This thesis is concerned with different aspects of laser cleaning of stone sculpture including the interaction of laser radiation with stone, evaluation and monitoring of the cleaning process and the development of a practical cleaning tool. Results show that a Q-switched Nd:YAG laser is the most suitable type of laser for the removal of black encrustations from marble and limestone surfaces. Measurements of the absorptivity of polluted and clean stone surfaces at 1.06 itm show that a typical black crust, resulting from atmospheric pollution, absorbs 2.5-3.5 times as much of the incident radiation as typical clean limestone and marble surfaces. This is shown to lead to the controlled removal of black crust material, by several thermal and mechanical mechanisms, without damaging the underlying stone. Measurements of ablation thresholds, by deflection of a probe beam, reveal that material is removed from a black crust at a fluence approximately half that required to remove material from a clean limestone or marble surface. Monitoring of the acoustic pulse generated during the laser beam/stone interaction is used to characterise the cleaning process. Results also show that cleaning is more efficient when a thin layer of water is applied to the surface prior to irradiation. Surface analysis techniques show that if cleaning is carried out below the damage threshold of the stone there is no modification to its physical or chemical structure. Cleaning of limestone sculpture from Lincoln Cathedral and terracotta sculpture from the Victoria and Albert Museum shows laser cleaning to be more refined than conventional cleaning techniques and to have considerable advantages in the area of health and safety.

Στην περιοχή της Ελευσίνας, στη Δυτική Αττική, υπάρχει το Ιερό της Δήμητρας στο οποίο τελούνταν για αιώνες τα Ελευσίνια Μυστήρια. Η ιστορία του Ιερού χρονολογείται ότι ξεκινά ήδη από τους προϊστορικούς χρόνους, τουλάχιστον από τις αρχές της 2ης π.Χ. χιλιετηρίδας και τελειώνει τον 4ο αιώνα μ.Χ. Τα ερείπια του Ναού είδαν το φως σταδιακά μετά τις πρώτες ανασκαφές (1812). Από το 1875 και έπειτα, λόγω της θέσης της, η πόλη μετατράπηκε σε βιομηχανικό κέντρο με πολύ υψηλές τιμές ρύπανσης στο θαλάσσιο και το ατμοσφαιρικό περιβάλλον της περιοχής. Συγκεκριμένα, η ατμοσφαιρική ρύπανση αντανακλάται στα λίθινα στοιχεία του αρχαιολογικού χώρου. Σκοπός της εργασίας είναι να αναγνωρίσει και να καταγράψει τις μορφές διάβρωσης που επικρατούν στο χώρο και να τις συνδέσει με πιθανά αίτια που τις προκάλεσαν. Αρχικά, γίνεται πλήρης περιγραφή των 5 κύριων λιθοτύπων (3 ασβεστόλιθοι και 2 μάρμαρα) που χρησιμοποιήθηκαν στις διάφορες φάσεις ανέγερσης του Ιερού και των χώρων που το περιβάλλουν, μακροσκοπικά και με τη βοήθεια της οπτικής και ηλεκτρονικής μικροσκοπίας. Έτσι, προκύπτει ότι οι κυριότεροι ενδογενείς παράγοντες φθοράς των δομικών λίθων του Ιερού της Δήμητρος είναι το υψηλό πορώδες, η ορυκτολογική σύσταση και κυρίως η παρουσία αργιλικών ορυκτών. Στη συνέχεια, καταγράφονται μακροσκοπικά οι φθορές που έχουν υποστεί οι δομικοί λίθοι του μνημείου με κυριότερη τη δημιουργία τσιμεντιτικών κρουστών σε επίπεδες επιφάνειες. Σημαντικές είναι και οι διάφορες πάτινες βιολογικής και άλλης προέλευσης που αλλοιώνουν χρωματικά τα λίθινα στοιχεία καθώς και οι επιφάνειες με έντονα φαινόμενα κυψέλωσης και γυψοποίησης. Δε λείπουν περιπτώσεις εκλεκτικής διάβρωσης λόγω ορυκτολογικής ετερογένειας. Η μελέτη των διαβρωσιγενών μορφών που οφείλονται σε περιβαλλοντικούς παράγοντες συνεχίζεται μικροσκοπικά όπου παρατηρούνται αλλοιώσεις των επιφανειών βάσει της σύστασης, της μορφολογίας και της μικροδομής. Έτσι, εντοπίζονται αποθέσεις οργανικού υλικού, θραυσμάτων SiC και άλλων βιομηχανικών ρύπων με υψηλή περιεκτικότητα σε βαρέα μέταλλα, αποικίες βακτηρίων και άλλων μικροοργανισμών, πάτινες αργιλοπυριτικής σύστασης, σκληρές τσιμεντιτικές κρούστες, γύψος, οξειδωμένες σιδηρούχες φάσεις, μικροκαρστικά φαινόμενα και τέλος, φερτά αντικείμενα θαλάσσιας και ατμοσφαιρικής προέλευσης. Η ακτινογραφική μελέτη επιβεβαιώνει το φαινόμενο της γυψοποίησης σε σημεία των λίθων που δεν υπάρχει ελεύθερη ροή νερού και διαφοροποιεί τα δείγματα λάσπης ή σκόνης που προέρχονται από κοιλότητες πετρωμάτων που εγκλωβίζουν νερό και αυτά που ελήφθησαν από ωμές πλίνθους ως προς το κυρίαρχο ορυκτό που στην πρώτη περίπτωση είναι ο ασβεστίτης και ακολουθεί ο χαλαζίας, ενώ στη δεύτερη περίπτωση συμβαίνει το αντίστροφο. Η γνώση των λιθοτύπων και των διαβρωσιγενών μορφών αποτελούν κλειδιά για την επιλογή μεθόδων αποκατάστασης των φθορών που συναντώνται στο χώρο της μελέτης. Έτσι, προτείνονται μηχανικές μέθοδοι για σκληρές κρούστες, πάστες και κομπρέσες για αδιάλυτα και διαλυτά άλατα και βιοκτόνα για βιολογικές επικαθίσεις. Για ρωγμές με διάμετρο μικρότερη των 0,2mm απαιτούνται εργασίες στερέωσης και αδιαβροχοποίησης με οργανικά ή ανόργανα υλικά που εφαρμόζονται ανάλογα με το είδος του λίθου. Για μεγαλύτερα ανοίγματα τα κενά συμπληρώνονται με κονίαμα, ενώ τέλος η αισθητική αποκατάσταση περιλαμβάνει τη συγκόλληση τεμαχίων ή απωλεσθέντων κομματιών, ενίοτε με τη χρήση συνδέσμων. / In Eleusis, a town in western Attica, lies the site of the Demeter Sanctuary, the sacred temple where the Mysteries of Eleusis took place in the antiquity. The history of the Sanctuary starts in the beginning of the 2nd millennium B.C. and ends during the 4th century A.C. The first time that some of the ruins came into light was in 1812, the year when the excavations started. Since 1875 the town got industrialized because of its geographical position with very high levels of marine and atmospheric pollution. In particular, atmospheric pollution reflects on the weathered stones of the ruins. This work aims to recognize and record the weathering forms that exist in the Demeter Sanctuary and to connect them with possible causes that engendered them. Primarily, 5 lithotypes (3 limestones and 2 marbles) got described macroscopically and microscopically (optical and electronic microscopy). From that, one can conclude that high pororsity, mineralogical composition and mostly clay minerals are the main intrinsic factors that play an important role in stone weathering. Thereinafter, the weathering forms that are observed macroscopically on the stones of the monument are written down. The most important of those forms is the case of the cementitious crusts in horizontal surfaces. Also important are the different patinas of biological or other origin as well as surfaces with intense phenomena of alveolization and gypsum formation. Preferential deterioration attributed to mineralogical heterogeneity can’t be omitted. The research on the weathering forms is continued microscopically where one can observe alterations due to composition and due to morphology or microstrusture. Thus, we note depositions of organic matter, SiC and other industrial pollutants such as heavy metals, biological activity attributed to bacterial and other microorganisms colonies, patinas rich in Al and Si, hard cementitious crusts, gypsum, oxidized ferrous phases, microcarstic phenomena and specific objects imported from the nearby marine environment or the atmpsphere (shells or rounded particulate matter). The study of the X-ray diffraction results confirms the gypsum formation in places protected against rainfall, and also demarcates the samples that were collected from water enclaving stone cavities and those that were picked from raw plinths, as far as their mineralogical composition is concerned˙ in the first case calcite is the main mineral and quartz follows, while in the second case, quartz is in abundance and calcite happens in a smaller percentage. Recognizing the lithotypes and the weathering forms and combining them, constitutes the key to choose the conservation methods for the treatment of the decay patterns that are encountered in the ruins of the Demeter Sanctuary. Thus, mechanical and chemical methods can be proposed for the hard crusts, different pastes and compresses for soluble and isoluble salts and biocides for biological deposits. Ultimately, consolidation with organic or inorganic material and water repellents applying is proposed for application in cracks with diameter less than 0,2mm, always taking the lithotype into consideration. For larger gaps, the voids are supplemented with mortars.

The Back to Nature project was developed as a collaborative doctoral award between the British Museum and the School of Geography and the Environment, University of Oxford. The British Museum has a large collection of limestone artefacts, some of which have deteriorated to an extent that a consolidation treatment, to improve internal cohesion, has been deemed necessary to ensure the value of the artefacts is maintained. Previous consolidation practices within the museum had centred on the use of organic consolidants. In the past, inorganic consolidants had been considered ineffective chiefly due to poor penetration depth and inconsistent deposition within the stone. Recent scientific developments within conservation, particularly the development of nano-limes, coupled with a trend towards treatments offering physical and chemical compatibility, led to the British Museum reconsidering the use of inorganic consolidants. Three inorganic consolidants: ammonium oxalate treatment, Calcite In-situ Precipitation System (CIPS), and CaLoSil have been investigated by the Back to Nature project to assess their potential for use on the British Museum’s deteriorated limestone artefacts. The investigation involved a developmental sequence of four experimental phases to assess the effectiveness of each consolidant upon freshly cut, artificially pre-weathered, and naturally weathered stone samples. In all experiments an organic silane Wacker SILRES BS OH 100 was used as a comparison. A variety of analytical methods were used including: Equotip, GrindoSonic, Scanning Electron Microscopy, Karsten Tubes, Drilling Resistance Measurement System, spectrophotometry, X-ray Diffraction and Ion Chromatography. Experiments show that the CIPS treatment could hold promise for use as a consolidant. Ammonium oxalate only appears to provide consolidation at the surface level, and CaLoSil would need modifying and more investigation before using in the museum environment. The differences noted between experiments on artificially weathered and naturally weathered stone indicate that a two-step process, involving testing on both types of samples, would give the best indication of consolidant effectiveness. High variability between samples shows a large number are needed to give an accurate interpretation of change due to consolidation.

Shelters, as preventive conservation methods, have traditionally been considered a better option than leaving the site exposed. However, there has been limited research on their effect on the preservation of heritage materials and, as a result, there is no clear scientific evidence to support sheltering. This study aims to provide the first rigorous scientific assessment of the effect of lightweight, open shelters on limestone deterioration at archaeological sites. A method based on the use of low-cost environmental monitoring equipment and limestone blocks and tablets (as indicators of decay) has been developed to determine the degree of protection provided by the shelters at the Bishop' Palace (Witney, England) and Hagar Qim (Malta). Preliminary visual assessments of the field sites were followed by 12-18 month exposure trials. Temperature extremes and fluctuations, frost events, relative humidity extremes and fluctuations, NaCl crystallisation events, solar radiation, wetting events, salt content, atmospheric pollutants and dust deposition were monitored. In addition, stone decay was studied by analysing changes in weight, elasticity, surface hardness, ultrasonic pulse velocity, surface colour, moisture content and general appearance (microscopic and macroscopic pictures) in stone samples. An exhaustive assessment of the shelter at the Bishop's Palace was carried out using Chalk, Cotswold and Portland limestone blocks as well as Portland limestone tablets (specifically for studying dissolution, soiling and biological growth). Additionally, a comparative assessment of the effects of the two shelters in contrasting climatic environments, the Bishop's Palace (temperate maritime) and Hagar Qim (Mediterranean), was undertaken by monitoring Globigerina and Coralline limestone blocks simultaneously at both sites. The research has shown that lightweight, open shelters do not exclude decay completely but minimise it. However, there are some areas at higher risk of decay, i.e. top parts of the walls and the periphery. In addition, problems with the shelter design can enhance some decay mechanisms, such as biocolonisation on the periphery at the Bishop's Palace and dust deposition under the shelter at Hagar Qim. Therefore, the effectiveness of shelters should not be assumed.