Monument Future

Conclusions

The results of the investigations on the objects have shown that, at least in some areas, there is a correlation between the surface hardness measurements and the ultrasonic velocity. In these areas a structural damage can be assumed with low values.

Both values can be positively influenced by consolidation. The intensity of weathering apparently also plays a role here. With heavily softened objects such as tombstone III, the results can be interpreted to mean that the consolidation was only 268partially successful in some areas. If necessary, the amount of consolidant used was not sufficient.

Figure 8: The three tombstones after restoration with the hot lime reaction mortar.

Figure 9: The ultrasonic velocity correlated with the surface hardness of the three tombstones before conservation.

Practical consolidation measures on stone objects can be checked with the presented examination methods. They can provide valuable information on the success or failure of the consolidation method used and are therefore suitable as a control or quality inspection procedure.

The results of the salt reduction showed that there is a clear correlation between the level of electrical conductivity and the intensity of the weathering. This leads to the assumption that salts are primarily responsible for the weathering observed. The intense weathering on tombstone III is probably also due to the high hydric dilatation of the rock material. Clay minerals are probably responsible for the latter, as further investigations should clarify.

Acknowledgements

We would like to thank Ms. Birgit Busse from the Green Spaces Department of the city of Göttingen for trusting our expertise and for her engaged commitment to the maintenance of the Bartholomew cemetery.

References

Domaslowski, W. (2003) Preventive conservation of stone historical objects. Torun.

Wedekind, W. (2016 a) Verwitterung und Salzreduzierung von monolithischen Baukörpern aus Sandstein. Vortrag auf der Tagung „Aktuelles aus Forschung und Praxis zum Thema Salz“, Dresden 1.4.2016, doi: 10.5165/hawk-hhg/291.

Wedekind, W., Lopéz-Doncel, R., Ruedrich, J., Siegesmund, S. (2016b) Evaluation of innovative treatments and materials for the conservation of the strongly salt-contaminated Michaelis church in Zeitz, Germany. in: Hughes, J. J., Howind, T. (Eds.) Science and Art: A Future for Stone. Proc. of the 13th Internat. Congress on the Deterioration and Conservation of Stone. Paisley 2016, Volume II, p. 981–990.

Kracke T., Müller C., Krinninger S., Wedekind W., Ruedrich J., Siegesmund S. (2007) Buntsandsteine Göttingens: Verwendung und Verwitterungsverhalten am Beispiel des Bartholomäus Friedhofs. Zeitschrift der Deutschen Gesellschaft für Geowissenschaften (ZDGG), 158/4, 957–984.

Kracke, T., Ruedrich, J., Wedekind, W. Mueller, C., Siegesmund. S. (2008) Weathering Behavior and the Effects of Consolidation Approaches on the Buntsandstein: a Case Study from the Bartholomew Cemetery in Göttingen. In: Jadwiga W. Lukaszewicz & Piotr Niemcewicz (Eds.) Proceedings of the 11th International Congress on Deterioration and Conservation of Stone. 15.–20. September 2008, Torún. Torún 2008, Volume I, p. 677–684.

Teipel, M., Pötzl, Chr., Wedekind, W., Middendorf, B., Siegesmund, S. (2020) Approach on developing stone replacement mortars fort he cultural heritage of Armenia.

269

MONITORING AND EVALUATION OF DAMAGING OF TRADITIONAL CLEANING TECHNIQUES OF STONES WITH 3D OPTICAL MICROSCOPY PROFILOMETRY

Cristina Tedeschi, Mariagiovanna Taccia

IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

Politecnico di Milano, Department of Civil and Environmental Engineering, Italy, cristina.tedeschi@polimi.it

Abstract

In cultural heritage cleaning techniques are very important and delicate operations, and they are necessary every time the surface of the stone is covered with mostly soiling material to avoid further degradation or alteration. The problem is that often during the cleaning operations, not only the alteration and the degradations are removed, but also part of the materials of the building stones and ornaments, thus damaging it irreversibly.

This paper investigates the effectiveness of 3D optical microscopy, used to monitor the surface of some natural stones, such as Carrara marble, the Noto Limestone and the Serena Stone subjected to mechanical cleaning subjected to mechanical cleaning by micro-sandblasting.

Therefore, we focus on the evaluation of the effects of cleaning on new natural stones taken from the quarry, through 3D mapping and the roughness profile. The selected samples were analysed with the Alicona IF-Portable optical profilometer before and after cleaning to monitor any morphological changes induced on the marble substrate.

Introduction

In this paper we will analyse the accuracy of 3D optical microscopy to evaluate the impact of a common cleaning technique, called micro-sandblasting.

The cleaning of the stones with a view to preserving cultural heritage is a crucial phase that shows some risks, it is an irreversible process and sometimes can alters the surface and original finishing layer of the building work. There are no real procedures that quantify the degree of cleaning but only a guideline (Mecchi et al. 2008), (Revez, M. J.; Delgado Rodrigues, J., 2016), (Demoulin, T.; et al).

For years, the efforts of research in this sector, have been aimed at finding increasingly respectful solutions towards the materials treated and which, of course, are always in more or less precarious conditions.

To evaluate the effectiveness of the process of removing undesired materials (soiling, surface deposits, alterations of the surface due to degradation, etc.) (ICOMOS, 2008) laboratory tests are frequently carried out, which provide the removal of small portions of material to be subjected to laboratory analysis. (BS8221-1:2012, ASTM C1515-14, ASTM E1857-97).

The possibility of evaluating the removal of soiling, effectiveness in situ in a totally non-destructive way and at the same time quantifying any alterations of the original surface due to degradation is an objective that every conservator should set.

270The Alicona IF-Portable, used for this research, is a portable instrument, therefore it has the possibility of being mounted on a frame or on a trailer in order to carry out on-site acquisitions, thus able to meet one of the minimum requirements that allow the operator to verify the effectiveness of the intervention by monitoring the degree of cleaning and the harmfullness of the treatment, with the possibility of interrupting the operation at any time.

The Alicona IF-Portable microscope proves to be a suitable instrument for this purpose as there is no interaction with the surface, allowing a rapid acquisition.

An attempt was made to evaluate, by means of the Optical Alicona IF-Portable Profilometer acquisition, the efficacy and impact (harmfulness) of the cleaning following micro-sandblasting on stones of different hardness, as such is known to increase surface roughness in some cases (Carvalhão M., Dionísio A., 2015), (Pozo-Antonio J. S., et al. 2016).

The Alicona IF-Portable Optical Profilometer is generally used to perform high resolution 3D surface measurements for the evaluation of production quality in research and development in the laboratory. The key applications for this tool are used for surface analysis and characterization of materials in mechanical engineering, for example for the construction of tools and moulds, for precision mechanics, in the aerospace sector, in the automotive industry, in the field of science materials of all kinds, for the evaluation of corrosion, in electronics and for the development of medical devices. Thanks to its technical specifications, the Focus-Variation technique is used for both shape and roughness measurements (Avagliano R. et al. 2013).

Cleaning methods

There are different cleaning methods: the choice of the same must be from time to time, through laboratory tests, based on the nature of the substance to be removed, the type of surface and object to be cleaned, the nature of the stone material, the type and of the degree of alteration. It is therefore clear that cleaning involves both extremely delicate aesthetic and technical problems that only a specialized technician can solve.

The cleaning techniques most frequently used in the stone conservation sector can be distinguished in chemical, mechanical or laser methods.

In general terms, the cleaning of a historical monument should meet minimum requirements such as: 1) the absence of harmful products which, remaining on the stone, could compromise its future conservation; 2) a cleansing action that does not produce surface irregularities, micro-fractures, increase in porosity, dissolutions, mineral transformations or colour changes in the original material; 3) a sufficiently slow action over time to allow the operator to continuously check the degree of cleaning and stop the operation at the desired time; 4) moreover, they must be gradual and selective processes with affordable costs. (Pozo-Antonio J. S., et al. 2016), (Turk, J., et al. 2019), (Gulotta, D., Toniolo, L., 2019), (Perez-Monserrat, E. M. et al.), (Doehne, E. F.; Price, C. A., 2011).

In this research, only micro-sandblasting was evaluated as it is a recommended technique for cleaning all types of natural stone. Unlike water-based cleaning techniques that are usually applied in the conservation of buildings without historical and artistic interest, dry systems are suitable for the cleaning of stone monuments, in particular those covered with atmospheric particles and other crusts. The use of regular sandblasting is accepted only in certain circumstances, for example on large stone surfaces and on areas without any artistic or historical interest, due to its high aggressiveness caused by high pressure and high hardness of the projected sand (Pozo-Antonio J. S., et al. 2016).

Material and methods

The possible harmfulness of the cleaning method with regard to the stone material is generally assessed only with laboratory tests, water absorption and water absorption coefficient by capillarity. The method in question was applied, with techniques 271as close as possible to those that will be adopted on site, on perfectly shaped specimens, perfectly polished 50 × 50 × 20 mm plates.

Table 1: Porosity properties of the three materials tested.

In this research, three types of stones of different hardness and abrasion resistance were chosen:

Carrara marble: a white fine-grained crystalline marble widely used for decorations, it is a compact stone with low porosity and very low resistance to chemical attack, high resistance to weathering, an apparent density of 2.69 g/cm³, and a total abrasion measure of 22.0 mm (UNI EN 14157:2017).

Stone Serena: a gray sandstone, is a sedimentary stone with a fine to medium-large grain size, compacted with a clay matrix. With an almost uniform light cerulean color, it shows shiny dots marked by the presence of mica flakes. The blue-gray color due to a chemical reaction in the stone, it turns to into a red color. This is due to oxidization in the clay matrix of the stone. The Pietra Serena used widely for architectural details in Italy during the Renaissance is commonly used for outdoor flooring in Northern Italy, it is a hard stone with low porosity. It has a high resistance to chemical attacks and erosion, and low resistance to atmospheric agents due to pollution that leads to the formation of black crusts. Its bulk density is 2,65 g/cm³, and the measured abrasion is 18,0 mm.

Noto Limestone: a yellow sedimentary sandstone mainly composed of organogenic limestone (formed by sediments deriving, in different ways, from living organisms) with fossil inclusions and polygenic gravel, formed in the Miocene period. The Noto Limestone is commonly used for historical constructions in Southern Italy, it is a soft stone with high porosity (33 %) and low resistance to chemical attacks. Due to its easy workability, it is also used as a decorative element typical of the Sicilian Baroque. Very low resistance to weathering due to its low effectiveness in withstanding damage, an apparent density 18,56 g/cm³, abrasion measure of 42.00 mm.

In table 1 the porosity characteristics of the three materials used for the tests.

Before carrying out the cleaning by micro-sandblasting, the 50 × 50 × 50 stone samples were superficially polished to obtain a surface as homogeneous as possible, in order to better quantify any alterations related to cleaning. The polishing was carried out through wet petrographic polishing using abravise papers. The Noto Limestone and the Stone Serena, we were unable to polish them perfectly.

The precision micro-sandblasting was carried out with a precision micro-gun (Colibrì 8) with a 2 mm nozzle that shoots siliceous river sand (no powder < 0,063 mm) with grains of about 0.25 mm diameter an with an outlet pressure of 2 bar and an operating time of 1 and 2 minutes.

For the assessment of damage due to mechanical cleaning, an Alicona IF-Portable, a three-dimensional optical device based on non-contact Focus-Variation was used.

Position volume (X, Y, Z): 50 mm × 50 mm × 26 mm, lens magnification used: 5X, lateral measuring range (X): 3.52 mm, lateral measuring range (Y): 2.64 mm, and vertical resolution: 7.58 µm.

Experimental Results and Discussion

The surface of each specimen (50 × 50 mm) was divided into four areas of 25 × 25 mm, thus dividing the surface into 4 quadrants.

Cleaning was not carried out on one of the 4 quadrants, because it was left as the control field surface of the untreated material, while the second quadrant was treated by cleaning with water.

Before starting the cleaning of the three quadrants, a Plexiglas mask was placed on the surface 272of the sample to avoid damaging the part not to be cleaned.

Figure 1: Three types of stone of different hardness and abrasion resistance: Carrara Marble, Serena Stone and Noto Limestone.

Figure 2: The Alicona IF-Portable Optical Profilometer used to perform high resolution 3D surface measurements.

The two remaining dials were treated with a micro-sandblaster, the first at a constant pressure of 2 bar for 1 minute, while the other dial was treated for 2 minutes. The two remaining quadrants were treated with a micro-sandblaster, the first at a constant pressure of 2 bar pressure for 1 minute, while the other quadrant was treated for 2 minutes. The sandblaster was held at a distance of 40–50. The 3D scans with profilometer and the optical observation with Alicona and static contact angle test were carried out before and after the treatment.

In order to assess the impact of micro-sandblasting on the samples, all areas were scanned. The 3D scans were acquired with 5x magnification from which the profiles that allowed to compare the differences in surface roughness.

Carrara marble: the untreated surface appears as a homogeneous and has a roughness in the range of 10 µm. The treated surface has an increase in roughness of 20–30 µm and the creation of some porosities with a maximum diameter of 300 µm, and a depth of 80 µm. a table with the results would be important.

The hardness of the material preserves it from the loss of mass, the extreme delicacy of the marble is also highlighted, a small variation in the surface roughness is clearly reflected on the macroscopic aspect of the object by altering its brightness. In fact it is possible to see from Figure 1, how the specimen appears damaged on the surface, but it is only a change of opacity without loss of material, opacity that is also obtained by water cleaning. Serena Stone: The 3D acquisition highlights a non-homogeneous granular material with a surface roughness on the range of 30 µm and the presence of numerous natural pores the roughness depends very much on the production or preparation process.

The cleaned surface does not show an increase in roughness and porosities with a depth of 50–60 µm 273are highlighted. Cleaning did not have a great impact on the total roughness and therefore also a limited macroscopic effect, but observing the profile data, we find some variations in the pore diameter and therefore in the surface porosimetric distribution.

Noto Limestone: From the 3D acquisition, the material is immediately very rough and uneven, with a roughness ranging from a few µm to 40–50 µm. The acquisition after treatment highlights a conspicuous loss of material and an increase in roughness this is typical for this type of limestone. Its very soft nature makes it the most threatened sample by micro-sandblasting.

Figure 3: 3D images representing the Carrara marble sample before and after cleaning by micro-sandblasting and its corresponding graph of surface roughness before and after cleaning. The blue line shows the measurement line.

Figure 4: 3D images representing the Carrara marble sample before and after cleaning by micro-sandblasting and its corresponding graph of surface roughness before and after cleaning. The red line shows the measurement line.

Figure 5: 3D images representing the Carrara marble sample before and after cleaning by micro-sandblasting and its corresponding graph of surface roughness before and after cleaning. The red line shows the measurement line.

Conclusions

The analysis with 3D Optical microscopy profilometer has highlighted quantitatively how a routine mild cleaned can alter the surface of a stone material.

The analyses have highlighted that in an abrasion-resistant material such as marble the effect of the impact of the grains of sand on the surface varies in relation to the angle of impact of the single grain, which in turn affects the roughness of the material by altering its surface polishing.

In this research, three materials of different hardness and abrasion resistance were analysed, and in fact it was observed that according to the morphology of the samples, the alterations generated by the treatment increase the roughness and in the case of the samples that were not very resistant to abrasion, such as the Noto Calcarenite Lime stone, also involved significant loss of material compromising its macroscopic appearance.

In an abrasion-resistant material, such as Serena stone, which apparently does not show any alteration of the surface visible to the naked eye, an increase in roughness is observed microscopically which increases the surface absorption.

This new technique is applicable as an alternative and/or in addition to optical color measurements during laboratory tests and complementary to investigations such as SEM (Schroettner et al. 2006).

274As a future perspective of the research, the possibility of creating an algorithm for the evaluation of surface porosity variations is being evaluated by comparing the data obtained with Alicona, mercury porosiment and micro-CT.