Restoration

"New Methods in the Conservation of Natural Stone"

Abstracts of papers presented at StonTec'99 German Stone Fair in Nuremberg, June 3, 1999

CONTENTS:

Article 1 -- A NON-DESTRUCTIVE METHOD FOR MEASURING THE DAMPNESS IN NATURAL STONE MASONRY USING A TIME DOMAIN REFLECTOMETER
By Friedrich Gruner, Gabrielle Grassegger, and Markus Stacheder

Article 2 -- NEW METHODS FOR THE RESTORATION AND CONSERVATION OF NATURAL STONE MASONRY
By Michael Auras

Article 3 -- SCOPE AND LIMITATIONS OF THE STRENGTHENING OF NATURAL STONE WITH SILICIC ACID ESTER
By Markus Boos, Josef Grobe, Georg Hilbert, and Eberhart Wendler

Article 4 -- ELASTIC SILICIC ACID. ESTERS IN THE CONSERVATION OF NATURAL STONE: FUNDAMENTAL THEORIES DEVELOPMENT AND EFFECTIVENESS
By Achim Wolke

Article 5 -- MORTARS FOR CONSOLIDATION AND REPORTING OF NATURAL STONE MASONRY
By Karin Kraus

Article 6 -- LASER CLEANING OF NATURAL STONE
By Dr. Heiner Siedel

Article 7 -- THE USE OF ULTRASONIC ANALYSIS FOR DAMAGE ASSERTATION AND QUALITY CONTROL IN RESTORATION OF CULTURAL HERITAGE
By Wolfram Kohler

 

Article 1 -- A NON-DESTRUCTIVE METHOD FOR MEASURING THE DAMPNESS IN NATURAL STONE MASONRY USING A TIME DOMAIN REFLECTOMETER

By Friedrich Gruner, Gabrielle Grassegger
Forschungs-und Materialprüfungsanstalt Baden-Württernberg,
Referat 32: Erhaltung historischer
Bauwerke, Pfaffenwaldring 4, D-70569
Stuttgart, Germany
Markus Stacheder
IMKO-GmbH, Im Stock 2, D-76275
Ettlingen, Germany

Time Domain Reflectometry (TDR) is a physics-based method for measuring dampness in porous building materials such as bricks, natural stone, or concrete. The key to TDR is the high dielectric constant (DC) of water, as the DC of humid porous substances is mainly a function of the water content, thus allowing for a non-destructive test.

Originally developed for locating cable defects in the telecommunications industry, this method is utilized for measuring high frequency pulses in metal conductors of a defined length in contact with the material in question. The quotient of the length of the conductor and the time delay determines the velocity of the pulses, which, in non-magnetic materials are, in turn, a function of the DC. The relationship between DC and water content of a porous substance is established with the help of easy-to-calculate empirically derived calibrations. The standard method is the Darr-Test, which determines the relative dampness of a substance by continuously heating it in a drying oven to 1100°C, at which point all water will have evaporated.

Improvements of the TDR method, such as the TRIME method, for instance, were customized for the needs of the construction industry in the form of small, battery-powered handheld devices which allow an in-situ analysis by utilizing either deep probes doing only little damage or completely non-destructive surface probes. The measurement field enters the top 3-8 cm of the sample, depending on the method used, and needs to be applied for only a few seconds. The degree of dampness can be obtained directly in mass-percent by correlating the internal calibration to the density of the type of rock in question.

One of the most important advantages of this method is its comfortable handling and high precision. The measurement is only marginally influenced by factors like salinity, temperature or a water film on the surface, which are very problematic for other indirect test methods.

The investigations presented in this paper are carried out on several architectural monuments consisting mostly of natural stone masonry. Additional test series were performed at the laboratory. Tests of concrete and natural stone masonry of different degrees of porosity in which the analysis with the TDR method is compared to conventional drill core sampling and drying via the DARR method. Furthermore, the test series were set up in a manner as to obtain results concerning the handling and limitation of both device and method.

These practically orientated investigations showed the high degree of accuracy of this method as well as its practical applicability.

Article 2 -- NEW METHODS FOR THE RESTORATION AND CONSERVATION OF NATURAL STONE MASONRY

By Michael Auras
Institut for Steinkonservierung, Grosse Langgasse 29, D-55116
Mainz, Germany

The primary goal of this congress is the presentation of results of new and practically oriented research from the field of stone conservation to non-scientific specialists working in architectural monument conservation. Research in this area of study is necessary in order to continually expand our knowledge concerning natural stone and its characteristics as well as developing a broad range of preserving agents and methods, thus enabling the conservation of our cultural heritage.

Apart from the transfer of knowledge from science to practice, it is also important for the scientist to get a resonance feed back from the people actually working with the different tools and materials. This resonance feed back enables the scientist to react to newly arising problems and possibilities. Thus the congress should be a forum for both sides of this dialogue.

In Germany, important basic research has been done in the field of stone conservation over the last 15 years. This took part in the framework of an extensive research program sponsored by the German Ministry of Education and Research (BMBF, Bundesministerium for Bildung und Forschung) with approximately 300 million DM.

Weathering of natural stones and its causes were just as much subject to research as techniques for cleaning, conservation and restoration. Additionally, the program was expanded to include other materials such as historical mortars and plastering, brickwork, glass painting and other topics as supporting the stability of historical stone masonry.

Based on this research, additional, more practically oriented research projects were needed after the expiration of the BMBF program. In many cases, this was made possible by support of the German Environmental Trust (DBU, Deutsche Bundestiftung Umwelt). Most of the results presented here belong to projects supported by the DBU to whom I gratefully express my thanks, and the thanks of my colleagues.

Conservation and restoration of architectural heritage is a wide field and thus it is not sufficient to merely develop new conservation agents and techniques. It is also necessary to fine-tune the proper tools for the diagnosis of the material in question, for recognition of the damaging processes and last, but not least, for quality assessment of conservation work.

At the moment it is almost always necessary to utilize destructive methods for material analysis of building monuments and their deteriorating. This is a situation which has to be improved upon as soon as possible. Therefore, I am glad to introduce two non-destructive diagnostic methods. The determination of dampness in masonry by microwave techniques has just completed the prototype stage and awaits its introduction to the market. The structural examination of natural stone by ultrasonic measurements, has been in use for several years in the field of stone conservation.

In addition, the most recent development in cleaning of natural stone, cleaning by laser, is presented. The huge potential but also the limitations of this method are discussed Institute for Inorganic Chemistry under Prof. GROBE (University of Münster) and REMMERS Bauchemie (Loningen) with financial support by the German Environmental Trust (Osnabruck). The SAE (silicic acid ester) kit may be applied to the following problems of stone conservation:

• strengthening of natural stone
• injection mortars
• mortars used to seal off transition zones
• color paints and transparent paints

Article 3 -- SCOPE AND LIMITATIONS OF THE STRENGTHENING OF NATURAL STONE WITH SILICIC ACID ESTER

By Markus Boos
Research and Development, Remmers Bauchemie
Loningen, Germany
Josef Grobe
Institute for Inorganic Chemistry, University of Münster
Münster, Germany
Georg Hilbert
Zentrale Objekt Abteilung, Remmers Bauchemie
Loningen, Germany
Eberhart Wendler
Laboratory for Conservation Issues in the Preservation of Monuments,
Munich, Germany

Introduction / Summary

Apart from certain methods of decreasing the rate of weathering, such as a reduction of hygric swelling or waterproofing, a sensible approach to the preservation of natural stonework will have to rely in most cases on the consolidation of the surfaces with a strengthening agent. To this end, silicic acid ester-based products have been used worldwide for decades now and their application can be judged as positive, without forgetting to take into account the limitations of this method, however. The following text will describe the current state-of-the-art of silicic acid ester research along with its scope and limitation as well as the development of new materials in order to expand the practical applicability of this method.

Weathering Profiles of Natural Stone

The different types of natural stones show enormous differences in composition, structure, pores, etc. In the same way differences in the natural processes of weathering and the weathering profiles, which mean the change of mechanical and hygric parameters from the (weathered) surface to the (unweathered) internal material. A detailed classification of typical strength profiles including an assessment of possible techniques for strengthening the material in question is described elsewhere (3). The scope of this classification shows that for certain profiles a successful strengthening of the structure is nearly impossible. This belongs to the limitation of the method that common silicic acid esters can not be used to strengthen structures with large pores. These limitations may be categorized according to its source as follows: 1) Distribution of the pore radii (i.e. tuff) 2) Development of micro-cracks in the structure in response to weathering (hygric swelling, trachyte) 3) Development of subcrustal weakened zones as a result of weathering due to anthropogenic influences

The Silicic Acid Ester Kit

WENDLER offers different approaches to the solution of the problems addressed above. A practice-oriented kit is being tested at the moment. This kit is based on the use of silicic acid ester (SAE) as a strengthening agent as well as a binder for mortars. It is currently being developed in cooperation between the Laboratory for Conservation Issues in the Preservation of Monuments WENDLER (Munich).

Article 4 -- ELASTIC SILICIC ACID. ESTERS IN THE CONSERVATION OF NATURAL STONE: FUNDAMENTAL THEORIES DEVELOPMENT AND EFFECTIVENESS

By Achim Wolke
Anorganisch-Chemisches- Institut, Universität Münster
Wilhelm-Klemm-Strasse 8, D-48149
Münster, Germany

Silicic acid ester (SAE) is, formally seen, the ester of silicic acid and an alcohol (equation 1). For industrial production, however, SiCI4 is used instead of silicic acid (equation 2), since the reaction according to equation 1 yields only small quantities of the ester.

The consolidating effect is based on the fact that SAE condenses in the presence of water to an amorphous silica gel in the form of small (10-20 mm) flakes which connect the components of a natural stone or another porous material and thus strengthen its inner structure. The advantages of using SAE are its weather-resistance, its ability to penetrate deeply into a porous structure and its permeability for water vapor.

The ability to penetrate into porous materials originates in the relatively small size of the SAE molecules. Its permeability for water is caused by a secondary porosity of the gel, which results from incomplete condensation of the SAE as well as from mechanical stress on the gel due to shrinking. Shrinking is caused by the loss of alcohol and water, the by-products of the condensation reaction. Shrinking and crack formation are responsible not only for the water vapor permeability but also for the smallish size of gel particles. in several types of weathered natural stone pores and cracks with diameters up to 500 mm can be observed. If those structures are subjected to mechanical stress, the small gel particles are no longer able to establish a strong grain-grain interconnection, thus making a consolidation of the structure impossible.

Sponsored by the German Environmental Trust, the group around Prof. Grobe, (University of Münster) in co-operation with Firma Remmers Bauchemie GmbH and the Laboratory for Conservation Issues in the Preservation of Monuments (Munich) developed elastic silicic acid esters (ESAE).

These ESAE are able to form bridges of bigger gel flakes, which, in combination with a limitation of the degree of inter-connectivity, leads to better "elastic" characteristics. Furthermore, the ESAE are able to connect fillings in order to achieve the desired strengthening effect in the case of extreme structural damage. Figure I shows the theoretic model of the mechanism of ESAE.

Investigations as to the capability of water vapor diffusion, water absorption, dynamic modulus of elasticity as well as flexural strength have shown that the theoretical model is fit for practical application, thus constituting an important milestone in the field of natural stone consolidation.

Article 5 -- MORTARS FOR CONSOLIDATION AND REPORTING OF NATURAL STONE MASONRY

By Karin Kraus
Institut for Steinkonservierung e.V.

Over the last ten years, it became more and more evident that natural stone masonry can only be restored satisfactorily by utilizing lime mortars. Lime mortars are, after all, the original substance used, and it is obvious that it is not only aesthetically but also from technical point of view advantageous to utilize materials similar to the original in restoration. Technical advantages include high flexibility and water permeability as well as their relatively low strength. These characteristics guarantee durability and compatibility in natural stone masonry. Even though these advantages should be obvious, especially when considering the century-long durability. of historical lime mortars, cement mortars were predominantly used after the discovery of Portland cement in the middle of the last century. This, however, oftentimes led to the need of restoring these restorations after only a few decades.

The hardening of mortars with high lime content is mostly the result of carbonation, a fact that definitely needs to be taken into consideration. Rapid carbonation is decisive for a successful application of lime mortars and thus recipe, application, and curing must be dealt with accordingly. With respect to the recipe, it is important to make sure that the grading curve is at the same time steady but not too steep in order to achieve optimum porosity for carbonation. For the application, it is important not to utilize too large amounts of mortar, as drying and carbonation can only be achieved on a relatively small exposed area on the very surface of the mortars used in filling the joints between the stones. Depending on the climate, curing needs to take place to ensure a continuous moistening of the top level of the mortar in question as the process of carbonation cannot continue its progress towards the interior under a dry and dense outer crust. On the other hand, it necessary to ensure continuous protection from rain for outdoors stone work freshly restored with lime mortars in order to prevent a washing out of the binder and too much moisture in the mortar during the first winter. In general, restoration work with lime mortars will not be possible during late fall or winter.

The types of usable lime based binders range from pure lime over pure natural hydraulic lime to limes mixed with cement or puzzolanic components, the choice of materials depending on the respective circumstances, such as the material characteristics of the object or its exposition to the weather.

Information on recipes may be found in the German standard DIN 1053 (1). Mortars of mortar group I are ideal for repointing work, while those of mortar group 11 serve for consolidation. In addition, lime mortars were over the last couple years repeatedly subject to scientific investigation (2),(3),(4). Mixtures of mortars examined in these publications supplement the list of existing mortar recipes. Mortars of higher groups, i.e. IIa or III are generally not usable as they are too hard and dense and thus incompatible to historical natural stone work. The same holds true, of course, for the use of ready-mixed mortars for consolidation and repointing.

Article 6 -- LASER CLEANING OF NATURAL STONE

By Dr. Heiner Siedel
Institut for Diagnostik und Konservierung
Dresden, Germany

Laser cleaning of natural stone surfaces is possible on the basis of some physical properties of laser radiation (extremely monochromatic, highly coherent light of high brightness and low divergence; selective absorption in materials). They allow one to focus on relatively high energies over very short time periods (pulses of some nanoseconds) on the soiled or encrusted stone surface. The absorption of laser radiation in soil layers and black crusts generates very high temperatures over a very short time and leads to vaporization and ejection of the soil material away from the impact site without any remarkable heat flow into the stone surface.

Within the last few years some laser systems have been specially designed for and used in the field of conservation of artworks. Most of them are Nd:YAG lasers with a wavelength of 1064 nm (nano meter). They are successful especially in the case of black encrusted limestone and marble surfaces, as shown by case studies in France (Notre-Dame in Paris and others), and in Italy and Germany as well (e.g. sculptures from the Cologne Cathedral).

Compared with the "classical" mechanical and chemical cleaning techniques in stone conservation and restoration, the laser shows the following advantages:

• Cleaning without any mechanical touch to the surface (suitable for cleaning loose scales, removing crusts from deteriorated stone surfaces etc.)
• Selective removing of very thin (microns) layers without affecting, the underlying original surface.
• "Self limiting effect" in the case of dark layers or crusts on light stone surfaces (e.g. white marble). Better absorption in the dark layers leads to their ablation while the light stone surface reflects most of the laser light at a certain level of energy density.

The efficiency of laser cleaning in comparison with the classical mechanical cleaning techniques is still too low on a lot of substrates to clean a whole facade. For that reason, the laser technique nowadays is mainly used in the case of restoring sculptures and building elements of higher artistic rank and quality, where good cleaning results (without damaging original surfaces) compensate for the longer working time.

Some problems may rise when painted (polychrome) stone surfaces are cleaned by laser. Various combinations of pigments and binders are sensitive in their interaction with laser light and may change their colors and chemical structure. At the moment there are investigations in progress at several institutes to evaluate these reactions and to avoid undesired side effects. Nevertheless, laser cleaning can be used even in the case of polychrome stone surfaces if the materials are known and sensitive binder-pigment systems can be localized. Thus, the laser technique can be combined in selected parts of the monument with other cleaning techniques as recently shown at the "Riesentor" of the Vienna Cathedral, Austria.

Laser cleaning is a special technique which should be carried out by restorers with some experience in this field. Some work has still to be done to find a scientific explanation for all reported effects of laser radiation in contact with soiled stone surfaces. The German Federal Foundation for the Environment is supporting two national projects dealing with laser cleaning, one for stone and the other for stained glass windows. These will be finished in 2000 and should give a summary of knowledge about the possibilities and the limits of laser cleaning for the most common German natural building stones.

Article 7 -- THE USE OF ULTRASONIC ANALYSIS FOR DAMAGE ASSERTATION AND QUALITY CONTROL IN RESTORATION OF CULTURAL HERITAGE

By Wolfram Kohler
Labor Kohler Bergblick 17, D-14558 Bergholz
Rehbrücke, Germany

Determination of the ultrasonic velocity within a certain type of natural stone allows deductions concerning the conditions and structures such as cracks within the rock itself as well as dampness or salinity. Furthermore, peculiarities in the sonic image indicate the use of preservative or strengthening agents near the surface. Moreover, it is possible to control the success of treatments with stone strengthening agents.

The most basic parameter in ultrasonic. analysis is the ultrasonic velocity which is a factor calculated by measuring the amount of time a soundwave needs to cover a certain distance. The most important factor of the ultrasonic method, on the other hand, is the velocity of the longitudinal shockwave vi.

VI = I / tI

VI = longitudinal shock wave velocity in km/s
I = distance in mm
tI = run time of longitudinal wave velocity in us

Mebstrecke in cm

US-Geber US-Empfänger Examples of practical work done with this method are investigations of the "Jungfrauenportal" of the Magdenburg Cathedral in which the ultrasonic method was used for damage assessment and investigations concerning the progress control of conservation treatments on the Monastery "Unser Lieben Frauen" in Magdenburg.

Figure 1: Ultrasonic analysis

US-Geber: ultrasonic emitter
US-Empfänger: ultrasonic receiver
Mebstrecke: covered distance