LASER CLEANING OF PAPER - A STEP TOWARDS OPTIMISATION
V. S. Šelih*1, M. Strlič1, J. Kolar2, D. Kočar1, B. Pihlar1
1
University of Ljubljana, Faculty of Chemistry and Chemical Technology, Aškerčeva 5, 1000
Ljubljana. Slovenia
2
National and University Library, Turjaška 1, 1000 Ljubljana, Slovenia * corresponding author:
vid-simon.selih@uni-lj.si
1. Introduction
In conservation, laser cleaning is becoming more and more popular. Commercial laser cleaning
systems have become available during the last years and are now being increasingly used in
conservation studios across Europe, where well over 20 such systems are available today.1
Figure 1: Example of laser cleaning of a paper document.
Laser-based cleaning is a well controllable method for removal of soiling from the surface of a
substrate.2 It is furthermore highly selective, contact- and reagent-less. In many cases it gives the
conservator a level of control not achievable with the traditional cleaning methods. Two general
approaches to laser cleaning arc used; dry and wet (water assisted) approach. While in the case of
dry laser cleaning only interaction of light with soiling leads to a cleaning effect, wet laser cleaning
takes advantage of interaction of laser light with water deposited on soiling. For paper, dry laser
cleaning is used.
During laser cleaning, removal of soiling should in principle proceed without alterations of the
underlying substrate of an artefact. This is possible only if light absorptivity of soiling is
considerably higher than that of the substrate and if there is no interaction between soiling particles
and the substrate. In case of sensitive organic materials, e.g. paper, parchment and textiles, this is
frequently not the case and surface modification after laser cleaning may be observed, exhibited as
discolouration or yellowing.
Yellowing as a result of laser cleaning is a particularly disturbing phenomenon and it has been
noticed in a variety of applications.1 Interactions between the substrate and laser light may be such
that both formation of new chromophorcs (discolouration or yellowing) and destruction of already
existing chromophores (bleaching) may occur simultaneously4,5, especially if the substrate is a
complex material, e.g. lignin-containing or gelatine-sized paper. Yellowing is a common
phenomenon observed when fibrous materials arc cleaned using ordinary laser cleaning parameters
(Nd: YAG - 1064 nm or 532 nm; 0.1 -1 J/cm2, repetition rate 10-50 H/., 5-10 ns pulse duration).
Formation of chromophores during dry laser cleaning of paper is not a sufficiently understood
phenomenon and a higher level of knowledge could lead to better optimised cleaning parameters,
thus reducing undesirable side effects. This was the scope of our work.
2. Experimental
Purified cotton linters cellulose paper (Whatman N° I filter paper) was used as a model. To obtain
an exaggerated soiled model, well defined charcoal powder (low content of impurities, uniform
particle size) was used as model soiling in high surface density. It was deposited onto paper by
filtering aqueous suspension through paper sheets. Viscomctry according to standard procedure,6
using fresh cupriethylenediamine solvent was used to determine the degree of polymerisation (DP)7.
Accelerated light ageing studies were performed in Xenotest Alpha light ageing chambers.
Accelerated thermal ageing (up to 160 h. 90 °C, 65% RH) was performed in a Vötsch VC0020
climatic chamber. Chemiluminesccncc experiments in N, atmosphere were performed with Lumipol
2 instrument. Colorimctric measurements of samples were performed with a Minolta CM-36KM
diffuse reflectance VIS spectrophotometer with the specular component excluded. The reflectance
was measured in % relative to polymeric Minolta standard. CIE L*a*b* system8 was used to
evaluate the colour changes.
Two Q-switched Nd-YAG lasers at fundamental (1064 nm) or doubled frequency (532 nm) were
used in this study. Soiled samples used for chemiluminomclric analyses and study of light ageing
stability were treated with 1 J/cm2 fluence and 8 mm spot diameter laser pulse. For optimisation of
cleaning process other laser with lower fluenccs (0.05 and 0.1 J/cm2). 5 mm spot and 1 and 10 shots
per second repetition rales was used.
3. Results and discussion
With chemiluminometry we showed, contrary to Rudolph et al.,9 that changes in substrate
immediately after laser treatment can be observed. In a dynamic experiment, chemiluminescence
activity of samples immediately after laser treatment is evident already at low (<100 °C)
temperatures. This indicates the presence of reactive species, formed during the process of laser
cleaning and gradually decomposed, as can be seen in Fig. 2, curves a, b, c It is evident that the
species is quite long lived in darkness at room conditions (22 °C), but is easily destroyed by
oxidation (Fig 2. curve d). as a consequence, limited chemiluminescence emission, close to
background, is observed at low temperatures (<100°C).
Figure 2: Chemiluminescence emission in nitrogen atmosphere during dynamic experiments
(temperature gradient: 2.5 °C/min) after a 15-min period of flushing, both in N2 atmosphere. All
samples were soiled and laser-cleaned (Nd:YAG 1064 nm. 1 J/cm2) and stored in darkness for: a)
20 min; b) 22 h; a) 95 h. Sample d) was, 98 min after the cleaning, oxidized in O2 at 100 °C for 30
min and then subjected to the same chemiluminometric experiment.
Figure 3: Changes in b* and DP during photo ageing (l>340 nm) of a non-treated and a soiled and
laser-cleaned (Nd:YAG 1064 nm, 1 J/cm2) cellulose sheet.
Results, obtained with si/.c exclusion chromatography, chemiluminometry, FTIR, accelerated photo
and thermal ageing experiments all support the fact that chemical changes do take place and will in
long term destabilize structural integrity of the substrate. Light-induced ageing processes are of
particular importance for objects which are exhibited alter laser cleaning.
The data in Figure 3 demonstrate that as a result of laser cleaning, the yellow component b*
increases substantially. However, extensive bleaching of chromophorcs takes place even during
irradiation with λ>340 nm, the difference amounting to 7 units in 7 days. Long-term instability
towards chain-scission, however, is also impaired and the laser-treated material degrades more
quickly than the original non-treated one. Similar results can be demonstrated for thermal ageing.
Figure 4: Degree of polymerisation during accelerated ageing (80 °C, 65% RH) of a non-treated
and a soiled and laser-cleaned (Nd:YAG 1064 nm, 1 J cm-2) cellulose sheet.
The rate of thermal degradation at 80 °C, 65% RH is also significantly changed - it is evident that
the stability of laser-cleaned paper will be impaired in the long term.
It was already shown9,10 that laser cleaning at 532 nm may in several cases be preferable to 1064
nm, and that one pulse of 1 J/cm2 is belter than several pulses of lower fluence4. Considering that
cellulose is a thermal insulator, the heat generated during interaction of light with particles of
soiling, is accumulated in the treated area.
4. Conclusions
Due to differences in type of soiling and type of paper itself, universal conditions for laser cleaning
can not be put forward.
The research results obtained with an exaggerated model (high surface density of charcoal soiling,
highly sensitive cellulosic material), indicate that laser cleaning of paper may result in its increased
instability.
However, it should be stressed that even if optimal laser cleaning conditions are achievable, with
minimal yellowing, the ccllulosic substrate will still be destabilized in the long-term, both during
thermal and photo ageing. It is doubtful whether large-area applications are thus acceptable, while
laser cleaning of localised areas may still be the cleaning method of choice in certain instances (low
mechanical stability, hindered access, unavailability of other treatments). Furthermore, since paper
is a complex material, the behaviour of a particular artefact during laser cleaning is difficult to
predict, and testing before use is essential.
5. References
1. Artwork conservation by laser in Europe database http:// alphal.infim.ro/cost/pagini/TEXTBD.htni
2. S. Georgiern, Adv. Polym. Sei.. 2004, 168, 1-49.
3. V. Verges-Belmin, C. Dignard, Journal of Cultural Heriatage, 2003, 4, S238-S244
4. M. Strlič, J. Kolár, V.-S. Šelih, M. Marinček, Appl. Surf. Sei.. 2003. 207, 236-245.
5. V. R. Botaro. CG. dos Sanots, G. Arantes Junior, A. R. da Costa. Appl. Surf. Sei., 2001, 183,
120-125.
6. SCAN-CM 15:88: Viscosity in Ciipri-Ethylenediaminc Solution, Scandinavian pulp, paper and
board testing committee. 1988, I-7.
7. R. Evans, A. F. A. Wallis. 4[l] Int. Symp. Wood Chem. 1987, 201-205.
8. K. McLaren. JSDC. 1976.338-341.
9. P. Rudolph, F. Ligterink, J. L. Pedersoli Jr., M. Van Bommel. J. Bos, H. A. A/i/, J. B. G. A.
Havermans, H. Schölten. D. Schipper. W. Kautek, Appl.Phys.A, 2004. 79, 941-944.
10. J. Kolár. M. Strlič. S. Pent/ien. W. Kautek, Appl. Phys. A, 2000,71,87-90.