Siegfried Siegesmund - Monument Future

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).

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).

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/cm 3, 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/cm 3, 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/cm 3, abrasion measure of 42.00 mm.

In table 1the 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.