Feasibility of ultrafast picosecond laser cleaning of soiling on historical leather buckles
© The Author(s) 2016
Received: 22 March 2016
Accepted: 12 August 2016
Published: 10 October 2016
KeywordsVegetable tannin Buckles Soiling Laser Picosecond Cleaning Microscopy DSC FTIR
While cleaning of sensitive organic objects, such as leather, should be accomplished with much care and well-assessed technical studies, the choice of a suitable cleaning method will need many basic tests to avoid damage and accept the minimal risk. Removal of soot and soiling from the leather surface is important for future restoration and consolidation treatments . Several chemical and mechanical cleaning methods have been tested with leather to remove unwanted materials with high risk of leather damage [2, 3]. Currently, laser cleaning is considered one of the least invasive studied cleaning techniques and it has been proved more beneficial in case of sensitive organic materials. The control of laser cleaning processes has been improved due to the development of laser wavelengths and pulse widths. Reports have indicated successful cleaning of soiling from collagen artefacts using nanosecond UV/IR Nd:YAG lasers with the first harmonic (1064 nm) and fourth harmonic (266 nm) [4–6]. Past studies showed the removal of dirt over model vegetable tanned leather using high fluences (~4–7.5 J cm−2) and high number of pulses (20 pulses) of Q-Switched Nd:YAG nanosecond laser emitting at 1064 nm could induce significant side effects (such as melting and burning) of the leather substrate while successful laser cleaning of dirt on ancient leather has been performed at 3.2 J cm−2 without damaging the underlined layers . Among the most commonly used lasers which have been proposed for the successful removal of dirt is the Nd:YAG laser which is emits in the near infrared at 1064 nm and strongly absorbed by the dirt in contrast to the substrate. This wavelength provided more satisfactory cleaning results comparing to the 532, 355 and 266 nm . Recently, the development of industrial applications of laser technology offers the option of ultrashort pulse durations which used successfully to minimize thermal and mechanical side effects during laser cleaning [9–11]. The practicability and effectiveness of ultrashort picosecond lasers for cleaning artworks has demonstrated before with satisfactory and gentle cleaning results [12–14].
Leather has been used since ancient and medieval times for many purposes [13–15]. The corium is the raw material used for leather manufacturing which consists of a tight network of collagen fibres and fibrils . Animal skin has been tanned through the ages using different tannages such as vegetable tannins [17, 18]. Vegetable tanning is based on the treatment of animal skins using natural sources of condensed tannins (catechols) found in acacia, mimosa and pine, as well as hydrolysable tannins (pyrogallols) such as oak wood, gulls and sumac leaves [1, 16–19]. Animal skin collagen protein interacts with the large molecules in vegetable tannins which transform the skin into leather with greater stability and resistance to microbial attack, comparing to non-treated skins .
The accumulation of dust and soot particles on the leather surface can be a serious problem for leather conservation since dust particles are abrasive and can be difficult to remove from the irregular surfaces. These accumulated particles accelerate leather deterioration through promoting microbial growth and the absorption of moisture from the surrounding environment . Also, vegetable tannins can deteriorate due to exposure to UV radiation as vegetable tannins are strongly absorbing between 270 and 350 nm . Oxidation, combined with the effects of atmospheric pollution, can also lead to acidification and darkening of the leather surface . Presence of pollution gases, which could be transformed into acids by the reaction with moisture, can oxidise the lubricating agent and dressing layers on the leather surface and among the collagen fibres leading to the darkening of leather surface. Furthermore, acidification causes leather decomposition, which will be manifested in a grain layer which appears powdery .
The aim of the research is to present a system recently developed and used for automated cleaning of artworks and to examine the suitability of using this ultrafast and precise computed-scanning picosecond laser (1064 nm) with a repetition rate of 10 kHz and a temporal pulse length of 10 ps for the removal of soiling from leather buckles without damaging the leather substrate. Preliminary tests will be performed with the model samples to determine the leather damage threshold fluence and the soiling ablation threshold fluence before using a laser for the removal of the soiling from a historical leather buckle.
Objects and samples
Model vegetable tanned leather was exposed to thermo-oxidative ageing. Heat ageing of the leather will allow a loss of moisture, reducing the flexibility, change as well as the dimensions of the leather and alter the colour. Artificial ageing is expected to produce deterioration similar in historical leather and tt is an absolute necessity to evaluate the effect of conservation treatments. The model samples were undergone dry thermal ageing in a standard laboratory oven with a dry heat at 100 °C for 11 days, at which point the leather weight is constant [1, 23, 24].
The ps laser parameters: the tested fluences, repetition rate and the number of scans along with the pulse duration
Fluence (J cm−2)
Repetition rate (kHz)
Number of scans
Investigations and analysis
Optical microscopy (Leica, UK) and scanning electron microscopy were employed to study the morphology of the leather surface and assess the laser cleaning. Microscopic investigations were carried out using an Environmental Scanning Electron Microscope (FEI Quanta 200, Netherlands, with accelerating voltage 30 kV). For the elemental analysis of the deterioration products and assessment of laser tests, Scanning Electron Microscope (Carl Zeiss Ultra Plus Field Emission) connected with an energy dispersive X-ray spectroscopy (EDS) detector (Oxford Instruments X-Max) was employed. The instrument is associated with an ultra high-resolution secondary and backscatter electron imaging (1 nm), utilising new in-lens detector technology, and a charge compensation system for the imaging of non-conducting samples.
Chemical spot tests
Identification of the tanning material of the historical leather is needed to understand the manufacture technology of the leather as well as the interaction of the laser radiation and tanned leather. The Ferric Spot Test [23, 28] was used to determine the type of tannin (vegetable or not) using fibres from the historical leather samples. The fibres samples were wetted with distilled water and soaked in (2 % w/v ferric chloride and water, FeCl3·7H2O). Positive result for the presence of vegetable tannins is indicated by dark colouring of the fibres. The Vanillin spot test for the testing the presence of condensed tannins was carried out using leather fibres samples (4 % vanillin in 99 % ethanol, and the hydrochloric acid HCl). The presence of a red colour is an indication of the presence of condensed tannin. The Rhodanine spot test for testing the hydrolysable tannins was employed using leather fibres samples, 2 N H2SO4, Rhodanine (0.667 % w/v in 99 % ethanol) and 0.5 N potassium hydroxide (KOH). A red colour yields a positive reaction.
Measurement of the denaturation temperature of leather (Td)
The denaturation temperature (Td) is a measure of the physiochemical change of the collagen structure within the leather causing disruption of the structure (e.g. by heat or radiation or solvents used for cleaning). Analysis of the denaturation temperature provides information with regards to the temperature at which the collagen triple helix in the leather collapses and is transformed into a random coil. Differential scanning calorimetry (DSC) was applied as a thermoanalytical technique to investigate the hydrothermal stability of the leather and measure the denaturation temperature of the model leather samples (fresh/aged/laser irradiated) [29, 30] as a function of the various conditions to evaluate laser cleaning. This will help assessment of the laser cleaning of the model leather samples because any damage or breakdown of the collagen network should decrease the denaturation temperature . A reduction in the denaturation temperatures is an indicator of the degradation of the triple helix of the collagen molecule. However, various factors such as the type and age of animal skin, tanning material and water content can affect the leather Td [31, 32]. Td measurements were carried out using a Mettler Toledo DSC822e calorimeter. The model leather samples (~10 mg) were placed in a conditioning room at 20 °C and 60 % relative humidity for a minimum 48 h before measuring Td. The leather samples were hermetically sealed in aluminum pans (40 µl Al pans withstand pressure of up to 0.2 MPa and temperature of up to 640 °C. The crucibles were sealed with a sealing press with a sealer plate and plunger set for standared crucibles. The selected temperature range was 0–140 °C and the heating rate was 5 °C min−1. During heating the fibers will start to shrink at a certain temperature, depend on the stability of the collagen. The DSC was previously calibrated using “Check ʌexo In or ʌexo Zinc” method with Indium/Zinc calibration pans.
Measurement of moisture content in leather
The samples (fresh/aged/aged-laser irradiated model vegetable tanned samples, used in Td measurements) have been employed to measure the moisture content of the leather which was determined by weighting before and after drying in a laboratory oven (Abinghurst LTd, UK) at 100 °C for 24 h or until a constant weight was achieved. The weighting of the samples was carried out using (A&D balance, USA, readability to 0.001 mg). Measuring the moisture content of leather could be an indicator for assessment of the thermal effect of laser cleaning of leather.
Fourier transform infrared spectroscopy (FTIR) was carried out using a Thermo Nicolet iS10 FT-IR Spectrometer in reflectance mode using an attenuated total reflectance (ATR) slide-on accessory with diamond crystal, spectral range 4000–400 cm−1 and resolution 4 cm−1, to attempt and elucidate the composition of soiling on the surface.
Spectrodensitometer (X-Rite, Incorporated, 500 Series, Germany) was employed to measure the colour changes of the different leather conditions of model leather (new, aged, soiled and laser cleaned) based on changes observed on the L scale (Luminosity), a* scale (red/green) and b* scale (yellow/blue). Seven measurements were averaged to obtain one data point.
Results and discussion
Microscopic investigation and qualitative chemical spot tests of the historical leather buckle indicates that the buckles leather of vegetable tanned leather using condensed tannins . Vegetable tannins may have different chemical structures, but share some common properties as being insoluble in organic liquids, miscible with water, and being hygroscopic, amorphous substances, which are sensitive to oxidation and reduction . FTIR spectra of fresh and aged leather show standard collagen peaks including a small band at 1377 cm−1 which is more evident in aged leather and could be related to the accumulation of an unidentified degradation product in the leather. Leather samples contain spectra typical of leather (collagen) with Amide-I (1634 cm−1) and Amide-II (1535 cm−1) bands.
Show the results of investigation and analysis of leather samples
Moisture content (% w/w)
F dam (J cm−2)
F abl (J cm−2)
Model non-aged reference
82 (−25 %)
1377, Amide-I (1634) and Amide-II (1535)
Model laser cleaned
3 (3 scans)
1.02 (5 scans)
Soling: 878, 1422 (CaCO3). 795, 1012 (clay)
Soling: C, Ca, S, Si, Cl, Al, Na, Fe
2 (6 scans)
2 (3 scans)
Laser cleaning of leather
Colorimetric measurements of model leather samples in different conditions
Upon an extensive optimization and analysis on model vegetable tanned samples, Microscopic investigation of laser tests with the historical leather in the buckles showed that the soiling particles could be removed satisfactory without damaging the leather using the commonly used laser wavelength 1064 nm  at fluence of 2 J cm−2 using 3 scans(Fig. 6, right). At this fluence, one scan was not sufficient for the complete removal of the soiling (Fig. 6, left). Increasing the number of scans provides better cleaning results but if the number of scans is higher than 5; damage of the leather (denaturation) may occur.
differential scanning calorimetry
Fourier transform infrared spectroscopy
scanning electron microscope connected with an energy dispersive X-ray spectroscopy
- F dam :
damage threshold flunce
- F abl :
ablation threshold fluence
AE developed and coordinated the work and access to the objects. AE, PF and KW performed the laser cleaning tests with the ps laser system. AL and YF assisted in tanning identification, DSC analysis and moisture content measurements. All co-authors contributed to the writing of the manuscript and results interpretations. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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