The discoloured appearance of laser cleaned surfaces, which may appear under certain conditions, poses limitations to the practice and wide application of laser technology in CH conservation and thus urges for thorough investigation. Laser-induced colour changes are directly dependant to the involved materials and can be distinguished as follows:
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Discoloration on stonework: yellowing alteration is mainly associated with the use of infrared (IR) radiation (1064 nm) of nanosecond (ns) pulse duration to remove pollution encrustations from stonework [3–6]. Revealing of pre-existing historic layers and/or patinas [3], ‘staining’ of the original surface (as a result of the migration of the yellowish fraction which is present within the pollution crust and originates from polar organic compounds) [7–9], and selective vaporization of the various dark-coloured airborne particles (which are embedded in the gypsum-rich matrix of the pollution crust) at rather low laser fluences [4, 10], were among the scenarios introduced to explain such undesired coloration. In parallel, discoloration into grey has been also reported upon use of ultraviolet (UV) radiation to treat pollution crusts [4, 11].
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Darkening phenomena observed on painted surfaces upon intense and direct laser irradiation. The sensitivity of most pigments to the direct exposure to laser irradiation is an important issue, which requires careful study and thorough approach. Relevant research [12–16] has pointed out that darkening or, in the worst case scenario, blackening of painted surfaces is a material related alteration which occurs upon direct exposure to laser radiation. Although initially it was believed that such undesired alteration is irrelevant to the binding medium as it affects both paints and pigments in raw form, recent studies on the basis of different ablative methodologies using lasers of various wavelengths and pulse durations, proved that the presence of a binding media that absorbs highly the employed laser wavelength may safeguard the pigment itself [16]. Therefore for cleaning applications that involve painted components or surfaces a number of parameters must be thoroughly considered.
Discoloration issues have been extensively and long studied and a number of publications have been focused on their understanding [3, 17–19], while emphasis has been also given to the development of eliminative and remedial approaches on the basis of careful choice and fine-tuning of the operative laser parameters (such as the laser wavelength [11, 18, 21–23] and pulse duration [17, 24].
The two-wavelength methodology has been introduced initially to overcome yellowing discoloration of sculpted marbles. The cautious “blending” of two laser beams, which overlap in space and time, has been suggested in an attempt to bridge the inefficient and unsuccessful cleaning result from the individual use of the two beams. An important parameter that must be taken into account in this methodology is the relative ratio of the intensity of the two beams, which has a decisive role as regards the prevailing ablation processes and thus the cleaning result. A brief description of background work that influenced the development of the methodology is highlighted in the following:
Laser-assisted encrustation removal using IR wavelengths (1064 nm) of ns pulse duration is a well established cleaning methodology for removal of dark-coloured over-layers from light-coloured substrates with numerous applications worldwide [1, 2, 11, 25, 26, 27, 28, 29] mainly on stonework. In this cleaning regime “photo-thermal” mechanisms (selective explosive vaporization and spallation) are responsible for material removal, while its success is attributed to its “self-limiting” nature based on the fact that the majority of encrustation usually encountered on stonework absorbs in this wavelength significantly higher than the stone substrate (typical absorption coefficients of pollution crusts in the 1064 nm are four times higher than the marble substrate ones). As a result the energy density (fluence, F) threshold value for encrustation removal is significant lower than the one for substrate/marble damage and thus selective and self-limiting cleaning is feasible. Typical F threshold values determined for 1064 nm ablation of black pollution crust and Pentelic marble are 0.8 and 3.5 J/cm2, respectively [18].
Nevertheless, the use of IR laser beams both on technical samples simulating pollution encrustation, as well as on real marble fragments with pollution crusts, often resulted in surfaces discoloured to yellow–brown. A series of preliminary experiments, which took place on technical samples made of gypsum (CaSO4 2H2O) and 5–10 % wt of charcoal particulates (with size in the range of 50–150 μm) in a simplistic simulation of pollution crusts, aimed at studying the removal mechanisms of the pollution crusts in this IR regime (1064 nm, 30 ns). The irradiated surfaces appeared yellow, while it was evident that discoloration is more intense for higher charcoal percentages (10 %) [20]. Thus it was related with preferential charcoal particulate removal and consequently insufficient ablation of the bulk material (gypsum). Furthermore, the fact that IR irradiation of gypsum pellets without any particulates does not cause any colour or chemical (on the basis of Raman analysis [20]) changes suggest the thermal dissociation of the charcoal particulates as a reason for this alteration [20]. Indeed assessment of the treated surfaces under the optical microscope (OM) and the scanning electron microscope (SEM) pointed out the presence of voids/craters of various sizes without any significant damage to the gypsum crystals, indicating that at fluence values close to the ablation threshold of the crust, charcoal particulates, either individually or in clusters (Fig. 1a, c), are preferentially removed.
Similar results were also observed on real fragments bearing thin homogeneous pollution crusts which have been cleaned with 1064 nm of ns pulse duration. Evaluation of the treated surfaces pointed out that at F values slightly above the ablation threshold the crust is not totally removed and a thin layer of matrix material still remains on the surface (Fig. 2a, c). This layer, which has lost its initial dark colour and has a beige-yellow coloration, cannot be easily removed. Its removal is possible using significantly high fluence values, which are close to the ablation threshold of the marble and thus such an intervention cannot be considered “self-limited”.
Conversely no yellow–brown discoloration is observed upon UV irradiation. Tests on the technical gypsum-charcoal samples using 355 nm with pulse duration of few ns [5, 6, 11, 21] resulted into relatively faded/bleached surfaces. Under the OM minimal void formation and relatively homogeneous surface relief is observed indicating that material removal takes place on the basis of the “layer-to-layer” ablation model and the whole structure (gypsum-charcoal) is gradually removed. It has to be noted that for effective material removal in this regime high fluence values are required, which may result into damaged gypsum crystals as confirmed with SEM (Fig. 1d).
On the other hand attempts to remove real pollution crusts from marble fragments using 355 nm were not satisfactory as regards the cleaning efficiency and the degree of control. Although thin and homogeneous crusts could be removed without yellow discoloration their cleaning rate was rather slow and inefficient, especially on areas with micro-relief. Furthermore thick and inhomogeneous pollution accumulations could not be eliminated satisfactorily resulting into irregular surfaces. It has to be noted that in the UV ablation regime the difference between the absorption coefficients of encrustation and substrate is not enough to ensure significant differences to the ablation thresholds of the two materials and thus a ‘self-limiting’ cleaning process [21].
To avoid discoloration and overcome the above issues the combined use of the two laser beams was suggested. Initially the attention was focused on their sequential (SQ) use with the intension to employ the UV laser beam to correct the discoloration induced by the IR beam [30]. The result was not satisfactory; the discoloration could be rectified to a certain degree, however the surface morphology appears seriously uneven under the microscope. This can be explained due to the fact that the “new” crust surface, which resulted upon the IR irradiation, shows different physicochemical properties to the untreated crust and the ablation effect with the UV beam is thus different from the one that has been reported on the crust itself. Similarly insufficient was the encrustation removal obtained when the IR beam was applied to rectify the effect of UV irradiation.
The synchronous (SN) use of the two wavelengths in spatial and temporal overlapping was then tested [10, 30, 31]. Tripled QS Nd:YAG lasers emit simultaneously in 1064 nm, as well as their 2nd (532 nm), 3rd (355 nm) etc. harmonic frequencies and thus offer a convenient basis to exploit the possibility to combine the different cleaning mechanisms (in this case the IR at 1064 nm and UV at 355 nm). Key feature is the adjustment of the energy density ratio of the two beams (FIR/FUV) and thus the regulation of the contribution of the individual ablation mechanism for different encrustations and substrates. Moreover, another important aspect that must be taken into account is the total F value \(({\text{F}}_{\text{total}} = {\text{F}}_{\text{IR}} + {\text{F}}_{\text{UV}} )\). In order to avoid damaging effects upon cleaning the sum of the F values of the two beams must be lower to the ablation threshold of the underlying original surface, while for ensuring an efficient cleaning the total F must be higher to the IR ablation threshold for dark pollution encrustation removal (i.e. FIR_crust ~ 0.8 J/cm2).
An illustrative example of the different effects upon laser ablation of pollution crusts from individual, sequential and synchronous use of the two wavelengths is shown in Fig. 3. In this Figure a series of laser irradiation experiments on a real fragment of marble is shown, which was added as a corner complement to replace missing elements and reinforce the Parthenon Frieze blocks during their restoration intervention in the 1960s. This piece of new marble of the same origin to the ancient marble pieces (Pentelic quarry) shows the same encrustation to the rest of the Parthenon Frieze and thus could be used for the purpose of this study. Previous studies [32, 33] have determined the ablation threshold values for the thin pollution crust in the IR and UV regime to be respectively FIR_crust = 0.8 J/cm2 and FUV_crust = 0.6 J/cm2, while the corresponding values for damaging the marble are FIR_marble = 3.5 J/cm2 and FUV_marble = 1.2 J/cm2. The fragment was treated dry with 20 pulses of various laser beams and combinations as shown in the schematic of Fig. 3.
Irradiation with the IR laser beam at 1064 nm (left column, areas 1, 2 and 3) shows yellow discoloration. Treatment at F values close (FIR_2 = 0.8 J/cm2) and below (FIR_3 = 0.4 J/cm2) the ablation threshold of the crust shows insufficient crust removal while the final surface is yellow. For F above the ablation threshold (FIR_1 = 1.5 J/cm2) cleaning is efficient but the marble surface has still a slight yellow hue. Similar insufficient result is obtained upon irradiation with the UV laser beam at 355 nm (second to the left column, areas 4, 5 and 6). In this case for F values in the range of 0.1–0.2 J/cm2 the crust appears rather discoloured into grey, while at F slightly higher (FUV_4 = 0.4 J/cm2) damage of the marble substrate is visible as broken shiny marble crystals are revealed.
The SQ use of the two beams has been also comparatively studied and is shown in Fig. 3 (the two columns on the right). Areas 10, 11 and 12 have been irradiated initially with the IR laser beam (at the same conditions to 1, 2 and 3) and then, following a ~5 mm shift to the right for comparison purposes, with the UV beam (at the same conditions to 4, 5 and 6). It is obvious that the result of the SQ irradiation varies significantly to the SN one on every aspect (colour-wise and efficiency).
The synchronous (SN) use of the two wavelengths in spatial and temporal overlapping is shown in the middle column (areas 7, 8 and 9). In this case the exact parameters of the individual IR and UV beams have been combined effectively with the aim to reach an optimum cleaning result without discoloration or other alterations. As seen in Fig. 3 areas irradiated with the synchronous beam are less discoloured to the ones treated with the individual beams. For example the colour of area 9 (which is the combination of the beams that treated areas 3 and 6) is closer to the colour of the untreated crust. Indeed, the final surface of area 9 appears less yellow to area 3 (FIR_3 = 0.4 J/cm2) and less grey to area 6 (FUV_6 = 0.1 J/cm2), although it has to be mentioned that all three areas are considered under-cleaned. In this example area 8 appears to have an optimal cleaning level and final surface (judging on its colour and surface morphology) while area 7 is undoubtedly over-cleaned.
The irradiated areas shown in Fig. 3 illustrate the superiority of the two-wavelength laser cleaning methodology as regards the final colour, surface morphology and homogeneity of the cleaned areas, while indicating the limitations of the individual IR and UV cleaning regimes, as well as of their SQ use. The methodology has been thoroughly tested through a series of studies, both on technical samples and real fragments [18, 30, 31], in order to determine the optimal conditions for its application. These results have allowed its adaptation and fine-tuning for the cleaning challenges of the Athens Acropolis Sculptures and Monuments with success. However it must be underlined that the choice of the laser parameters and cleaning methodology relies strongly to the materials involved and the specific conservation challenge. The combinative use of the two beams is an option that may answer difficult and demanding cleaning issues. A couple of such examples is highlighted in the following including its application on the Athens Acropolis.