertaking conservation work of the finds from the 2019 archaeology field school as part of her masters in Conservation Practise at Cardiff University.


The lead cames (these are slender, grooved bars of milled lead used to hold panes together in stained glass or leaded lights) arrived with a significant layer of corrosion and dirt on their surface.


How do lasers work in cleaning?


The lasers used in conservation to clean objects emit short pulses of infrared radiation. The dirt layer will absorb this wavelength of radiation very efficiently, with energy being transferred to the dirt causing it to ablate. There are multiple mechanisms at play during dirt removal. The most important mechanism at play for a lower fluence (effectively a lower energy density) is the rapid thermal expansion of the dirt particles. As they absorb energy their temperature will rapidly rise, and they will expand. This quick expansion generates enough force to remove the particle from the surface. For a high fluence beam (so a higher concentration of energy) the dirt can actually be vapourised, but this is a riskier method as it is less controllable.


The surface of the material should not respond like the dirt does and instead reflects the beam. This means the process is somewhat self-limiting as it will remove the dirt without causing damage to the object. However, the settings of the laser must be appropriate to ensure that this self-limiting process takes place. This is particularly important for metals, such as the lead cames, as if the laser provides a pulse for too long or at a high enough energy the surface could melt and damage could occur. For lead in particular there needs to be enough of a break in between pulses to allow the metal surface to cool before the next pulse arrives or overheating could occur.

The laser worked really well at removing the white crusty carbonate layer on top of the lead. Lasers are useful as they’re very selective at what they remove compared to when cleaning by hand. It is also much quicker than cleaning by hand, which can take a long time to remove a thick corrosion layer.



Using the laser to clean one of the cames. The vacuum is removing the corrosion products from the air after they're removed from the cames' surface


Testing


After researching different cleaning options I decided to trial using a laser on some test pieces of corroded lead in the lab so I could compare it to mechanical wet cleaning with a scalpel. I had to test the laser at different distances from the lead piece to see what the most effective distance would be that wouldn’t cause damage to the lead underneath. I had to decide on very specific settings so that the carbonate layer (white and crusty) would be removed but the oxide layer (dark grey) wouldn’t as this provides a protective layer for the metallic lead surface.



Testing the different distances between the laser and the lead piece


The wet cleaning test piece


Using the lasers


After trialling the two methods and continuing research I decided to use the laser on the window cames. Lasers pose a health and safety risk to the eyes and skin, and lead products can be very dangerous if inhaled or ingested so full PPE is needed during the laser cleaning process.



Goggles, ear defenders, dust mask, gloves and lab coat are all needed to protect yourself when using the laser!


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