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After co-presenting my own research about modifying the energy output of an Nd:YAG laser, I was excited to hear the Gardner conservators speak about their continuing experiences with Er:YAG laser treatments. I remember hearing about their use across the street while at MFA Boston during my pre-program years and looked forward to an update on their work. As the talk began with a sultry image of George Stout, I was not to be disappointed.
Stout, I learned, encouraged and recognized the potential of lasers in the treatment of objects in the early 1900s and, since he founded the conservation department at the Gardner in 1934, it is fitting that the lab continues experimenting and developing approaches with this versatile instrument. As a brief overview, Holly and Jess spoke about treating a painting, fragile gilded surfaces, and the stone Farnese sarcophagus, emphasizing the controllability of the slow, delicate pulses of an Er:YAG unit at 2940 nm. The Er:YAG laser targets material with hydroxyl (-OH) groups particularly well, as these materials are highly absorptive of energy at this wavelength.
In terms of equipment, they highlighted the pros and cons of two lasers in the Q&A session – the Er:YAG Lynton unit and the Fotona Lasers LLS. The Lynton unit has no aiming beam, making it more difficult to determine the exact location of treatment prior to testing. It also has a finger trigger and allows for both the changing of spot size and energy. The Fotona laser has a handy articulated arm and a foot pedal, but has less operator control, as it has a fixed spot size. Something else to keep in mind is that lasers are directional and work best when delivered perpendicularly to a surface. The light doesn’t naturally turn corners and won’t get into undercuts.
I will attempt to describe the non-objects treatments, as an objects conservator, wishing to avoid misinformation about reducing coatings and varnish on paintings. Suffice it to say, a very low fluence (0.55 J/cm2) and pre-wetting the surface with an alcohol to deliver the -OH groups, was enough to break up natural resin varnish. A hazy effect was created, instead of complete removal through ablation, and the cloudy, translucent varnish could be swabbed away without the use of harsh (to the user) solvents. By manipulating the fluence and number of treatment passes, they also removed metal flaked overpaint and other restoration coatings leaving behind *no residues* and producing comparable SEM images of the surface between laser and gel cleaning of the same material. This is exciting, if you have a laser and the training to operate one.
In sum, no use of harsh solvents, no difficult-to-clear residues, and no SEM-perceivable difference in AT surface treated with a laser versus the same one treated with a gel.
For objects, we listened to a short introduction of an earlier cleaning of friable stone with the delicate touch of a Nd:YAG 1064 nm laser. I was familiar with issues common to this treatment, such as discoloration and suggested avoidance of original paint layers. They described a stone surface that was structurally delicate and had traces of gilding and Egyptian blue pigment (tracked with VIL imaging), obscured in places by dense, well-adhered, black corrosion crusts. They were able to strip extremely thin layers of this crust away slowly with the Er:YAG laser (at 1.3 J/cm2) to reveal more original paint (again, tracked with VIL imaging). The side-by-side VIL images were wonderful and demonstrated the ability to reveal hidden original surfaces.
As a final note, the speakers brought up work at the Worcester Art Museum testing the use of gels as the hydroxyl group to hold the laser action *just* at the surface in the same way gels are being used to restrict solvent or aqueous cleaning methods to the surface. I’ll be looking more closely at papers delivered at the LACONA conferences to stay abreast of the latest research in this area. I think a full list in one place of available laser units on the market and input about their usability quirks, such as those mentioned by Jess and Holly, would be a helpful resource for labs interested in acquiring a laser.
Many thanks to the self proclaimed Laser Nerds of the laser learning pod for their continued dedication to new techniques and to sharing these applications widely!
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