Getting Closer...

Today, we (Lei and I) succeeded in getting the shutter functioning with some help from Rod. After consulting the engineering guys, and then (and more importantly) consulting Rob, we figured out that the trigger signal we were trying to use had a frequency far too high for the shutter. We got a signal generator from Rod, and we now have the shutter configured so that it can open and shut as quickly as possible on command. The next logical step is to figure out how fast the shutter is, but we don't have a beam yet.
We're now missing a translation stage driver, two lenses, a CCD camera (which Lei knows how to find) and a beam. Rod's crew are tinkering with a pulse shortening mechanism, so we don't have a cleaning beam yet. However, when their mechanism is finished, we're potentially looking at pulses as short as 5-6 fs, which would be pretty cool. Rod also gave me a tour of their setup, which was cool, but a lot of it went over my head because I know nothing about plasma physics. Also, the Japanese (mon) coins that Bianca purchased came in today, so I'll be stopping by Carrefour tonight and picking up Q-tips to do some very very light mechanical cleaning as a prelude to ablation.

I also read three papers today, which I will summarize now.

Characterization of Laser Cleaning of Limestone

M.I. Cooper et al, Optics & Laser Technology Vol 27 No 1 1995

Limestone, which was a commonly used building material in Europe especially for monuments is composed primarily of calcium carbonate and is chemically susceptible to sulphur dioxide and nitrous oxides, which are common pollutants from the combustion of fossil fuels. Chemical reaction between the stone and pollutants lead

to the formation of a gypsum crust. From there, a buildup of soot particles with small amounts of metal, rubber, and asphalt results in the formation of a black crust, the removal of which is desirable.

The authors made use of the different absorption spectra of the limestone and crust; at 1060 nm (Nd:YAG) clean limestone absorbs about 30% of the radiation while the black crust absorbs 90%. The cleaning is thus a self-limiting vaporization process. The authors used a Nd:YAG laser with a 300mJ/pulse limit, pulse duration 6ns, and a rep rate of 10 Hz focused to "a desirable" (i.e. unstated, and probably not measured) fluence. The authors also extolled the virtues of applying a thin layer of water, the explosive evaporation of which removes particulates at lower fluences. When the crust is removed, boiling of w ater due to heating of the limestone is minimal.

The experimental layout used by the authors is shown below.

The calorimeter was used to monitor the output energy of the Nd:YAG laser. The HeNe laser was used to monitor the amount of removed material by the intensity of HeNe scattering recorded. The acoustic monitoring was an interesting idea. Ap parently when material is ablated and rapidly released from a material, the ejection creates an audible shock pulse. The authors found a linear relation between material ablated and shock pulse amplitude, with the slope being greater by an order of magnitude for "wet" cleaning. However, the shock amplitude is dependent on the angle from the sample's normal at which the microphone is held, so care has to be taken to maintain angle during the experiment. In terms of laser ablation, the experiment was successful, and preserved detail on the limestone quite well, with the cleaning being more effective upon the addition of water.

Laser Cleaning in Art Restoration

Gobernado-Mitre et al, Applied Surface Science (1996) 474-8

The author and his compatriots used a 7ns pulse Nd:YAG laser at 1064 at a rep rate of 20 Hz in an attempt to clean limestone from the Santa Cruz Palace. They characterized their samples using X-ray diffractometry, IR spectroscopy, and micro-Raman spectroscopy. The authors also used optical microscopy and SEM to morphologically analyze their samples.
The authors found that the samples they were using were mainly composed of dolomite CaMg(CO3)2, small amounts of gypsum CaSO4 x 2H2O and quartz, SiO2. The gypsum was probably due to weathering of the building to airborne pollutants, leading to chemical degradation via exposure to SO2 in the air or rain.

The authors used various numbers of incident pulses and studied ablation depth. The results are below.

The author observes that a natural "over-painting" exists on the cleaned stone for fluences of 200 mJ or less, but which is ablated by 2 400 mJ pulses.

The last paper I read was on plasters, but I'm falling asleep at the keyboard now, so any summary I write is going to be unintelligible. Will update tomorrow.