1 INTRODUCTION Applications of laser beam techniques in the field of Art and Archaeology have been widely developed since a few decades. Both analytical and imaging methods, such as Raman micro-spectrometry, laser induced breakdown spectroscopy (LIBS), laser radar, interferometry and holography, provide useful in-formation related to the object composition, conser-vation state and defects. Laser-based restoration techniques, which mainly consist of surface cleaning (varnish removal on paintings, elimination of gyp-sum crusts on rock materials), use either pulsed or Q-switched laser radiations.

This work attempts to apply continuous wave (cw) laser radiation for the restoration of darkened red lead-containing paintings. Such an alteration phenomenon is commonly observed in mural paint-ings. Previous works showed that red lead darkening often result from the pigment transformation into black lead dioxide (plattnerite, β-PbO2). The degra-dation process is mainly controlled by both envi-ronmental factors, such as humidity, light, tempera-ture or sulphur-containing pollutants. Besides, influence of intrinsic parameters, including composi-tion and grain structure of the pigment, has been stated.

Restoring blackened red lead-containing paintings may consist in recovering the original red colour. In most cases, removing the black layer is inconceiv-able, considering the low amount of remaining pig-ment. The most sustainable solution might be based on the reversion of the alteration products into red lead. The main component of traditional red lead pigment is similar to minium, the mineral mixed va-

lence lead oxide of formula Pb3O4. However, as a consequence of the manufacture process, significant amounts of lead monoxide may remain in the pig-ment. Both crystalline forms, i.e. orthorhombic (massicot) or tetragonal (litharge), have been identi-fied in red lead-containing artworks.

Reversion of plattnerite into minium may be achieved by either chemical or physical routes. Due to the complexity of the Pb-O system, which in-cludes a number of distinct minerals, a chemical re-duction of β-PbO2 into Pb3O4 is challenging.

Lead dioxide, however, is able to undergo thermal reduction through successive oxygen losses which lead to the formation of lower oxides. In particular, plattnerite evolution into minium is spontaneous over ca. 375°C. This thermal transformation can be achieved using a laser irradiation (Burgio et al., 2001). In order to profit by this property for a possi-ble reversion of darkened red lead, a series of laser irradiation tests were carried out.

2 EXPERIMENTAL

2.1 Samples

The effects of laser irradiation on plattnerite were investigated on both raw samples (powdered β-PbO2, Merck, 99.95%) and blackened red lead-containing samples taken from experimental wall paintings (Morineau & Stefanaggi 1995; Aze 2005). Details related to both mural alterations and red lead discolorations have been described in previous pa-pers (Aze 2006; Aze et al. 2007).

Towards the restoration of darkened red lead-containing mural paintings: a preliminary study of the β-PbO2 to Pb3O4 reversion by laser irradiation

S. Aze, P. Delaporte LP3, Marseille, France

J.-M. Vallet CICRP, Marseille, France

V. Detalle

LRMH, Champs sur Marne, France

O. Grauby, A. Baronnet

CRMCN, Marseille, France

ABSTRACT: Red lead pigment darkening in paintings is generally caused by the pigment alteration into platt-nerite (β-PbO2). Plattnerite reversion into minium may be achieved by heating over 375°C. This reaction may be achieved using continuous-wave (cw) laser irradiation. Laser-induced photo-thermal reduction of plattnerite into minium was investigated through irradiation tests on both pure plattnerite powder samples and darkened red lead paint samples taken from experimental wall paintings. The effects of both visible (514 nm) and near infrared (1064 nm) sources were investigated. This study shows the high potentiality of an innovative technique based on cw-laser irradiation for the restoration of darkened red lead-containing paintings.

2.2 Methodology

Irradiation tests were carried out with both visible and near-infrared radiations, using a cw-Ar

+ laser

(514 nm) and a cw-Nd:YAG laser (1064 nm), re-spectively.

The laser beams were set-up using a set of optical devices, including both converging and diverging lenses, diaphragms and mirrors. Resulting laser beam size, measured using a CCD device, was ap-proximately of 1.8 mm

2.

The effect of the main irradiation parameters, namely, irradiation time and laser power density were investigated. The effect of each irradiation test on the plattnerite samples was estimated through ob-servations of the sample surface by means of optical microscopy.

2.3 Characterization method

Local analyses of both sample surface and cross-sections were carried out using a micro-Raman spec-trometer. Spectra of ca. 1 µm

2 areas were recorded

using a Renishaw inVia system equipped with a Spectra Physics Ar+ Laser (514.5 nm, 20 mW) and a Renishaw GaAs diode Laser (785 nm, 300 mW) calibrated using the 520.5 cm

-1 line of a silicon wa-

fer. Laser power, optical magnification and irradia-tion time were selected so that no degradation occurs due to photo-thermal effects. Spectral separation of the scattered photons was performed using both a Notch filter and grating monochromators (1800l/mm with 514.5 nm Laser, 1200l/mm with 785 nm Laser). Photons were collected over the [100-3000] cm

-1

range with a spectral resolution of 1 cm-1 using a Peltier-cooled charge-coupled device (CCD) detec-tor.

3 RESULTS

3.1 Ar+ laser irradiation tests

Visible effects of the laser irradiation appeared for a laser power higher than 0.90W (Table 1).

Table 1. Visual effects of Ar+ irradiation tests on pure platt-nerite powder samples as a function of the laser power (P) and irradiation time (I)* ______________________________________________ P (W) 0.75 0.90 1.00 1.10 1.25 I(s) ______________________________________________ 1 none none R R Y 5 none slight R R Y Y 10 none slight R R Y Y 30 none slight R R Y Y 60 none slight R R Y Y 120 none slight R R Y Y ______________________________________________ *R: reddishing, Y: yellowishing.

A slight reddishing of the plattnerite grains was

observed in these conditions for irradiations longer than 5 seconds. For longer irradiation times (up to 120 seconds), no visible increase of the reddishing intensity was observed. At higher laser powers, the reddishing was noticeably more intense, until a strong yellowing took place. For irradiation times over 5 seconds, this phenomenon occurred at laser powers higher than 1.10 W.

Micro-Raman spectroscopic analyses of the red and yellow phases showed the reduction of plattner-ite grains into Pb3O4 (minium) and β-PbO (massi-cot), respectively (Fig. 1).

Figure 1. Raman spectra obtained from (a) the red areas, (b) the yellow areas obtained after plattnerite irradiation using Ar+ la-ser. The spectra match with the reference spectra of Pb3O4 (minium) and β-PbO (massicot), respectively.

3.2 Nd:YAG laser irradiation tests

A slight reddishing of the plattnerite powder samples occurred at 0.15 W (Table 2). For long irradiation times (t>5 seconds), the reddishing was more intense for laser powers between 1.0 and 3.6 W. According to microscopic observations, all visible plattnerite grains were transformed for laser powers higher than 2 W. Yellowing of the plattnerite sample was ob-served over 3.60 W.

Table 2. Visual effects of Nd:YAG irradiation tests on pure plattnerite powder samples as a function of the laser power (P) and irradiation time (I) ______________________________________________ P (W) 0.10 0.15 1.00 3.00 3.60 I(s) ______________________________________________ 1 none slight R slight R R Y 5 none slight R R R Y 10 none slight R R R Y 30 none slight R R R Y 60 none slight R R R Y 120 none slight R R R Y ______________________________________________

According to Micro-Raman analyses of the red phase obtained at 3.0 W, most of plattnerite grains have been reduced into minium (Fig. 2). The pres-ence of an additional band near 342 cm

-1 may be at-

tributed to the lead sesquioxyde of formula Pb2O3.33 (Aze 2005).

Figure 2. Raman spectrum of the red phase obtained from plattnerite irradiated with cw-Nd:YAG laser at P=2W, com-pared to the reference Raman spectrum of minium Pb3O4.

4 DISCUSSION

Plattnerite reduction into minium spontaneously oc-curs over 375°C (Clark et al. 1937). On the other hand, minium itself is reduced into massicot over 512°C (Ciomartan et al. 1996). Depending on the ir-radiation parameters, plattnerite may thus be trans-formed into either minium or massicot.

Irradiation of plattnerite by Ar+ laser (514 nm)

initially leads to the formation of minium; over a certain power density threshold, minium is reduced into massicot, due to the high absorption of the green laser light.

When irradiated by near-IR laser beam (Nd:YAG, 1064 nm), similar phenomenon occurs. Minium pro-duced by plattnerite reduction, however, has a rela-tively low absorbtivity in the IR spectral domain. As a consequence, the laser power density threshold for minium reduction into massicot is much higher with Nd:YAG irradiation than with Ar

+ irradiation.

Influence of the irradiation time appears to be negligible over few seconds. We thus suppose that local temperature of the irradiated material quickly rises until it reaches a maximum corresponding to a stable thermal regime.

5 CONCLUSION

Plattnerite irradiation using Ar+ laser (514 nm) leads

to the formation of pure minium within a slight laser power density range. Due to the absorption of laser light by minium, massicot is readily produced. On the contrary, Nd:YAG irradiation (1064 nm) pro-duces minium over a large power density range.

Such irradiation tests show the high potentiality of continuous wave Nd:YAG laser for the possible reversion of blackened red lead pigment in paintings

6 ACKNOWLEDGEMENTS

The authors wish to acknowledge the French Minis-try of Culture and Communication for financial sup-port, and the PLANI group from the LILM labora-tory (CEA) which supplied the experimental set up.

7 REFERENCES

Aze S. 2005. Altérations chromatiques des pigments au plomb dans les oeuvres du Patrimoine, PhD thesis, University of Marseille.

Aze S., Vallet J.-M., Baronnet A., Grauby O. 2006. The fading of red lead pigment in wall paintings: tracking the physico-chemical transformations by means of complementary mi-cro-analysis techniques. European Journal of Mineralogy 18(6): 835-843.

Aze S., Vallet J.-M., Pomet M., Baronnet A., Grauby O. 2007. Red lead darkening in wall paintings: natural ageing of ex-perimental wall paintings versus artificial ageing tests. European Journal of Mineralogy 19(6): 883-890.

Burgio, L., Clark, R. J. H., Firth, S., 2001. Raman spectroscopy as a means for the identification of plattnerite (PbO2), of lead pigments and of their degradation products. Analyst 126(2): 222-227.

Ciomartan D.A., Clark R.J.H., McDonald L.J., Odlyha M. 1996. Studies on the thermal decomposition of basic lead (II) carbonate by Fourier-transform Raman spectroscopy, X-ray diffraction, and thermal analysis. Journal of Chemi-cal Society - Dalton Transactions 18: 3639-3645.

Clark G. L., Schieltz N. C., Quirke T. T. 1937. A New Study of the Preparation and Properties of the Higher Oxides of Lead. Journal of the American Chemical Society 59(11): 2305-2308.

Morineau A., Stefanaggi M., 1995. A statistical approach to the problem of frescoes: The French experience. Statistical Methods and Applications 4(1): 37-53.