Day 5.1 (summaries of articles I read yesterday)

"Femtosecond Laser Cleaning of Painted Artefacts; Is this the Way Forward?"

P. Pouli, G. Bounos, S. Georgiou, and C. Fotakis

Springer proceedings in physics

LACONA VI Proceedings, Vienna, Austria, Sept 21-25 2005

This article primarily discussed femtosecond lasers versus nanosecond lasers as cleaning tools for direct ablation of varnish. The author states that it is known that fs lasers are more efficient because:

1. They are able to process even nominally transparent substrates

2. They minimize thermal diffusion throughout the substrate (diffusion in extreme cases can lead to melting and the creation of an even more difficult to clean mess)

3. The pulses are so short that there is no plasma shielding

The author the proceeds to talk about photochemical interactions, which are, as the name might suggest, chemical reactions which are initiated by the presence of light. These are especially important for the case of painted artifacts, due to the photolability (ability or likelihood of a material to change chemically due to interaction with light) of the painted substrates.  

The author next proceeds to define effective cleaning as based on:

1. Spatial resolution (etching efficiency) and ablation threshold

2. Extent of induced photochemical modifications

3. Morphology of ablation spots.

In general, fs lasers can ablate materials at much lower fluences than ns lasers. According to the experimental data presented in this paper, the ablation rate is roughly material independent.

The authors monitored the photoproducts formed in the varnish via LIF (Laser-induced Fluorescence). Apparently the disassociativity and activity of the dopant used are extremely sensitive to alterations in the polymer environment; in other words, the photoproduct activity can lead to a good understanding of photochemistry in the varnish layer. The authors found that product formation via photochemistry was much reduced for fs pulses. The two pulses of different widths behaved similarly under the ablation threshold, but above it, fs length pulses exhibited less thermal effects and less photochemical effects.

In terms of surface morphology, the authors found that fs pulses were also more effective. The fs lasers produced clean, small cuts, while the ns lasers exhibited some melting effects.

In  short, the authors found that fs pulses are better in all respects. Unlike Tam’s article, this process is applied directly to the varnish, probably because the paints would react in a way unpleasing to conservators if the steam cleaning method or some other substrate heating method were attempted. The authors found that the etching depth for both varnishes they used was much less for fs pulses, allowing for more precise cleaning.

“Experimental Study on the Effect of Wavelength and Fluence in the Laser Cleaning of Silvering in Late Roman Coing”

Vlachou-Mogire, Drakaki, Serafetinides, Zergioti, Boukos

14^th International School on Quantum Electronics: Laser Physics and Applications

SPIE Vol. 6604, 66040W (2007)

Due to economic turmoil in the later periods of the Roman Empire, previously silver Roman coins began to be made from an alloy of copper containing led, tin, and silver. The coins were then coated with silver in a film a few micrometers thick. Wear and corrosion can destroy any detail from the silver layer, but more importantly, mechanical cleaning of the coins can easily remove the silver layer, thus the necessity for more precise methods.

The authors irradiated the coins with a Q-switched Nd:YAG laser with 6ns pulses, 10 Hz repition rate, and between .1-7 J/cm^2 possible fluence.  They used both 532nm and the second harmonic 266 nm pulses, comparing the two in terms of cleaning utility.

The authors found that a fluence in the neighborhood of .93-1 J/cm^2 was most effective for the removal of the corrosion products.  For both silver and copper, the diffusion length is typically less than the corrosion layer depth, but the diffusion length is modified by the presence of the corrosion products, so such an approximation is not always valid, and the user must procced with extreme care (use the lowest effective fluence possible) to remove the corrosion products. The authors also found that at fluences well below the threshold, the material blackened. They postulated that this was due to another chemical reaction in which Cu2OàCuO. In other words, too low a fluence as well as too high a fluence resulted in deterioration of the coin surface

Finally, the authors found that the 532 nm light cleaned far more effectively than the 266 nm light, and postulated that this is due to metals absorbing strongly in the UV as compared to the IR, which would lead to surface melting.