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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
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).
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.
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
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