3rd article

A summary of the laser paper I read yesterday

"Lasers Cleaning of Patrimonial Plasters" (LaserS not a typo)

E. Tanguy, N. Huet, A Vinçotte

Due to mineralogical and physical characteristics, plaster becomes dirty quickly. Moreover, traditional plaster cleaning techniques often remove fineness of detail within the artwork.

 The authors fabricated a plaster sample, mixing the powder with only water (no additional additives). As a natural dust contamination process would be extremely lengthy, the authors deposited powdered carbon graphite as a contaminant.

 The authors thermally treated samples of their plaster to determine what modifications of phase could be induced by the ablation pulses. The samples were viewed with SEM after treatment, and were then ground and analyzed via X-ray diffraction. The results are below.

 The authors first attempted using the first harmonic of an Nd:YAG laser (1064), which resulted in "an intense yellowing" of the plaster, while use of the 3rd harmonic (UV) recovered a color "close" to the original plaster.  The fluences used were 2.88 J/cm^2 for the 1064 nm light and .72 J/cm^2 at 355. The discrepancy, not alluded to by the authors, may be enough to cause the yellowing of the plaster. The authors note that cleaning by the fundamental harmonic also morphologically modifies the surface, although the exact modification is not stated. The authors estimate the ablation thresholds of the "dirt" as .16 J/cm^2 and .23 J/cm^2 at UV and IR, respectively. The authors then cleaned a plaster pieta piece using UV laser light at .41 J/cm^2 and achieved "satisfactory results," preserving the details on the piece.