Bureaucracy is a French Word. Efficiency isn't.

Today I went to see Yvres-Bernard to get the laser safety exam and watch a laser safety video after which I'm allowed access to the laser lab. However, we discovered two things:
1. The video is currently MIA, or at least somebody has it borrowed from Yvres-Bernard
2. I need to get a retinal scan/eye exam before I can even take the exam.

I'm not concerned about the exam. I've studied for it a bit, and the laser safety stuff we did at CUOS seems to more than cover the material. The retinal scan issue came as a surprise, and apparently there's a problem requisitioning the proper paperwork because apparently I'm technically LOA and Corinne (who's been extremely helpful so far) is ILE... or maybe I have that backwards. In any case, it took all of today to figure out who was actually going to cover the exam, and since we can't make an appointment until we have a "bon du command" which I take to be a declaration of payment, that's going to get done tomorrow. I'd be ok covering it up front with cash and then being reimbursed, but apparently that's even more of a bureaucratic pain. I was looking forward to actually getting some lab time in, but it doesn't look like that's happening within the next few days.
So instead, I spent most of today reading articles.
Article from SPIE proceedings vol. 7027, 2008. (all images taken from above article)

This group of researchers from Athens conducted research on laser-based cleaning of coins, using a Roman coin (minted c. 120 AD) and a Byzantine coin minted some time in the 6th century A.D. The Roman coin was relatively well preserved, but covered in a thin green corrosion product the authors identified as Copper Chlorite, while the Byzantine coin showed evidence of multiple different colors and types of corrosion products.The group used XRF (X-Ray fluorescence) to monitor the amount of each chemical compound in the corrosion products; that is to say XRF was used to monitor the removal of certain products by change in emission spectra.

The group attempted cleaning with several different laser systems, including an Nd:YAG which they used at 266, 532, and 1064 nm with a pulse width of 6ns and a rep rate of 10Hz, an Er:YAG of 2.94 um wavelenth with 190ns pulses at a rep rate of 1 Hz, and a TEA CO2 laser 10.6 um, 80 ns pulses, 1-5 Hz rep rate. The pulsed beams were focused by a lens of f=50 onto the coins. According to the authors, fluences ranged from 2-19.2 J/cm^2.

In terms of general results, the authors found that using a larger number of pulses resulted in a greater display of thermal effects in the material. Lower fluences were preferable, except that below a certain threshold the coins were poorly cleaned, or not cleaned at all. The authors also tested the coins in "wet" and "dry" conditions, although they failed to define what those definitions meant. My understanding is that "wet" means they applied a thin film of liquid to the coins before irradiating them, but the paper was not explicit.

The authors also mentioned an additional way to characterize the cleaning. The Roman coin in question was a quaternary alloy of Tin, Lead, Silver, and Copper. Apparently there exist two X-rays in Tin's spectrum, denoted La and Ka, where the La X-rays are significantly absorbed if some material exists above Tin, but which radiate strongly if Tin is in strong concentration at the substrate surface. In other words, once the ratio is large ("large" being not well described in the article) the cleaning has made significant process.
Ultimately, the authors found the presence of water helpful, and their best results were obtained by the 532nm Nd:YAG at fluences of about .93-1 J/cm^2

Proceedings of SPIE, vol 7391, 2009

The same heroes from last time, plus a few new faces, study the cleaning of ancient coins, here a coin minted in Cologne around 260 AD and a Roman coin minted in Alexandri in 315 AD. Both coins contain a high concentration of copper; the Cologne coin (designated NMW56 in the paper) was 91% Cu, 3% Ag 3% Sn and 3% Pb roughly, while the Roman coin (R2) was 83% Cu and 17% Ag. The authors stated that composition and structure of the corrosion layers had not
been fully investigated, but that they were mainly comprised of copper corrosion products. The authors used optical microscopy, SEM, and a stylus profilometer to determine cleaning efficiency. The authors used and compared 4 different lasers:

With repetition rates of 10, 1, 490e6, and 5e3 Hz respectively.

For the Nd:YAG laser at 532 nm, the authors reused data from a previous paper (the one about cleaning coins as a function of wavelength). They stated the result of most successful cleaning between .93 and 1 J/cm^2 but stated that the nonhomogeneity of the corrosion products complicated cleaning.

For the Nd:YAG at 1064, in a microscopic photo, the uncleaned area of the coin actually appears more homogeneous that the cleaned area due to melting/reforming of copper and budding exposure of Ag. Most of the corrosion products were successfully removed successfully.

For the GaAlAs Diode laser, the beam had to be tightly focused only to achieve a max fluence of ~.06 J/cm^2. Also, with such a high repetition rate, many more pulses hit each area than with the Nd:YAG laser (appx 10^8 pulses/spot according to the author). Given the author's previous claim that multiple shots lead to thermal effects, I have a hard time seeing this measurement as being comparable with the Nd:YAG results. In terms of results, the authors found that the diode laser successfully removed corrosion products but was not intense enough to remove cover covering the coin's silver outer coating. Personally, I think this sounds quite useful, as it managed to blast off the corrosion products without possibly harming the delicate (um thick) silver coating.

For the Ti:Sapphire laser, the authors found that the pulses removed not only the corrosion products, but also the copper AND delicate silver layer they were trying to protect, and concluded that for this specific operation, it wasn't the tool to use. The pulse rate was such that, at the two scanning speeds the authors attempted, either 1 or 4 pulses hit the same spot on the coin. The authors concluded from the variations in the Nd:YAG trials that perhaps the corrosion product behaved significantly differently for visible and IR radiation, which may have also affected the Ti:S's utility.

It is interesting to note that while this paper purports to compare cleaning as a function of pulsewidth, the repetition rate of the ps laser was such that a great number of pulses hit each spot, and also that the wavelength of the fs laser was such that cleaning may have been impractical regardless of pulse width. The authors even mention in the second to last paragraph that more work needs to be done since the test wasn't a fair comparison of fs, ps, and ns lasers.

As something of an aside, I find the focusing mechanism used in these papers interesting. Every time, the author just seems to mention "a lens of focal length" whatever which causes the beam to focus. But I can imagine that using just one lens would lead to significant beam aberrations especially in larger beams, especially from spherical and chromatic aberrations, particularly for short pulses which have a larger bandwidth. I would be interested in seeing the result of using a well-corrected lens system as opposed to a singlet lens used to focus the beam.