1- Instituto de Nanociencia y Materiales de Aragón (CSIC - University of Zaragoza), c/María de Luna 3, 50018 Zaragoza, Spain 2- Laboratorio de Evolución Humana and Unidad Asociada de I+D+i al CSIC “Vidrio y Materiales del Patrimonio Cultural (VIMPAC)”, Departamento de Historia, Geografía y Comunicación, Universidad de Burgos, Plaza Misael Bañuelos S/N, 09001 Burgos, Spain 3- Área de Historia del Arte and Unidad Asociada de I+D+i al CSIC “Vidrio y Materiales del Patrimonio Cultural (VIMPAC)”, Departamento de Historia, Geografía y Comunicación, Universidad de Burgos, Pº Comendadores S/N, 09001 Burgos, Spain 4- Área de Didáctica y Dinamización, Museo de la Evolución Humana, Paseo Sierra de Atapuerca nº2, 09002 Burgos, Spain 5- Archéosciences Bordeaux Laboratory UMR 6034, CNRS, University Bordeaux Montaigne, France 6- HERCULES Laboratory, University of Évora, Largo Marquês de Marialva 8, 7000-809 Évora, Portugal Md. Ashiqur Rahman 1,3,6 , Germán F. de la Fuente 1,* , José Miguel Carretero 2 , Mª Pilar Alonso Abad 3 , Rodrigo Alonso Alcalde 4 , Rémy Chapoulie 5 , Nick Schiavon 6 , Luis A. Angurel 1 Ultrashort pulsed laser cleaning in the conservation of archaeologically significant bones and flints Introduction Materials Results and Discussion References Acknowledgements Conclusion and Future Perspectives Laser cleaning techniques could be considered amongst the most noteworthy contributions of Physics towards the conservation of artworks 1,2 . Although having established themselves as a striking field of research for the last three decades, these techniques have attracted limited attention by the archaeological conservation community. Laser-assisted removal of contamination and deterioration products in archaeological bones 3 and flints is a subject of great interest to improve recovery and conservation of these most valuable museum artifacts, and may highlight the use of laser cleaning methodology in this field. This research work reports on studies aimed to evaluate the application of two different ultrashort pulsed lasers for the elimination of contaminants on significant Pleistocene bone and Neogene flint surfaces from Sierra de Atapuerca(Spain) 3,4 in an attempt to safeguard their archaeological value and origin. 1. T. Maiman, “Stimulated Optical Radiation in Ruby.” Nature 187, 493–494, 1960), doi: https://doi.org/10.1038/ 187493a0 2. R. Lahoz et al., “Laser Applications in the Preservation of Cultural Heritage: An Overview of Fundamentals and Applications of Lasers in the Preservation of Cultural Heritage.” In Conservation Science for the Cultural Heritage: Applications of instrumental analysis; E. Varella, Ed.: 294–332, 2013, doi: 10.1007/978-3-642-30985-4 3. M. A. Rahman et al., “Sub-ns-pulsed laser cleaning of an archaeological bone from the Sierra de Atapuerca, Spain: a case study,” SN Appl. Sci., vol. 3, no. 12, 2021, doi: 10.1007/s42452-021-04850-8 4. J. L. Arsuaga et al., “Sima de los Huesos (Sierra de Atapuerca, Spain). The site.” J. Hum. Evol., vol. 33, no. 2–3, pp. 109 –127, 1997, doi: 10.1006/jhev.1997.0132 Project supported by H2020-MSCA-ITN-EJD/ED-ARCHMAT action funding under the Marie S. Curie grant agreement, No 766311 and by Gobierno de Aragón (research group T54_20R). Femtosecond (fs) Laser Sub-nanosecond Laser Wavelength 343 nm 1064 nm Pulse duration 238 fs 800 ps Pulse repetition rate 200 kHz – 1 MHz 200 – 800 kHz Average power 9.33 W 8 W Maximum pulse energy 46.6 µJ 40 µJ Beam diameter 30 µm 80 µm Distance bet. adjacent laser passes 15 µm 20 µm Laser Cleaning Systems and Parameters Material: Pleistocene bone (Bear rib) Archaeological site: Sima de los Huesos Chronology: Pleistocene (430,000 years) Objective: Remove the hard blackish stains without altering the surface. Fig. 1: Illustration of the laser cleaning apparatus used for the present study (left), where the laser x-y scanner head is shown above the archaeological artifact sample and a fume extraction device. The upper left inset illustrates the ideal sample behavior under laser irradiation, where the contaminant layer is removed, while the protective patina (green) is preserved. The upper middle inset depicts the connection between laser intensity and various definitions of beam waist for a Gaussian beam profile, emphasizing the 1/e 2 criteria utilized in this investigation. The lower and right side inset illustrations depict the laser output intensity at a specific place as a function of time for the burst pulse and beam scan mode used to control thermal damage. These insets visually depicts the pulse width, pulse-to-pulse (interpulse) spacing and how thermal incubation occurs during the subsequent pulse irradiation procedure. Intensity Beam Waist Peak Intensity Interpulse Separation I0 Time Heat Diffusion Heat Incubation I0/2 I0/e I0/e 2 Pulse Width Beam Diameter Distance Between Lines Beam Scan Direction Distance Between Pulses Laser Beam Material Plasma Dirt Layer Contamination Products Archaeological Artefact Vaporized residue Suction Channel Laser Device Gaussian Shape Beam Profile Beam Waist Applying the 1/  Criterion Intensity I0/2 I0/e I0/e 2 I0 Peak Intensity Interpulse Separation Pulse Width I0 Time Intensity Burst Pulse (Group of 4 micro-pulses per site) Interval between Bursts Heat Diffusion Cumulative Heat Incubation Burst Pulse Mode Beam Scan Mode Material: Cretaceous flint Archaeological site: La Paredeja Chronology: Upper Pleistocene Objective: Clean the dark brownish - yellowish encrustations without damaging the original surface. Material Region Laser Application mode Cleaning thresholds Damage thresholds Fluence (J/cm 2 ) Irradiance (GW/cm 2 ) Fluence (J/cm 2 ) Irradiance (GW/cm 2 ) Bone Fig. 2b 800ps n-IR Beam Scan 0.20 – 0.31 0.25 - 0.39 0.36 0.45 Fig. 2d 800ps n-IR Burst pulse 0.14 – 0.16 0.18 – 0.20 0.17 0.22 Fig. 2f 238fs UV Beam Scan 0.29 – 0.56 1248.27 – 2377.66 0.66 2793.75 Flint Fig. 2h 238fs UV Beam Scan 0.29 – 0.47 1248.27 – 1961.57 0.66 2793.75 Table 1: The cleaning threshold ranges and damage threshold values observed in the study. Original flint surface Laser treated flint surface 50 μm 50 μm 3mm x 3mm 3mm x 3mm c d 800ps n-IR laser in burst mode 20 μm 3mm x 3mm 3mm x 3mm Original bone surface Laser treated bone surface 20 μm 800ps n-IR laser in beam scan mode a b 3mm x 3 mm e f 238fs UV laser in beam scan mode 100 μm 100 μm 3mm x 3 mm 3mm x 3mm 3mm x 3mm g h 100 μm 100 μm 238fs UV laser in beam scan mode Fig. 2: Optical microscopy and SEM images of the bone and flint cleaned by both 238fs UV laser and 800ps n-IR laser, associated with Table 1: ‘a, c, e, and g’ presents the original outlook of the sample, where ‘b, d, f and h’ presents the best cleaning threshold outcomes. In all cases, only 1 treatment has been executed. Two-Theta (deg) Intensity (Counts) (a) (b) (c) Fig. 5: Representative XRD peak identification of the flint sample with and without treatment of fs UV laser: ‘a’ presents the original flint surface and ‘b & c’ presents the satisfactorily cleaned laser treated surface. The JCPDS-2000 database utilized to determine the phases. Wavenumber (cm ) -1 Absorbance Units Untreated area Laser treated: 0.68 TW cm -2 Laser treated: 1.24 TW cm -2 Laser treated: 1.96 TW cm -2 750 v 1 PO 4 3- v 2 CO 3 2- v 3 PO 4 3- v 3 CO 3 2- , B v 3 CO 3 2- , A Amilde III Amilde II Amilde I C-H Bonds Fig. 4: ATR-FTIR spectra of untreated & laser treated bone area with irradiance levels of 0.68, 1.24 and 1.96 TW cm -2 . Fig. 3: Representative raw XPS survey spectrum of the Bear rib bone: green & red colored bands obtained from the ‘as received non treated’ surface, where blue & black colored bands correspond to the 800ps n-IR laser- treated cleaned and damaged surface accordingly. Binding Energy (eV) Count Per Second (CPS) Fe 2p Fe 2p- Mn 2p Mn 2p- Original bone covered with hard blackish encrustations Laser cleaned surface using n-IR 800ps laser Yellowish colored original non-treated bone surface Laser damaged bone surface The results indicate that fs UV laser irradiation is significantly safer and more efficient at cleaning than sub-nanosecond laser irradiation, owing to the controllability of laser irradiation parameters, which allow for a systematic and accurate parameter description of an actual laser cleaning intervention. Further research on ultrashort pulsed laser-surface interaction using a variety of pulse durations and emission wavelengths, based on the advancements achieved within this study will pave the way towards future respectful and environmentally advantageous conservation practices. * Corresponding author: german.delafuente.leis@csic.es