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Treating the untreatable in art and heritage materials: Ultrafast laser cleaning of “cloth-of-gold” Mitsuhiko Kono, Kenneth Baldwin, Alison Wain, and Andrei Rode Langmuir, Just Accepted Manuscript • DOI: 10.1021/la504400h • Publication Date (Web): 05 Jan 2015 Downloaded from http://pubs.acs.org on January 11, 2015

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Treating the untreatable in art and heritage materials: Ultrafast laser cleaning of “cloth-of-gold” Mitsuhiko Kono,1,2* Kenneth G. H. Baldwin,2 Alison Wain,3† Andrei V. Rode1‡ 1

Laser Physics Centre, and 2Atomic and Molecular Physics Laboratories, Research School of Physics and Engineering, Australian National University, Canberra, ACT 2601, Australia 3

Centre for Creative and Cultural Research, University of Canberra, Canberra, ACT 2617, Australia

ABSTRACT Laser cleaning provides art and heritage conservators with an alternative means to restore objects when traditional chemical and mechanical methods are not viable. However, long (> nanosecond) laser pulses can cause unwanted damage from photothermal processes and provide limited control over ablation depth. Ultrashort (< picosecond) pulse lasers are emerging as a more appropriate tool for cleaning historic artefacts because of their unique ability to avoid heat- and shock-wave generation, thus minimising collateral damage of the underlayers, and to

*

Curently with the Senri International School of Kwansei Gakuin, Japan

†

Formerly with the Australian War Memorial.

‡

Corresponding author A.V.R (e-mail: [email protected])

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remove material with near-nanometer precision.

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Here we demonstrate the effectiveness of

ultrashort pulses by cleaning 19th century military gold braid that was untreatable by conventional means, and without any detrimental effects on the gold foil or the underlying silk thread structure. The results are compared with nanosecond-pulse laser treatment that damages the surface structure. By introducing in situ feedback control of the laser ablation via laserinduced breakdown spectroscopy (LIBS) monitoring of the ablated plume, we are able to halt the cleaning process just as the contaminant layer is completely removed. This technique allows ultrafast laser ablation to extend the armoury of conservation treatments, enabling restoration of a range of complex and fragile heritage objects previously untreatable by conventional means.

I. Introduction Laser treatment of heritage artefacts generally has many advantages: it is a dry and contact-free process, it has the potential to selectively remove contaminants or over-coatings, it can minimise mechanical disruption of the surfaces to be preserved, and it generates minimal potentially hazardous waste [1-4]. Long (nanosecond duration) pulsed lasers have been used successfully for cleaning stonework on monuments and sculptures, and for the removal of acrylic overpaint [5]: these lasers are readily available and increasingly less expensive. However, they have two main drawbacks. First, long laser pulses generate heat- and shock-waves which penetrate into the bulk of the substrate, potentially damaging the underlayers and also causing the removal of large fragments of material, making precise depth control problematic [6]. Second, for nanosecond-pulse laser cleaning, parameters such as wavelength, intensity and pulse duration usually have to be carefully tailored to the specific materials and applications. Optimisation of these parameters is often a tedious and time-consuming process involving significant trial-and-


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error, because every heritage object has distinctive features requiring specific treatment conditions [5]. Ultrafast lasers with pulse durations around 100 femtoseconds (1 fs = 10-15 s) are able to overcome both these issues. First, ultrafast pulses are shorter than the time scale of most important energy relaxation processes, such as electron-to-lattice energy transfer and electronic heat conduction [6-8]. As a result, the absorbed energy is retained entirely within the thin surface layer only tens of nanometres thick, and is unable to form heat- and shock-waves that penetrate the bulk. Second, the significantly higher laser intensities generated by ultrafast lasers lead to a radically different laser-matter interaction mechanism, which is much less materialspecific than that produced by nanosecond lasers. The first salient feature of the ultrafast laser-matter interaction is that only the electrons are excited during the pulse, leaving the atomic lattice of the material unaffected at timescales less than that required for the transfer of the absorbed energy from the hot electrons to the lattice (the electron-phonon coupling time - typically longer than picoseconds [6-8]). As a result, the absorbed energy is accumulated by electrons in the ‘skin layer’ well before the energy can be transferred to the bulk by electron-phonon coupling and heat conduction. The cold atoms are then swiftly displaced from their positions in the lattice by the electrostatic field imposed by the gradient of electronic temperature across the skin layer [6-10]. This causes the thin surface layer to be ablated faster than the time required for energy dissipation from the surface into the bulk; consequently it is termed ‘cold’ ablation. Due to the absence of significant heat- or shock-wave generation, the laser-affected region near the surface is only a few hundred atomic layers thick, resulting in etch depths as low as 20 nm per pulse which significantly improves the ablation precision [8,11,12]. The advantages of precise removal of surface layers by ultrafast pulses have


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already been proven in applications that are sensitive to collateral damage, such as dentistry, eye surgery, and precise micromachining [7,13-15], and lately in the cleaning of painted works of art and discolouration of dyes [10,16-23]. The second important feature of ultrafast laser ablation is that the absorption of laser energy has a strongly non-linear character due to the avalanche and multiphoton ionisation processes occurring in the laser-absorbing layers. Further, as a result of the short timescales involved in the laser energy transfer, any phase transformations induced in the bulk of the solid lattice occur in non-equilibrium conditions, creating material properties drastically different from their longpulse equilibrium counterparts. As a result, the ablation threshold becomes more dependent on the electronic characteristics of the material, such as the Fermi energy and ionisation potential, and much less sensitive to bound-electron optical properties [9,24]. At a given laser wavelength, for example, the ultrafast laser ablation threshold for metals is lower (due to the much higher density of free electrons) than that for dielectric material. Consequently the ablation threshold is much less dependent on the material composition than it is on whether the material is a metal or a dielectric. A number of treatments have been proposed in the past for cleaning fabrics containing gold metal threads. Unfortunately, treatments that remove corrosion are generally damaging to either the metal or the organic components of the thread, or both. Past dry cleaning treatments have included abrading metal threads with glass beads (a delicate form of sandblasting), which removes the corrosion but produces a matte surface quite different to the thread’s original sheen [25-28]. This method, and brushing with a glass bristle brush, also quickly removes the thin coatings on plated metal threads, dramatically changing the visual appearance of the thread.


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These methods also leave glass fragments in the textile, which can abrade the organic threads [29]. Irradiating tarnished silver thread with UV/Ozone successfully removes tarnish and contaminants from the metal foil, but the effect of such irradiation on wool and silk includes loss of surface lipids, oxidation and yellowing, and the method was deemed too damaging for the organic components of the textile [30]. A range of aqueous and non-aqueous cleaning solutions have also been applied. However, the residues of such chemicals will continue to degrade the organic components of the textile after cleaning, leading to weakening and even complete disintegration [31]. Acidic and alkaline solutions, such as 3% ammonia in water, and 10% hydrochloric acid with thiourea and detergent, have been used to remove corrosion, but the substantial risk of deterioration and dye colour change if the solutions penetrated the organic components of the textile meant that treatment was restricted to the highest surfaces of the metal threads [32]. Similarly the use of metal polish left stains on the fabric that could not be removed and immersion of metal thread textiles in chemical baths allowed harsh cleaning solutions to penetrate to the core of organic components, from where they could not be thoroughly flushed away [29]. Macleod and Car [33] treated gilded silver alloy embroidery by immersing a textile in a pH buffered reducing sodium dithionite solution, with pH and voltage measurements used to confirm removal of all dithionite after treatment. Although good results were achieved on the test object, this treatment risks causing mechanical damage to the metal thread through swelling of the fibre core, and it could not be used for textiles that had water sensitive dyes, adhesives or other components. Non-aqueous organic acid solutions such as 3% formic acid and 2% thiourea in acetone have also been used on the basis that they may be less disruptive to silk elements [34], but while they


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removed corrosion effectively they raised the same problems as other solutions in terms of pH related changes, mobility of metal ions, degradation through residue contamination and physical disruption and delamination of components. Wet cleaning techniques are problematic in any case, as the penetration of liquid into organic elements can cause swelling and splitting of the metal thread [35] as well as de-lamination of metal threads that incorporate sandwiched layers of metal and paper, leather or gut [36]. Metal ions in solution can also be washed into the organic components, causing staining and potentially catalysing further degradation of the fibres [37], and contaminants left on the metal surface promote re-corrosion [38]. Restoration using long, nanosecond laser pulses has been satisfactorily applied to cleaning metal threads [39-41], which suggests that long-pulse laser cleaning, which is dependant on the original colour of the sample, may serve as a useful tool for efficient removal of dark carbonaceous soiling - superior to dry or wet cleaning of brittle and extremely fragile objects with organic material.

However, each particular sample requires trial-and-error tests to

determine the unique optimal combination of laser parameters involved (wavelength, number of pulses, fluence, pulse duration), which is essential to define the best laser condition for each particular case. Ultra-short pulse lasers differ from these previous treatments in that they provide a much more universal solution since the laser interaction with the material is based on the electronic properties (metal or dielectric) but not on the sample colour. Further, the effects of femtosecond lasers are limited to a very thin surface layer enabling fine control, and the treatment does not introduce contaminating or physically disruptive cleaning materials. Environmental controls after cleaning are required to prevent further corrosion by airborne pollutants, but the thinness of


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the layer removed (10’s of nm ablation depth compared to the 6 µm thickness of the gold foil) means that ultrafast laser cleaning of the metal thread can be repeated if necessary. In this paper we show that the difference in the ablation thresholds provides conservators with a unique capacity to selectively clean metallic surfaces such as gold foil, while leaving dielectric materials, e.g. silk thread, intact. We demonstrate this selectivity through the use of ultrafast laser pulses to remove corrosion from gold braid (so-called “cloth-of-gold”), which cannot be done by traditional mechanical or chemical conservation methods [25]. Additionally, the low infrared photon energy of the ultrashort laser we employ reduces the danger of discoloration through photodecomposition of pigments in the treated objects [2,5,22,26,27].

The results

presented pave the way for restoration of other complex artefacts composed of delicate materials, which cannot be treated by other means.

II. Experimental A. Sample overview Gold metal threads have been used in textiles for centuries [29,30,32,35,42], while other metal threads have been used to decorate textiles for thousands of years. The earliest known samples show metal filaments woven directly into the fabric, but the technique of winding a metal filament around a fibre core (usually silk) was developed around the fifth century BC, producing a more flexible and versatile thread. In the medieval period and the following centuries, a number of further metal thread manufacturing methods were developed, including the use of alloys of gold, silver and copper, and the production of wire using dies and drawplates [35,42]. These technical advances facilitated an increasing use of metal threads in tapestries, rich brocades, ecclesiastical textiles and, pertinently to this paper, military textiles.


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The sample (Fig. 1) employed in this work is a section of British army uniform gold braid from the Australian colonial period, c.1890 (manufacturer unknown). The gold braid is, at 45mm wide, quite large and was probably used to decorate a ceremonial “Sam Browne” style harness, in which an external belt is supported on the left-hand side by a shoulder strap [43]. The braid is decorated in an oak-leaf pattern with a gold thread, comprising a gold/silver/copper alloy foil wrapped around a silk core, and held in place with silk warp threads. Elemental analysis conducted using Energy-Dispersive X-ray Spectroscopy (EDX) revealed that the alloy is composed of 52-55% gold, 40-45% silver, and 3-8% of copper, with some traces of Al, Fe and Co; the composition had a few per cent spot-to-spot and sample-to-sample variation. The silk fibre of the core and warp threads was identified using optical microscopy and burn tests [44]. As seen in this sample, gold alloyed with copper and silver is prone to corrosion which changes the metal colour and reflectivity and significantly reduces the aesthetic impact of the thread [35].

As described above, previous treatments to remove the corrosion caused

unacceptable damage, but gentler treatments that removed only general dirt could not recapture the metal’s original brilliance [25]. The experiments reported in this paper aim to remove the tarnish and other surface contaminants that may catalyse further deterioration, and return the gold braid to its original sheen.


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Figure 1. The gold braid, showing different levels of laser treatment. The left hand side of the braid has been folded over in storage, protecting it from exposure to dirt and pollutants and preserving the original bright appearance of the metal thread. The right hand side of the braid has been contaminated with dirt and tarnish, and has become dull and brown in colour. Area “a” was left untreated as a control. The irradiated regions appear above the bars marked. The square area “b” was cleaned with 170-fs laser pulses, using an optimal laser fluence of 1.5 J/cm2. Square area “c” was cleaned with ultra-short laser pulses, deliberately using an excessive laser fluence of 5.0 J/cm2. It can be seen that area “c” is visibly damaged, whereas there is virtually no visual difference between area “a” which has been naturally preserved, and area “b” which has been cleaned using ultrafast pulses in the optimal regime of laser irradiation.

B. Laser irradiation conditions The laser system we employ here for ultrafast laser cleaning has been described in detail elsewhere [11,12]. The 170-fs, ~790-nm titanium sapphire laser provides ~3 mJ per pulse with a repetition rate of 250 Hz. The pulse bandwidth was measured to be 85 cm-1 FWHM, which corresponds to 172-fs pulse width assuming a Fourier-transform-limited Gaussian function.


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Figure 2 illustrates how the laser beam was expanded, collimated and scanned across the sample using an orthogonal pair of galvanometer scanners.

Figure 2. Schematic diagram of laser ablation and LIBS apparatus. Mx and My are the scanning mirrors, L is the telecentric focusing lens, BS is the long-pass dichroic beam splitter, and SP the spectrometer. The laser beam path is shown on the sample surface.

The fluence threshold for laser ablation of metals by sub-picosecond pulses is determined by the condition that the energy of electrons accelerated by the laser field exceeds the sum of the atomic binding energy εb and the work function εesc by the end of the pulse. For dielectrics an additional energy is required to ionise the atoms, i.e. to transfer the electron from the valence band to the conductivity band. Thus, threshold fluences for metals Fthm and dielectrics Fthd are defined as the following [6,9,10]: Fthm = ( ε b + ε esc )

l s ne ln ; Fthd = ( ε b + J i ) s e ; A A


(1)

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here ls is the skin layer thickness, ne is the electron density, Ji is ionisation potential, and A is absorption. At threshold, the density of the ionised electrons saturates at the atomic number density ne ≅ na; as the absorption of the laser light occurs in the skin layer, to a first-order approximation A ls ≅ 4π λ . Thus, for a Ti:sapphire laser at 800 nm, the threshold ablation fluence varies in the range from 0.2 J/cm2 to 0.5 J/cm2 for a wide range of metals (i.e. laser intensities above Il ~1012 W/cm2), yet typically is in the range 2.0 J/cm2 – 2.5 J/cm2 for dielectrics (Il ~1013 W/cm2), whether transparent or opaque [6,9]. The ablation threshold for gold predicted by this approximation is 0.5 J/cm2 which coincides well with the experimentally determined value of (0.45±0.1) J/cm2 [45].

As a general rule, the ablation threshold for

dielectrics in the ultra-short pulse regime is about five to ten times higher [6,9,10,46]. Surface ablation for longer pulses i.e. for durations greater than the electron-lattice energy transfer time (1 – 10 picoseconds) occurs when the generated heat wave propagates over distances greater than the skin layer (ltherm >> ls) during the pulse (duration tp). This defines the heat penetration depth (diffusion length) which, when combined with the maximum permitted temperature Tmax (typically the metal melting point), determines the ablation threshold fluence [6,47,48]:

(

Fth ≅ Dtht p

1/2

)

TmaxC L na M a ; 2A

(2)

where Dth is thermal diffusivity of the material, CL is the heat capacity and Ma is the atomic mass. For a 6-ns pulse, the ablation threshold fluence for gold (Dth = 1.277 cm2/s; Tmax =1,337 K; CL = 0.129 J/(g cm3); na = 5.9×1022 cm-3, Ma = 197; A = 0.04) is predicted to be ~7.3 J/cm2. For comparison with ultrafast treatment using the femtosecond laser described above, experiments were also performed with a long-pulse laser comprising an optical parametric


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oscillator (OPO) and two-stage Ti:sapphire amplifier, the details of which are described elsewhere [49,51]. The pulse duration was measured to be ~6 ns FWHM and the maximum pulse energy was ~40mJ at ~830nm. The beam was focused to ~250 µm diameter spot on the sample surface, and attenuated by a neutral-density filters to yield laser fluence levels similar to that used for the ultrashort pulse laser treatment. C. Scanning pattern One of the key requirements for even and precise removal of contaminants from the surface is a high degree of control of the focused beam position over the whole scanning area. The ultrafast laser scanning system [12] employed in-house software to create patterns with constant scanning speeds up to 20 m/s. The high scanning speed ensured that the number of laser pulses per spot could be controlled over a broad range starting with a single pulse per spot, and with the maximum speed limited by the inertia of the scanning mirrors. A telecentric lens with a focal distance of 275 mm was used to guarantee a flat-field focus across the sample. The focal spot diameter was measured to be ~20 µm FWHM, delivering maximum laser intensities on the sample surface up to Il ~7.5×1014 W/cm2 (fluence up to ~120 J/cm2), which was further attenuated to obtain the desired fluence level. The confocal parameter was measured to be ~1.5 mm, which determined the maximum permissible level of unevenness of the fabric samples. The position of the focal spot was controllable within ±10 µm to assure a constant laser intensity over the scanning area (2×2 cm2). A moving race-track scanning pattern [12] was chosen to provide the most uniform ablation, and applied to a sequence of near-square ablated areas for the test experiments to determine the optimum focal spot sizes and laser intensities.


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D. In-situ spectroscopic monitoring To exploit the full capability of ultrafast laser cleaning, the controlled removal of material requires careful monitoring of the etch depth at nanometre scales. The ablation process was monitored in-situ using laser-induced breakdown spectroscopy (LIBS) to detect the elemental composition of the ablated plume, and thus provide the feedback needed to halt the ablation process when the contaminant layer removal is complete [52-54]. The advantages of LIBS include simplicity of operation, the ability to provide remote in situ analysis through coupling into an optical fibre, and a fast response time which is essential for large-scale applications. Importantly, this is a minimally destructive analytical method because the amount of ablated material required is of the order 10-10 – 10-6 g per pulse, yielding a sensitivity as low as 10 ppm [54]. The optical emission was collected by the 80 mm diameter telecentric lens, and separated by a broadband beam-splitter located between the telescope and the beam scanning mirrors. Suppression of the scattered 780 nm laser radiation was provided by a 700 nm edge-pass filter. The optical emission was then focused into a spectrometer (Ocean Optics HR2000 with a 600 line/mm grating) covering the spectral range 350-850 nm and providing a wavelength resolution of ~1.0 nm. A key feature compared to conventional LIBS schemes is the retro-collection of spectra through the aperture of the focusing/scanning system (Fig.2), so that the emission spectra precisely follow the ablation path, enabling the emission from each successive ablation plume to be collected and monitored with the same efficiency. Changes in the relative intensity of the spectral lines originating from the contaminant species and from the underlying layer to be preserved thereby provide in situ feedback on the ablation process.


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Atomic and ionic emissions from the ablated plume yield characteristic line spectra determined by the composition of the ablated layer, and by the plasma temperature which is laser-intensity dependent. The spectroscopic signature used to determine when to halt the cleaning process was the ratio of the relative intensities of the silver (Ag) and carbon (C) lines at ~547nm and ~589nm respectively. At ~589nm there are two strong, nearby C lines used as the contaminant signature: the 2s24p 2P° 3/2 – 2s23d 2D 5/2 transition and the 2s24p 2P° 1/2 - 2s23d 2D 3/2 transition, with transition rates of ~3.2×107 and ~3.5×107 s-1 respectively [49]. The Ag line at ~547nm from the 4d105d 2D5/2 – 4d105p 2P03/2 transition with a transition rate of 8.6×107 s-1 was used as the underlying surface signature, since it is much stronger than the strongest gold (Au) lines in this spectral region e.g. the ~628 nm Au line with a transition rate of 3.4×106 s-1.

III. RESULTS AND DISCUSSION A. Single-shot laser cleaning studies First we determined the optimum single-shot fluence needed to remove the contaminant layer without damaging the underlying gold foil. The optical microscope images (a-c) show the thin, ~12-µm silk warp threads which hold the 200-µm gold threads in place, and which are directly exposed to the laser radiation. Figures 3 (d-f) show Scanning Electron Microscopy (SEM) images of the samples, indicating the long strips of gold foil 250 µm wide and 6 µm thick wound around the 200-µm diameter silk core.


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Figure 3. Single-shot ultrafast laser cleaning results. Optical microscope (a-c) and scanning electron microscope (d-i) images of gold thread samples treated with 170-fs laser pulses. (a), (d) and (g): sample before treatment; (b), (e) and (h): sample after laser cleaning at the optimal fluence 1.5 J/cm2. (c), (f) and (i): damaged samples after exposure to an excessive fluence of 5.0 J/cm2 (laser damaged spots of ~20 µm size can be clearly seen on the surface of the gold foil). The scale bars are 0.5 mm in the (a,b,c) optical images, 100 µm (d,e,f) and 10 µm (g,h,i) in the SEM images.

B. Evaluation of single-shot ablation depth In order to determine approximately the single-shot ablation depth on the gold braid and hence the degree of ablation control, we undertook experiments using a sample of pure (99.99%) uncontaminated gold, measuring the ablation depth simultaneously with a reference spectrum using the LIBS technique. The spectrum in Figure 4a shows the most intense characteristic


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emission lines occurring at 479.3, 523.0, 565.6, 583.7, 595.7, and 627.8 nm from excited gold atoms [55]. Fig. 4b also shows the single line intensity at 628 nm as a function of laser fluence (circles), where the solid line is a least-squares fit whose intercept at the abscissa (the LIBS detection limit) is 0.81 ± 0.37 J/cm2. The line intensities and error values (1σ) are calculated by a Gaussian fit to the spectral peak. The triangles show the measured ablation depth, where the dashed line is a theoretically predicted logarithmic dependence of the ablated depth on laser fluence [56]. The thickness of the gold ablated layer at 0.81 J/cm2 (corresponding to the zero line emission intensity LIBS detection limit) is less than 10 nanometres - the limit of the LIBS ablation depth resolution. The threshold ablation value given by the intercept of the dashed line is 0.67 J/cm2, just slightly higher than for previous measurements of gold ablated with ultrashort laser pulses (~0.5 J/cm2) [9].

Figure 4. Spectroscopy of the ablated plume. (a) LIB spectrum for pure gold at a fluence of 2.5 mJ/cm2. (b) Triangles: ablation depth of a pure gold target vs. laser fluence. The dashed line for the ablation depth is a theoretical prediction [9], and the horizontal axis intercept at 0.65 ± 0.2 J/cm2 represents the experimentally determined threshold fluence. Circles: 627.8 nm line intensity measured in LIB spectra vs. laser fluence. The solid line for the LIBS line intensity is a linear least-squares fit to the data.


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C. LIBS determination of the optimum cleaning fluence Next we studied the LIB spectrum for the plasma plume ablated from the contaminated gold braid, shown as a function of the single-shot laser fluence in Fig. 5.

EDX analysis of

contamination shows the presence of C, O, Al, Na, S, Cl, Mg, Si, and Ca in various locations, with carbon present in all areas and in most cases showing the strongest emission intensity. For this reason we have chosen carbon as the contaminant signature. The strong peak at ~589 nm is assigned to unresolved carbon (C) lines at 589.0 and 589.2 nm, while strong silver (Ag) lines are observed at 521 and 547 nm, with some weak gold (Au) lines at 584 and 628 nm. The ~589 nm C line was the dominant contaminant signature, while the ~547nm Ag line was used as the signature for the underlying surface. The LIBS indicate that over the fluence range investigated from 0.25 J/cm2 to 5.0 J/cm2, the contaminant C line starts to become visible above 0.35 J/cm2, while the Ag line appears above 0.75 J/cm2. In order to determine the correspondence between the spectroscopic signature and the optimal single-shot laser cleaning fluence, we compare the surface appearance at varying fluence levels using optical and electron microscopy (Fig. 3). Just above the optimum fluence range (1.25 – 2.0 J/cm2), the ratio of the C- to Ag-line intensity becomes greater than unity: this coincides with the maximum allowed single-shot fluence, and thus defines the spectroscopic threshold (Fig. 5b). Further, at ~2.5 J/cm2 the strongest Au line at ~628 nm appears, and also serves to indicate the upper fluence limit that will safeguard the integrity of the underlying gold foil. Once the optimum single-shot fluence was determined, we undertook experiments using multiple shots at the same position on the surface. Note that the surface damage threshold is not determined by the cumulative fluence but by the single-shot fluence – the electron microscope images indicate that additional shots simply continue to uniformly ablate further material from


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As can be seen from Fig. 6, the spectra after ~3 shots changes the relative

dominance of the Ag- / C-line emission intensity, thereby indicating that the contaminant layer is effectively removed.

Figure 5.

Single shot laser fluence dependence.

(a) Single shot LIB spectra from

contaminated gold foil in the fluence range 0.25 J/cm2 to 5.0 J/cm2; the background level for each spectrum is shifted for clarity. (b) Emission intensity ratio of silver (Ag 547 nm) to carbon (C 589 nm) spectral lines: the solid line is a linear fit to the data.


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Figure 6. Multiple shot spectra. (a) – Spectrum of the laser plume for 4 successive shots at the same surface position for a single-shot fluence of 1.25 J/cm2 (spectra shifted vertically). (b) – Relative intensity of Ag 547 nm to C 589 nm lines for successive laser shots the same surface position. D. Laser Ablation of Silk Due to the difficulty of removing and separately conducting tests on the silk thread in the gold braid sample which would have made assessment of the effect on the artefact problematic, we undertook damage threshold studies of separate samples of silk thread using commercially available samples.

In these experiments we employed the same laser pulses and the same

optical system in order to reproduce the conditions of the gold braid experiments. The results of the exposure to scanning laser treatment are shown in the optical microscope images in Fig.7 below. The untreated image (a) is almost indistinguishable from the 0.8 J/cm2 fluence image (b) apart from a very slight reduction in the colour intensity. At the highest fluence in image (d) using 2.7 J/cm2, damage to the silk thread structure is clearly visible, and this is also seen to a lesser degree in image (c) at 1. 7 J/cm2. The damage threshold is consistent with that seen in the gold braid samples at ~1.5 J/cm2.


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Further examination of the samples under an electron microscope yielded similar conclusions. Fig.8 shows the electron microscope images at varying fluence levels, with image (d) (2.7 J/cm2) showing clear evidence of morphological damage. As the fluence is reduced to first 1.7 J/cm2 and 1.3 J/cm2 the damage is less apparent, consistent with the optical images. In order to determine whether the elemental composition of the silk was affected by the laser treatment, energy dispersive x-ray spectroscopy (EDX) was employed during the electron microscope investigations. The EDX diagnosis showed no change to the elemental composition as a function of laser fluence compared to the untreated sample.

This indicates that

morphological change does not appear to be accompanied by any loss of specific elemental components in the thread.

Figure 7.

Optical microscope images of physical damage of silk by laser treatment.

Untreated (a) and treated silk at the following fluences: (b) 0.8 J/cm2 (c) 1.7 J/cm2 (d) 2.7 J/cm2. 100 µm scale bars are shown.


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The results of these experiments with silk demonstrate that laser fluences below ~1.3 J/cm2 should be employed for safe ultrashort laser cleaning of “cloth-of-gold”. In order to fully determine the long-term effects of any treatment process (short-pulse laser or otherwise), ideally this requires longitudinal studies of the effects of the treatment over many years, using both new and aged samples. Such studies are beyond the scope of the present experiments, but would be desirable to determine the ultimate effectiveness of this technique for artefact conservation.

Figure 8. Electron microscope images of physical damage of silk by laser treatment. Untreated (a) and treated silk at the following fluences: (b) 1.3 J/cm2 (c) 1.7 J/cm2 (d) 2.7 J/cm2. 100 µm scale bars are shown.


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E. Comparison with nanosecond laser treatment To compare laser-cleaning treatment using ultrafast and nanosecond pulses, we also performed single-shot experiments with the OPO/OPA system using the same fluence level (1.5 J/cm2 – 2 J/cm2). This is above the ablation threshold for gold for fs pulses (0.5 J/cm2), but well below the ablation threshold of ~7.3 J/cm2 determined in section II-B above for 6-ns laser pulses. The SEM images in Fig. 9 clearly demonstrate that while the contaminants were successfully removed from the surface, nanosecond pulses caused thermal melting of the gold foil surface layer. Several spots demonstrate cracking of the thin metal foil (Fig. 9f), most probably due to uneven heating and subsequent uneven cooling. Further, EDX analysis shows a considerable change in the composition of the surface layer, with a significant increase in the proportion of silver (up to 85%), and a concomitant reduction in gold (down to ~2%). The exposed silk also exhibits burning and decomposition.

The results clearly demonstrate the advantages of

femtosecond laser treatment over nanosecond pulses in both material selectivity and fine control.


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Figure 9. Comparative single-shot laser-cleaning results using ultrashort and nanosecond pulses at the same laser fluence (1.5 J/cm2 – 2.0 J/cm2). (a,d) Gold thread surface before laser treatment; (b,e) after treatment with 170-fs laser pulses, showing no damage; and (c,f) after treatment with 6-ns laser pulses showing surface damage. The scale bars are 100 µm in (a, b and c) and 10 µm in (d, e, and f).

IV. CONCLUSIONS We have shown that ultrafast laser cleaning effectively removes contaminants from gold braid without altering the integrity either of the gold foil, or of the silk core or warp threads. The integrity of the gold foil is maintained due to the fine ablation control (of the order of 10’s of nanometres) combined with the LIBS monitoring technique, while the integrity of the silk thread is maintained because of the ability of the ultrafast laser ablation process to provide strong selectivity between metallic and dielectric materials. The advance in ultrashort laser treatment presented in this study is the demonstration of successful treatment of complex and fragile heritage objects in a safe and controllable way, which further to previous fs studies [11, 16-22] is achieved via in-situ monitoring of the ablated plume to provide immediate feedback on the species removed from the surface by each laser pulse. These findings offer a wide range of opportunities for restoring historic textiles and other materials that are untreatable by other means. With the rapid development of powerful and compact femtosecond lasers, ultrafast laser ablation has the potential to become a standard tool in the conservation armoury, and a key technique for conserving previously untreatable artworks and heritage objects.


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ACKNOWLEDGEMENTS The authors would like to acknowledge support for this work from an Australian Research Council Linkage Project grant LP0668117 which included the Australian War Memorial, the Army Heritage Unit, the Naval Heritage Collection, the Art Gallery of New South Wales and Artlab Australia. The authors would also like to thank Jane Peek for providing information on the provenance of the gold braid, and Sarah Clayton and Bridie Kirkpatrick of the Textile Conservation Laboratory of the Australian War Memorial for assistance with identifying the textile elements of the gold thread decoration.

REFERENCES [1]

Nevin, A.; Pouli, P.; Georgiou, S.; Fotakis C. Laser Conservation in art. Nature Materials

2007, 6, 320-322. [2]

Luk’yanchuk, B. S.; Zafiropulos V. On the theory of discoloration effect in pigments at

laser cleaning. In: Luk’yanchuk, B. (ed.) Laser cleaning, 2002 (World Scientific, New Jersey,) Ch. 8, 393–414. [3]

Fotakis, C.; Anglos, D.; Zafiropulos, V.; Georgiou, S.; Tornari, V. Lasers in the

Preservation of Cultural Heritage 2007 (Taylor and Francis, New York). [4]

Kane, D. M.; (ed.). Laser Cleaning II 2006 (World Scientific, Singapore).

[5]

Pouli, P.; Oujja, M.; Castillejo M. Practical issues in laser cleaning of stone and painted

artefacts: optimisation procedures and side effects. Appl. Phys. A 2012, 106, 447-464. [6]

Gamaly, E. G. The physics of ultra-short laser interaction with solids at non-relativistic

intensities. Physics Reports 2011, 508, 91-243.


24

Page 25 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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[7] Gattass, R. R.; Mazur E. Femtosecond laser micromachining in transparent materials. Nature Photonics 2008, 2, 219-225. [8] Corkum, P. B.; Brunel, F.; Sherman, N. K.; Srinivasan-Rao, T. Thermal response of metals to ultra-short-pulse laser excitation. Phys. Rev. Lett. 1988, 61, 2886-2889. [9] Gamaly, E. G.; Rode, A. V.; Luther-Davies, B.; Tikhonchuk V. T. Ablation of solids by femtosecond lasers: ablation mechanism and ablation thresholds for metals and dielectrics. Phys. of Plasmas 2002, 9, 949-957. [10] Gamaly, E. G. Femtosecond Laser-Matter Interaction: Theory, Experiments and Applications 2011 (Pan Stanford Publishing, Singapore). [11] Rode, A. V.; Baldwin, K. G. H.; Wain, A.; Madsen, N. R.; Freeman, D.; Delaporte, Ph.; Luther-Davies B. Ultrafast Laser Ablation for Restoration of Heritage Objects. Appl. Surf. Science 2008, 254, 3137-3146. [12] Rode, A. V.; Freeman, D.; Baldwin, K. G. H.; Wain, A.; Uteza, O.; Delaporte Ph. Ultrafast pulse laser cleaning of paint: Scanning the laser beam. Appl. Phys. A, 2008, 93, 135139. [13] Rode, A. V.; Gamaly, E. G.; Luther-Davies B.; Taylor, B. T.; Dawes, J.; Chan, A.; Lowe, R. M.; Hannaford, P. Subpicosecond laser ablation of dental enamel. J. Appl. Phys., 2002, 92, 2153-2158. [14] Juhasz, T.; Frieder, H.; Kurtz, R. M.; Horvath, C.; Bille, J. F.; Mourou, G. Corneal refractive surgery with femtosecond lasers. IEEE Journ. Select. Topics Quantum Electronics 1999, 5, 902-910. [15] Sundaram S. K.; Mazur, E. Inducing and probing non-thermal transitions in semiconductors using femtosecond laser pulses. Nature Materials 2002, 1, 217-224.


25

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 34

[16] Pouli, P.; Bounos, I.; Georgiou, S.; Fotakis, C.; Doulgeridis M. Femtosecond laser cleaning of painted artefacts: Is this the way forward? In: 6th International Congress on Lasers in Conservation of Artworks Book of Abstracts 2005 (Academy of Fine Arts, Vienna) p.67. [17] Burmester, T.; Meier, M.; Haferkamp, H.; Barcikowski, S.; Bunte, J.; Ostendorf A. Femtosecond laser cleaning of metallic cultural heritage and antique artworks. In: Femtosecond laser cleaning of metallic cultural heritage and antique artworks, Proc. 5th Int. Conf. Lasers in the Conservation of Artworks (LACONA V) 2005, Ed.: Dickmann, K.; Fotakis, C.; Asmus, J. (Springer Proceedings in Physics 100, New York) p.61-69. [18] Pouli, P.; Paun, I.-A.; Bounos, G.; Georgiou S.; Fotakis, C. The potential of UV femtosecond laser ablation for varnish removal in the restoration of painted works of art. Appl. Surf. Sci., 2008, 254, 6875-6879. [19] Gaspard, S., Oujja, M., Moreno, P., Mendez, C., Garsia, A., Domingo, C., Castilejo, M.. Interaction of femtosecond laser pulses with tempera paints. Appl. Surf. Science 2008, 255, 26752681. [20] Walczak, M.; Oujja, M.; Crespo-Arcá, L.; García, A.; Méndez, C.; Moreno, P.; Domingo, C.; Castillejo, M. Evaluation of femtosecond laser pulse irradiation of ancient parchment. Appl. Surf. Science 2008, 255, 3179-3183. [21] Pouli, P.; Paun, I.-A.; Bounos, G.; Georgiou, S.; Fotakis, C. Laser assisted removal of synthetic painting-conservation materials using UV radiation of ns and fs pulse duration: Morphological studies on model samples. Appl. Surf. Science 2008, 255, 4955-4960. [22] Castillejo, M.; Martin, M.; Oujja, M.; Silva, D.; Torres, R.; Manousaki, A.; Zafiropulos, V.; van den Brink, O. F.; Heeren, R. M. A.; Teule, R.; Silva, A.; Gouveia, H. Analytical study of


26

Page 27 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

the chemical and physical changes induced by KrF laser cleaning of tempera paints. Analytical Chemistry 2002, 74, 4662-4671. [23] Oujja M.; Sanz M.; Rebollar E.; Marco J.F.; Domingo C.; Pouli P.; Kogou S.; Fotakis C.; Castillejo M. Wavelength and pulse duration effects on laser induced changes on raw pigments used in paintings. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 2013, 102, 7-14. [24] Kautek, W.; Krüger, J.; Lenzner, M.; Sartania, S.; Spielmann, Ch.; Krausz F. Laser ablation of dielectrics with pulse durations between 20 fs and 3 ps. Appl. Phys. Lett. 1996, 69, 3146-3148. [25] Toth. M. Lessons learned from conserving metal thread embroidery in the Esterházy Collection, Budapest, Hungary. Studies in Conservation 2012, 57, Supp. 1, 305-312. [26] Escudero, C.; Martínez, M. A.; Moreno, P.; García, A.; Méndez, C.; Prieto, C.; Sanz A. Study of laser cleaning of ancient fabric with femtosecond pulses.

In: Lasers in the

Conservation of Artworks, Proc. 7th Int. Conf. Lasers in the Conservation of Artworks (LACONA VII) 2008, Ed. Castillejo, M., Moreno, P., Oujja, M., Radvan R. and Ruiz, J. (CRC Press, London), 337-343. [27] Oujja, M.; Pouli, P.; Domingo, C.; Fotakis, C.; Castillejo, M. Analytical spectroscopic investigation of wavelength and pulse duration effects on laser-induced changes of egg-yolkbased tempera paints. Applied Spectroscopy 2010, 64, 528-536. [28] Gibson, B. M. The Use of the Airbrasive Process for Cleaning Ethnological Materials. Studies in Conservation 1969, 14, 155-164. [29] Withers, M.C; Markell, G.; Cavallo A. S. A Dry Method of Cleaning Metallic Yarns and Ornaments in Textiles. Studies in Conservation 1964, 9, 91-106.


27

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Page 28 of 34

[30] Hacke, A.-M.; Carr, C. M.; Brown, A.; Howell, D. Investigation into the nature of metal threads in a Renaissance tapestry and the cleaning of tarnished silver by UV/Ozone (UVO) treatment. Journal of Materials Science 2003, 38, 3307 – 3314. [31] The organic elements of a metallic thread textile cleaned by immersion in citric acid and thiourea were observed to have almost completely disappeared some years after treatment, presumably having crumbled to dust in storage. This was despite attempts to flush residual chemicals from the textile after cleaning. Sarah Clayton, textile conservator at the Australian War Memorial, personal communication 12.06.2013. [32] Doré, J. The Conservation of Two Eighteenth-Century English Court Mantuas. Studies in Conservation, 1978, 23, 1-14. [33] MacLeod, I. D.; Car, R. Conservation of a mid-18th-century Italian embroidered ecclesiastical cope and treatment of corroded metal threads using neutral dithionite solutions. In: ICOM-CC 17th Triennial Conference Preprints, Melbourne, 15–19 September 2014, ed. J. Bridgland, Paris: International Council of Museums 2014, art. 1306, 1-7 (ISBN 978-92-9012410-8) [34] Howell, D. Experiments with chemical cleaning for metal threads. Conservation of Metals: Problems in the Treatment of Metal-organic and Metal-inorganic Composite Objects. In: International Restorer Seminar, Ed. Járó, M., Veszprém, Hungary, 1-10 July, 1989, pp. 8789. [35] Timár-Balázsy, A.; Eastop D.; Járó M. Chemical Principles of Textile Conservation 1998 (Elsevier Butterworth-Heinemann, Oxford,). [36] Járó, M.; Gál, T.; Tóth A. The Characterization and Deterioration of Modern Metallic Threads. Studies in Conservation 2000, 45, 95-105.


28

Page 29 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

[37] Darrah, J. A. The microscopic and analytical examination of three types of metal thread. In: Conservation of Metals: Problems in the Treatment of Metal-organic and Metal-inorganic Composite Objects. Ed. M. Járó, International Restorer Seminar, Veszprém, Hungary, 1-10 July, 1989, pp. 53-63. [38] Járó, M. Re-corrosion of silver and gilt silver threads on museum textiles after treatment. In: Conservation of Metals: Problems in the Treatment of Metal-organic and Metal-inorganic Composite Objects, Ed. M. Járó, International Restorer Seminar, Veszprém, Hungary, 1-10 July, 1989, pp. 95-98. [39] Carmona N.; Oujjab M.; Roemichc H.; Castillejo M. Laser cleaning of 19th century Congo rattan mats. Appl. Surf. Science 2011, 257, 9935-9940. [40] Lee J.-M.; Yu, J.-E.; Koh, Y.-S. Experimental study on the effect of wavelength in the laser cleaning of silver threads. Journal of Cultural Heritage 2003, 4, 157s-161s. [41] Degrigny, C.; Tanguy E.; Gall, R. L.; Zafiropulos, V.; Marakis, G. Laser cleaning of tarnished silver and copper threads in museum textiles. Journal of Cultural Heritage 2003, 4, 152s-156s. [42] Rogerson, C.; P. Garside, P. Instrumental analysis of metal threads as an aid for interpretation and preservation of a fifteenth-century tapestry altar frontal and super frontal. In: Tapestry Conservation:

Principles And Practice, Eds. Lennard, F.; Hayward, M. 2006 (Elsevier Butterworth-Heinemann, Oxford). [43] Jane Peek, curator at the Australian War Memorial, personal communication, and http://www.army.gov.au/Our-history/Traditions/The-Sam-Browne-belt (accessed 12.06.2013).

[44] Corbman, B. Textiles: Fibre to Fabric 1985 (McGraw Hill Higher Education, London).


29

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Page 30 of 34

[45] Stuart, B. C.; Feit, M. D.; Herman, S.; Rubenchik, A. M.; Shore, B. W.; Perry, M. D. Optical ablation by high-power short-pulse lasers, J. Opt. Soc. Am. B 1996, 13, 459-468. [46] Tien, A.-C.; Backus, S.; Kapteyn, H.; Murname, M.; Mourou, G. Short-pulse laser Damage in transparent materials as a function of pulse duration. Phys. Rev. Lett. 1999, 82, 38833886. [47] Anisimov, S. I.; Imas, Ya. A., Romanov, G. S.; Khodyko, Yu. V. Action of high intensity radiation on metals. 1971 (National Technical Information Service, Springfield, Virginia). [48] Anisimov, S. I.; Luk’yanchuk, B. S. Selected problems of laser ablation theory. PhysicsUspekhi, 2002, 45, 293-324. [49] Kono, M.; Baldwin, K. G. H.; He, Y.; White, R. T.; Orr, B. J. Heterodyne-assisted pulsed spectroscopy with a nearly Fourier-transform limited, injection-seeded optical parametric oscillator. Opt. Lett. 2005, 30, 3413–3415. [50] He, Y.; Kono, M.; White, R. T.; Sellars, M. J.; Baldwin, K. G. H.; Orr, B. J. Coherent heterodyne-assisted pulsed spectroscopy: sub-Doppler two-photon spectra of krypton, characterizing a tunable nonlinear-optical ultraviolet light source, Appl Phys B 2010, 99, 609– 612. [51] Kono, M.; He, Y.; Baldwin, K. G. H.; Orr, B. J. Sub-Doppler two-photon spectroscopy of 33 Rydberg levels in atomic xenon excited at 205–213 nm: diverse isotopic and hyperfine structure. J. Phys. B 2013, 46, 035401. [52] A. W. Miziolek, V. Palleschi, and I. Schechter, (ed.). Laser-induced Breakdown Spectroscopy (LIBS): Fundamentals and Applications. 2006 (Cambridge University Press, New York).


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[53] D. Anglos. Laser-induced breakdown spectroscopy in art and archaeology. Applied Spectroscopy 2001, 55, 186A–205A. [54] Nevin, A.; Spoto, G.; Anglos, D. Laser spectroscopies for elemental and molecular analysis in art and archaeology. Appl. Phys. A 2012, 106, 339-361. [55] Ehrhardt, J. C.; Davis S. P. Precision wavelength and energy levels in gold. Journal of Optical Society of America 1971, 61, 1342–1349; and NIST ASD Team, NIST Atomic Spectra Database (ver. 4.1.0), 2011, National Institute of Standards and Technology, Gaithersburg, MD; http://physics.nist.gov/asd3 (accessed 1 October 2011). [56] Feit, M. D.; Rubenchik, A. M.; Kim, B. M.; Da Silva, L. B.; Perry M. D. Physical characterization of ultrashort laser pulse drilling of biological tissue. Appl Surf. Science 1998, 127-129, 869-874.


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FIGURE LEGENDS Figure 1. The gold braid, showing different levels of laser treatment. The left hand side of the braid has been folded over in storage, protecting it from exposure to dirt and pollutants and preserving the original bright appearance of the metal thread. The right hand side of the braid has been contaminated with dirt and tarnish, and has become dull and brown in colour. Area “a” was left untreated as a control. The irradiated region appears above the bars marked. The square area “b” was cleaned with 170-fs laser pulses, using an optimal laser fluence of 1.5 J/cm2. Square area “c” was cleaned with ultra-short laser pulses, deliberately using an excessive laser fluence of 5.0 J/cm2. It can be seen that area “c” is visibly damaged, whereas there is virtually no visual difference between area “a” which has been naturally preserved, and area “b” which has been cleaned using ultrafast pulses in the optimal regime of laser irradiation. Figure 2. Schematic diagram of laser ablation and LIBS apparatus. Mx and My are the scanning mirrors, L is the telecentric focusing lens, BS is the long-pass dichroic beam splitter, and SP the spectrometer. The laser beam path is shown on the sample surface. Figure 3. Single-shot ultrafast laser cleaning results. Optical microscope (a-c) and scanning electron microscope (d-i) images of gold thread samples treated with 170-fs laser pulses. (a), (d) and (g): sample before treatment; (b), (e) and (h): sample after laser cleaning at the optimal fluence 1.5 J/cm2. (c), (f) and (i): damaged samples after exposure to an excessive fluence of 5.0 J/cm2 (laser damaged spots of ~20 µm size can be clearly seen on the surface of the gold foil). The scale bars are 0.5 mm in the (a,b,c) optical images, 100 µm (d,e,f) and 10 µm (g,h,i) in the SEM images.


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Figure 4. Spectroscopy of the ablated plume. (a) – LIB spectrum for pure gold at a fluence of 2.5 mJ/cm2. (b) – Triangles: ablation depth of a pure gold target vs. laser fluence. The dashed line for the ablation depth is a theoretical prediction [9], and the horizontal axis intercept at 0.65 ± 0.2 J/cm2 represents the experimentally determined threshold fluence. Circles: 627.8 nm line intensity measured in LIBS spectra vs. laser fluence. The solid line for the LIBS line intensity is a linear least-squares fit to the data. Figure 5.

Single shot laser fluence dependence.

(a) – Single shot LIBS spectra from

contaminated gold foil in the fluence range 0.25 J/cm2 to 5.0 J/cm2; the background level for each spectrum is shifted for clarity. (b) – Emission intensity ratio of silver (Ag 547 nm) to carbon (C 589 nm) spectral lines: the solid line is a linear fit to the data. Figure 6. Multiple shot spectra. (a) – Spectrum of the laser plume for 4 successive shots at the same surface position for a single-shot fluence of 1.25 J/cm2 (spectra shifted vertically). (b) – Relative intensity of Ag 547 nm to C 589 nm lines for successive laser shots the same surface position. Figure 7.

Optical microscope images of physical damage of silk by laser treatment.

Untreated (a) and treated silk at the following fluences: (b) 0.8 J/cm2 (c) 1.7 J/cm2 (d) 2.7 J/cm2. 100 µm scale bars are shown. Figure 8. Electron microscope images of physical damage of silk by laser treatment. Untreated (a) and treated silk at the following fluences: (b) 1.3 J/cm2 (c) 1.7 J/cm2 (d) 2.7 J/cm2. 100 µm scale bars are shown. Figure 9. Comparative single-shot laser-cleaning results using ultrashort and nanosecond pulses at the same laser fluence (1.5 J/cm2 – 2.0 J/cm2). (a,d) – gold thread surface before laser


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treatment; (b,e) after treatment with 170-fs laser pulses showing no damage; and (c,f) – after treatment with 6-ns laser pulses showing surface damage. The scale bars are 100 µm in (a, b and c) and 10 µm in (d, e, and f).


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