SPECIAL ISSUE
Quality assessment of replacement stones for the Cologne
Cathedral: mineralogical and petrophysical requirements
B. Graue • S. Siegesmund • B. Middendorf
Received: 4 March 2011 / Accepted: 22 April 2011 / Published online: 19 May 2011
 The Author(s) 2011. This article is published with open access at Springerlink.com
Abstract Owing to its long building history, different
types of building stones comprised the construction of the
Cologne Cathedral. Severe damage is observed on the
different stones, e.g., sandstones, carbonate, and volcanic
rocks, especially when the different stone materials
neighbor the medieval ‘‘Drachenfels trachyte’’ from the
‘‘Siebengebirge’’. The question arises, ‘‘Is the insufficient
compatibility of the implemented building materials caus-
atively related to the strong decay of the Drachenfels
trachyte?’’ The present investigations focus on the
petrography and mineralogical composition of eight dif-
ferent stones from the Cologne Cathedral. Petrophysical
data, i.e., phase content, moisture and thermal character-
istics as well as strength properties are determined and
discussed in correlation to each other, showing that not
only in terms of lithology great differences exist, but also
the petrophysical properties strongly diverge. The ascer-
tained parameters are discussed in view of the deterioration
behavior and decay mechanisms of the different stones. To
evaluate the compatibility of original, replacement and
modern building materials, the properties of the investi-
gated stones are compared to those of Drachenfels trachyte
by means of constraints given in the literature. Besides
optical properties, petrophysical criteria are also defined as
well as strength values. It could be shown that primarily
moisture properties, i.e., capillary and sorptive water
uptake, water saturation, drying processes and moisture
dilatation can be addressed to the deterioration processes.
Keywords Stone decay  Cologne Cathedral 
Compatibility of building materials 
Requirements for replacement stones
Introduction
The Cologne Cathedral is one of the most important sacral,
political, and cultural monuments in Northern Europe. This
Gothic building has a unique position in an art historical
context. The cathedral has a very long construction history
in which different types of stone materials were used
(Figs. 1, 2). Since the Roman period the Drachenfels tra-
chyte from the quarries of the Siebengebirge was mainly
used as natural building stone for construction in Cologne.
The Rhine River provided an excellent means of trans-
porting good stone material from quarries along the Rhine
and connecting rivers (Wolff 2004). In Fig. 2, the litho-
logical survey map illustrates that the topic of stone
procurement was very important for the over 600-year
construction period of the Cologne Cathedral. The
increasing deterioration of the building materials from the
historic and more recent construction history has endan-
gered the structure of the cathedral. Construction scaf-
folding, which is always present as a permanent installation
indicates that preservation work is continuous at the
Cologne Cathedral.
The increasing emission of pollutants in our industrial
society has considerably accelerated the process of
weathering of building materials (for discussion, see
Siegesmund and Snethlage 2011). Generally, the
B. Graue (&)  S. Siegesmund
Department of Structural Geology and Geodynamics,
Geoscience Center of the University of Goettingen,
Goldschmidtstr. 3, 37077 Goettingen, Germany
e-mail: info@graue.org
B. Middendorf
Faculty of Architecture and Civil Engineering,
Department of Building Materials, TU Dortmund University,
August-Schmidt-Str. 8, 44227 Dortmund, Germany
123
Environ Earth Sci (2011) 63:1799–1822
DOI 10.1007/s12665-011-1077-x
assumption has been that acid-forming sulphur compounds
penetrate into the microstructure of the stone and then
become neutralized depending upon the rocks’ composi-
tion. These become concentrated as sulphate-rich salts
(especially gypsum enrichment) and are responsible for the
many damages observable (e.g., Knetsch 1952; Kraus
1985a; Kraus and Jasmund 1981). Cologne is a major city
with approximately one million inhabitants. Urban mobile
pollution sources, such as automobiles, trucks, railway,
etc., are the main contributors of air pollution in the city
today. Although the emission levels have dropped in the
last 30 years, dust pollution is still a major problem. The
observed values of air pollution can be correlated with the
increased number of chronic respiratory diseases (Wolf
2002). To¨ro¨k et al. (2010) investigated a series of samples
from the Cologne Cathedral, which were collected at about
30 m above the ground from the external walls. Very high
concentrations of lead (736 ppm) could be detected in dust
samples collected from different areas. The lead also
accumulates in the black crust, especially close to the
limestone crust interface. This indicates that either the crust
exhibits signals of past pollution levels or lead is being
mobilized from the surface to deeper zones. Even though
the SO2 content decreased in the atmosphere, the situation
in many industrial countries can be characterized as a
‘‘multi-pollutant’’ setting (CO2, NOx, VOC (volatile
organic compounds), dust, etc.).
In the coming years, upcoming restoration measures
should also demand the analysis of the natural building
stones, especially those selected as future replacement
materials. Not only should the weathering resistance be
investigated but also the material compatibilities need to be
analyzed. Recent observations at the Cologne Cathedral
lead to the assumption that the use of different building
materials, i.e., the historic and more recent replacement
stones, causes a negative interference in terms of their
deterioration and is of particular importance (Kraus 1985a;
Wolff 1992; von Plehwe-Leisen et al. 2007). The main
question arises, ‘‘Can the insufficient compatibility of the
used building materials be causatively related to the strong
deterioration of the Drachenfels trachyte?’’ Owing to the
fact that the Drachenfels trachyte quarry has been protected
Fig. 1 South elevation of the
Cologne Cathedral
1800 Environ Earth Sci (2011) 63:1799–1822
123
by conservation since 1922, no original material is avail-
able, and the search for adequate replacement stones is still
under discussion. Figure 2 documents that the medieval
part of the cathedral consists of Drachenfels trachyte,
whereas in later periods different stone types were used for
construction and repair purposes.
Since the middle of the nineteenth century, the building
stones of the Cologne Cathedral and their deterioration
behavior have been the subject of scientific scrutiny. Starting
with the observation of an ongoing decay of the building’s
structure, the aim of these investigations has always been the
search for suitable replacement stone material.
In this study, the mineralogical and petrophysical prop-
erties of the used rock types from the Cologne Cathedral are
investigated. On the basis of these investigations, the com-
patibility between the different stone types will be reflected.
Therefore, it is necessary to determine the porosity and the
pore size distribution (PSD), the water absorption and the
capillary loading behavior, the saturation coefficient, mois-
ture and thermal expansion as well as the mechanical rock
properties. In addition, the material should have similar
optical properties (e.g., color, de´cor, etc.).
Building history and the employment of replacement
stones
Construction of the Cologne Cathedral started in 1248. The
medieval part of the cathedral was built from Drachenfels
trachyte from the nearby quarry in the Siebengebirge. At
the beginning of the sixteenth century, the construction was
halted and recommenced at the beginning of the nineteenth
century. At that time, the Drachenfels trachyte was no
longer available.
The first replacement material used for the resumption
of construction work on the cathedral consisted of local
stone material available from the Siebengebirge. Initial
renovations were carried out with latite from the ‘‘Sten-
zelberg’’ and a few other materials from the quarries of the
Siebengebirge. In the middle of the nineteenth century, the
second construction phase used sandstone from ‘‘Schlait-
dorf’’, southern Germany. Later on, when the railway
connection between Cologne and Minden was built, the
‘‘Obernkirchner’’ sandstone from Lower Saxony could be
transported. The third construction phase started in 1918
and lasted until the beginning of WWII, where the
‘‘Krensheimer Muschelkalk’’ was the characteristically
used stone. In the 1950s, the decay resistant basalt lava
from ‘‘Londorf’’ was implemented. Presently, the trachyte
from ‘‘Montemerlo’’ (Italy) is used to replace the deterio-
rated Drachenfels trachyte; in addition, the sandstone from
‘‘Bozanov’’ (Czech Republic) is used to replace the
weathered Schlaitdorfer sandstone (Table 1) (Scheuren
2004; Schumacher 2004).
Since 1820, when the construction work and first repair
work resumed, a number of investigations for suitable
building materials for the Cologne Cathedral already
existed. A. von Lasaulx (1882) broached the subject of
Fig. 2 Lithological map of the
south elevation of the Cologne
Cathedral (Windscheid (2004)
after Wolff and Luckat 1973)
Environ Earth Sci (2011) 63:1799–1822 1801
123
weathering resistance in the implementation of building
stones. During the period under the supervision of master
builder, Hertel (1903–1927), a first systematic survey of the
deterioration behavior of the building stones of the
Cologne Cathedral took place (Hertel 1927). Besides the
works of Kaiser (1910a, 1910b, 1910c), Hirschwald (1910;
1912) was substantially involved in the development of
constructional investigations and the analyses of building
stones. He also laid the foundation for geoscientific anal-
yses in the preservation of cultural monuments.
Gru¨n (1931, 1933) showed that the condition of the
different building stones at the Cologne Cathedral varied
widely (Gru¨n 1931, 1933; Rathgen and Koch 1934).
The mortars used were considered as a potential source for
the deterioration and Gru¨n (1931) explicitly addressed the
environmental influences as deterioration factors.
Beginning with Knetsch (1952) emphasis was placed on
the geological and climatic context. The influence of air
pollutants and especially of flue gas on the deterioration of
natural building stones was detected in a program in the
1970s and potential preventive conservation treatments
were tested (Luckat 1973a, 1974, 1975, 1977, 1984; Wolff
and Luckat 1973; Wolff 1986; Wolff et al. 1988). Efes and
Lu¨hr (1976) accentuated the influence of environmental
pollutants as a significant factor for the stone decay. Fur-
ther studies dealt with the different deterioration processes
in several natural building stones from the Cologne
Cathedral (Kraus 1980; Kraus 1985a, 1985b; Kraus and
Jasmund 1981; Mirwald et al. 1987; Knacke-Loy 1988;
1989). Wolff (1992) first mentioned the negative interfer-
ences between the Schlaitdorfer sandstone and Londorfer
basalt lava, which mainly deteriorates the sandstone and as
a result of this the neo-Gothic building structure. This
raises the question again about which stone material with
comparable mineralogical, physical, and technical proper-
ties is suitable as a replacement material for the Cologne
Cathedral. The preservation of the Cologne Cathedral is
determined by the appropriate choice of a replacement
stone as well as the development of conservation treat-
ments and materials. Potential solutions were drawn for the
conservation of the Drachenfels trachyte (Dombauhu¨tte
Ko¨ln 2006; von Plehwe-Leisen et al. 2007). Until now, the
discussion about the preservation and conservation of the
stone materials used at the Cologne Cathedral is still in
progress.
Decay phenomena
The different building stones of the Cologne Cathedral
show a large variation of weathering phenomena. On a
representative survey area (Fig. 3a, b), the building mate-
rial and the deterioration phenomena have been mapped in
accordance with the classification by Fitzner et al. (1995)
and Siedel et al. (2011). Typical decay phenomena con-
sisting of surface deterioration, back-weathering, scaling,
structural disintegration, flaking and depositions are illus-
trated in Figs. 4 and 5. The individual deterioration phe-
nomena are assigned to the different building stones and
described in detail.
The detailed map of Fig. 3 shows a section of the
northern pillar of the North Tower, i.e., the medieval part
built with Drachenfels trachyte and the modern construc-
tion phase from the nineteenth century, when Obe-
rnkirchner sandstone was employed. The amount of
Drachenfels trachyte used in the mapped area is about
71%, Obernkirchner sandstone is used approximately 27%
as well as a minor percentage (*2%) of Schlaitdorfer
sandstone (Fig. 3a). While the complete surfaces of all
stones show a more or less distinctive deposition of dust
and aerosols, microbiological growth can only be observed
in the lower areas at the cornice. The localizations of the
deterioration phenomena are shown in the map presented in
Fig. 3.
Table 1 Main construction phases and building material used at the Cologne Cathedral (Scheuren 2004, Schumacher 2004)
Construction phase Period Building material
First construction phase 1248–1520/1530 Drachenfels trachyte
End of construction of the medieval cathedral 1530
Resumption of the construction work, first repairs 1820–1830 Wolkenburger latite, Stenzelberg latite, Heilbronner sandstone
Second construction phase [ sandstone period \ 1842–1860s Schlaitdorfer sandstone
From 1864 onward Obernkirchner sandstone
Third construction phase [ limestone period \ 1903–1945 Krensheimer Muschelkalk (1918–1940)
Caen, Savonnie`res limestone
Fourth construction phase From 1945 onward
Since 1952 Londorfer basalt lava
Since 2001 Bozanov sandstone
Since 2005 Montemerlo trachyte
1802 Environ Earth Sci (2011) 63:1799–1822
123
The main deterioration phenomena observable in the
Drachenfels trachyte are surface deterioration and back-
weathering (Fig. 4a–c) coexisting with flaking (Fig. 4f)
and structural disintegration to crumbling (Fig. 4d). Back-
weathered areas often display stronger further decay in
terms of microcracks, crumbling to total collapse. Scaling
is observable (Fig. 4b) and very often shows a granular
disintegrated zone on the reverse side. Formation of cracks
and fissures may also propagate many centimeters in depth
into the stone. Drachenfels trachyte is characterized by
large crystals of sanidines up to 7 cm in length. These may
cause a different weathering behavior between the matrix
and the phenocrysts. In the mapped area, the sanidines are
weathered-out (Fig. 4g), but only in the areas of the cor-
nices. The weathering behavior of the building stones is
also controlled by the rock fabric. In the Drachenfels tra-
chyte, the deterioration is more intense when the magmatic
foliation, defined by the preferred orientation of sanidines,
is parallel to the visible surface of the building stone
(Fig. 4e). A number of breakouts can be observed in the
Drachenfels trachyte, which are a result of the mechanical
impact of bombing during WW II. von Plehwe-Leisen et al.
(2007) have reported that flaking and scaling is often
observed. The flaking can occur in a very pronounced
fashion, which eventually leads to structural disintegration
and total fabric collapse. There are strong indications that
the decay phenomenon in the Drachenfels trachyte is
especially critical in the direct neighborhood of carbonate
replacement stones (Kraus 1985a; von Plehwe-Leisen et al.
2007). In many places, the decay starts from the joints,
which is indicated by gypsum crusts, flaking and scaling
(Fig. 5a).
In general, Obernkirchner sandstone is a very deteri-
oration resistant stone material (Grimm 1990). In the
area of the north tower the main deterioration phenom-
enon is the deposition of dust, forming grayish to black
crusts as well as the formation of gypsum crusts in
posterior areas (Fig. 5b). In some areas at the Cologne
Cathedral, the Obernkirchner sandstone was painted to
color adjust the stone to the Krensheimer Muschelkalk,
which was used for reinstatement work in the 1930s. In
connection with this paint layer, a surface parallel scal-
ing of very thin scales (thickness of 1–2 mm) can be
observed (Fig. 4a). Further severe damage is visible
Fig. 3 Map illustrating the
northern pillar of the North
Tower at the Cologne
Cathedral. a Stone distribution:
71% Drachenfels trachyte (red),
27% Obernkirchner sandstone
(beige), *2% Schlaitdorfer
sandstone (yellow). Joints:
mortar joints (brown), lead
joints (light blue), slate plates
(pink). b The main deterioration
phenomena are: surface
deterioration (light blue),
flaking (green), back-
weathering (orange), scaling
(pink), cracks (red), breakouts
(dark turquoise), crumbling
(brown), weathered-out
sanidines (dark blue),
microbiological growth (dark
red), gypsum crusts (light
green)
Environ Earth Sci (2011) 63:1799–1822 1803
123
along joints, where the sandstone shows breakouts due to
spalling, especially on the decorative parts, e.g., pilaster
strips (Fig. 5c).
Schlaitdorfer sandstone is a very problematic stone at
the Cologne Cathedral. Kraus (1985a) and Grimm (1990)
report that this stone characteristically disintegrates. The
carbonate cement (app. 14%) causes the problem, whereby
gypsum formation occurs that leads to massive scaling and
flaking phenomena as well as granular disintegration.
Moreover, another very typical deterioration phenomenon
for the Schlaitdorfer sandstone is rounding and notching
together with granular disintegration (Fig. 5d).
At present little is known about the deterioration
behavior of the Montemerlo trachyte at the Cologne
Cathedral, since this stone has only been implemented in
recent years. Very often intensive orange-brown discolor-
ation of the Montemerlo trachyte can be observed when it
is used as a replacement stone. These iron discolorations
have a negative aesthetic effect, but no structural impact.
However, Lazzarini et al. (2008) report exfoliation and
flaking, powdering and alveolic weathering for the
Montemerlo trachyte.
Stenzelberg latite and Londorfer basalt lava are very
resistant against weathering. Due to the high porosity, the
Fig. 4 Deterioration phenomena: a Overview: scaling and flaking of
Drachenfels trachyte (center); scaling of Obernkirchner sandstone due
to surface treatment (lower part); and Krensheimer Muschelkalk
(upper part); b Drachenfels trachyte: scaling; c Drachenfels trachyte:
back-weathering and structural disintegration (deteriorated pilaster
strip); d Drachenfels trachyte: pronounced decay in form of
microcracks, crumbling to total collapse; e Drachenfels trachyte:
pronounced surface deterioration due to the placement direction with
surface planar sanidines; f Drachenfels trachyte: flaking; g Drachen-
fels trachyte: weathering out of sanidines
1804 Environ Earth Sci (2011) 63:1799–1822
123
Londorfer basalt lava is susceptible to microbiological
action. The main deterioration phenomenon of Stenzelberg
latite is a typical formation of scales with a thickness of
2–3 mm (Fig. 5e). Furthermore, Grimm (1990) observed
exfoliation and contour-scaling, granular disintegration
into grus as well as powder, and breakout of mafic mineral
nests.
Bozanov sandstone shows spalling along edges when
mounted, which is problematic for masonry works. Prˇikryl
et al. (2010) reported on granular disintegration, scaling,
flaking, crust formation, as well as blistering, fracturing,
salt efflorescence and alveoli formation for the medium-
grained Bozanov sandstone.
In principal, the Krensheimer Muschelkalk is a deteri-
oration resistant stone. This carbonate building stone shows
massive gypsum crust formations as a result of acid rain
(Fig. 5f). This is visible in rain-protected areas, whereas on
surfaces exposed to rain, solution phenomena can be
observed, e.g., microkarst. In these situations, the blocks
express a surface roughness and it leads to a loss of shape
or form in detailed and decorative figural areas.
Joints are filled primarily with lime mortar, which often
have inserted slate plates. These are randomly visible due
to the back-weathered mortar. The present findings indicate
that these slate plates are used to cover the entire contact
surface of the building stones (Nussbaum and Lepsky
2010). The majority of the joints have been redone several
times with modern mortars during the different restoration
phases.
Building stones of the Cologne Cathedral
Petrography
The Drachenfels trachyte is a light gray, partially pale
yellowish or reddish, porous, porphyritic trachyte with
phenocrysts of sanidine up to 7 cm in size. The phenocrysts
Fig. 5 Deterioration phenomena: a Drachenfels trachyte: deteriora-
tion starting from the joint; b Obernkirchner sandstone: black dirt and
gypsum crusts; c Obernkirchner sandstone: breakouts due to spalling
along joints; d Schlaitdorfer sandstone: scaling, granular disintegra-
tion to sand and relief due to rounding and notching; e Stenzelberg
latite: scaling; f Krensheimer Muschelkalk: black gypsum crusts
Environ Earth Sci (2011) 63:1799–1822 1805
123
can show a preferred orientation in the matrix and traces a
magmatic foliation, which indicates the former flow
direction of the magma. According to Grimm (1990), the
rock comprises 50% sanidine, 24% plagioclase, 13%
quartz, 5% augite, 5% biotite, 2% ore and 1% apatite,
zircon and sphene. The fabric of the Drachenfels trachyte is
typical for volcanic rocks, which is characterized by
phenocrysts enclosed in a fine-grained matrix. Feldspar and
quartz comprise the groundmass (Fig. 6a). In places calcite
occurs. Volcanic glass fractions are in parts altered to
montmorillonite. More pyrite occurs in the groundmass
than what is generally expected for trachytes. Furthermore,
in some cavities pyrite and aggregates of pyrite-hematite
(limonite?) can be found. The visible pore space comprises
more than 10% (Grimm 1990).
The trachyte of Montemerlo shows an isotropic and
homogenous fabric (Fig. 6b). In some cases, the rock
exhibits a holocrystalline fabric with a weak parallel tex-
ture, where millimeter to centimeter large white feldspar
crystals, black biotite and dark, prismatic amphiboles float
in a gray, weakly yellow groundmass. Feldspar crystals can
attain sizes ranging from 0.5 to 10 mm. Biotites are smaller
than 2 mm. The composition was determined by Koch
(2006) as follows: K-feldspar (53%), plagioclase (15%),
quartz (8%), amphibole (8%), biotite (5%), pyrite (7%),
and calcite (4%). The phenocrysts can constitute about
Fig. 6 Cut sections of the investigated stones from Cologne Cathedral (a) Drachenfels trachyte, b Montemerlo trachyte, c Stenzelberg latite,
d Obernkirchner sandstone, e Schlaitdorfer sandstone, f Bozanov sandstone, g Krensheimer Muschelkalk and h Londorfer basalt lava
1806 Environ Earth Sci (2011) 63:1799–1822
123
40% of the total rock. Accessory minerals are zircon,
apatite, and sphene. The groundmass is microcrystalline
and mostly consists of anorthoclase, sanidine, plagioclase,
and seldom quartz. Plagioclase occurs as euhedral and
subhedral, zoned and twinned phenocrysts up to 6 mm in
size as well as in the groundmass. Alkali-feldspars are
predominantly anorthoclase and seldom occur as sanidine.
The Stenzelberg latite is a medium gray, porphyritic,
and in part porous latite (Fig. 6c). The micro- to crypto-
crystalline matrix (77%) is mainly composed of plagioclase
and sanidine. Plagioclase (14%), hornblende with individ-
ual grain sizes up to 10 cm (5%), augite (1%), and biotite
(1%) occurs as phenocrysts. Accessory minerals are apa-
tite, sphene, zircon and ore minerals. The mafic minerals
are often accumulated in clusters. Feldspar laths define the
flow fabric. The Stenzelberg latite has a low porosity
(8.5%), showing mainly intra- and minor inter-particle
pores with heterogeneous pore sizes (Grimm 1990).
The Obernkirchen sandstone is a medium-grained,
moderately to well-sorted quartz arenite showing a white to
orange color (Fig. 6d). The color is derived from iron oxide
and hydroxide. The detrital fraction (maximum grain size
300 microns) is composed of monocrystalline quartz
(98%), muscovite, zircon, tourmaline, rutile and opaque
minerals. Grains are rounded, spherical, and rod-shaped
with an aspect ratio up to four (Morales Demarco et al.
2007). Most of the grain contacts are concave-convex and
sutured. The fabric is grain-supported. The matrix (ca. 5%)
is composed of aggregates consisting of authigenic kao-
linite showing the characteristic booklet structure. The
cement consists of rare silica (probably syntaxial over-
growths) and iron oxide patches. Obernkirchen represents
pure quartz sandstone with a small amount of kaolinite
(Dienemann and Burre 1929; Grimm 1990).
The Schlaitdorfer sandstone is a whitish to yellowish
coarse-grained, well-sorted often parallel, angular and
cross-bedded rock (Fig. 6e). The detrital fraction (65%) is
represented by quartz (72%), rock fragments (12%), feld-
spar (2%) and cement (14%). The feldspar is often more
intensely weathered. The cement consists of coarse-grained
dolomite, in parts silica and rarely illite and kaolinite,
which show a dispersed distribution. The main accessories
(\1%) are apatite, zircon, tourmaline and opaque minerals.
Variegated marly clay is mostly accumulated in layers and
occurs parallel to the bedding. Grain contacts are mainly
along the long axes of the grains and they are often nar-
rowly intergrown (Grimm1990).
The Bozanov sandstone is a coarse-grained to medium-
grained arkosic sandstone (Fig. 6f). It is only weakly
cemented and shows a light gray to yellow color. Sporad-
ically clay cement participates in grain cementation as a
clay mineral seam or as a crack and gusset pore infilling
(smectite). Many grains display an initial interlocking grain
boundary with concave-convex grain contacts. The min-
eralogical composition is given by quartz (79%), rock
fragments (10%), feldspar (5%), clay minerals (smectite,
kaolinite, around 5%) and accessories like biotite, zircon
and opaque minerals. Very typical are cross-bedding, gra-
dations and shell-shaped cavities and impressions. The
feldspar content and its decoration by hematite crystals are
very characteristic features. The rock is poorly sorted
(Koch 2000; 2001; Prˇikryl et al. 2010).
The Krensheimer Muschelkalk is a light, brownish-
grayish fine porous limestone consisting of shell fragments
(Fig. 6g). It is classified as a grainstone according to
Dunham (1962) or as a densely packed bio(micro)sparite
after Folk (1962). Mussel- and brachiopod shells of
5–7 mm in size are densely packed in a fine calcite matrix;
pores are partially filled with calcite or often filled with
residual material. The components are oriented parallel to
the bedding, showing a moderate sorting. The composition
consists of 75% biogenic components mainly with micritic
rendering, 5% cement and 20% pores. The cement is spa-
ritic and partially micro-sparitic. Gusset pores characterize
the pore space, particle solution pores, and mould pores.
The fabric is grain-supported with mainly long and point
contacts (Grimm 1990; Siegesmund et al. 2010).
The Londorfer basalt lava is a brownish to bluish gray
basalt (Fig. 6h). Orange-brownish olivine crystals of
1–2 mm give the rock a weakly porphyritic appearance.
The fabric is fine-grained to medium-grained and around
the vesicles hyaloophitic. The rock is composed of pla-
gioclase (50%), clinopyroxene (25%), olivine (15%) and
ore minerals (ilmenite and magnetite around 10%).
Accessories of a cryptocrystalline nature (\1%) and par-
tially glass (up to 50%) also occur. The rock is highly
porous. Besides generally smaller pores, a less frequent
pore type of up to 6 mm in diameter is characteristic. The
pores are often coated by light gray-bluish secondary
zeolites. Inclusions of quartzite and claystone fragments up
to 5 cm can be observed (Grimm 1990; Steindlberger
2003).
Petrophysical properties
Analyses were carried out to determine density, porosity
and pore size distribution (PSD) on non-deteriorated sam-
ples from eight of the natural building stones used at the
Cologne Cathedral. Density was measured by buoyancy
weighing on cubic samples (65 mm). The dry mass, the
water saturation and the mass of the samples immersed in
water were measured to obtain information concerning the
porosity (DIN 52102 1988). Furthermore, the pore size
distribution was determined by using mercury intrusion
porosimetry (MIP) on cylindrical samples (Ø 12.5 mm)
(van Brakel et al. 1981; Siegesmund and Du¨rrast 2011).
Environ Earth Sci (2011) 63:1799–1822 1807
123
The investigated stones show medium porosities from
11.8% (Montemerlo trachyte) to 19.9% (Schlaitdorfer
sandstone), except the Stenzelberg latite, which belongs to
low porosity stones with a porosity of 8.5% (Table 2).
The stones investigated in this study have densities from
2.10 to 2.52 g/cm3 and can be grouped according to the
density. The three sandstones have lower densities.
Krensheimer Muschelkalk and the trachytes have slightly
higher densities. The highest density shows the basalt lava
(2.52 g/cm3) (Table 2).
According to Ru¨drich and Siegesmund (2006), the pore
size distributions (PSD) of the investigated stones (with the
exception of Krensheimer Muschelkalk and Londorfer
basalt lava) show a unimodal distribution. The Schlait-
dorfer and Bozanov sandstones have a broader distribution
of pores ranging 0.0064–82 lm with a clear peak of pores
at[10 lm (Fig. 7e, f). The Obernkirchner sandstone has a
narrower distribution (0.0064–64 lm) (Fig. 7d), and the
Drachenfels trachyte has even closer (0.0082–28 lm)
(Fig. 7a). Even strongly limited is the PSD of the Monte-
merlo trachyte from 0.0064 to 1 lm (Fig. 7b), and the
Stenzelberg latite with a PSD from 0.0064 to 0.28 lm
(Fig. 7c) with a relatively narrowed pore radii maximum.
Krensheimer Muschelkalk and Londorfer basalt lava show
a bimodal PSD (Fig. 7g, h).
Figure 8 shows the correlation of mean pore radius
and porosity. In general, it can be remarked that with a
higher porosity, the mean pore radius is also high. An
exception is the Obernkirchner sandstone, which has a
high porosity of 18.6% and a relatively small mean pore
radius of 0.82 lm.
Moisture properties
Capillary water absorption was measured according to the
standard DIN EN 1925 (1995), on cubic samples (65 mm).
The measurements were taken in two directions, parallel
and perpendicular, to the bedding of the stone. The w-value
is the amount of water taken up per area by the stone with
the square root of time (Wesche 1996).
The Drachenfels trachyte, Stenzelberg latite, and the
Londorfer basalt lava show low capillary water absorption
values (w \ 0.5 kg/m2Hh). The Montemerlo trachyte, the
Obernkirchner sandstone, and Krensheimer Muschelkalk
have a medium value (1–1.5 kg/m2Hh); Schlaitdorfer and
Bozanov sandstones show a high capillary water absorption
value (6.5–7 kg/m2Hh) (Snethlage 2005; Siegesmund and
Du¨rrast 2011). The values are listed in Table 2.
The Drachenfels and the Montemerlo trachytes show a
mode pore radius [the pore radius corresponding to the
region of the steepest slope (Aligizaki 2006)] in the range
0.1–1 lm (Fig. 7a, b; Table 2), which is the lower range of
capillary active pores according to Klopfer (1985). These
two rocks absorb water slowly with a low to medium
w value. The mode pore radius class of the Obernkirchner
sandstone ranges from 1 to 6.4 lm, which is within the
medium range of capillary active pore size (Fig. 7d;
Table 2). This sandstone shows slow but continuous water
suction with a medium w value. The Schlaitdorfer and
Bozanov sandstones show mode pore sizes in the range
10–100 lm (Fig. 7e, f; Table 2). These two rocks soak
water rapidly and have a high w value. Krensheimer
Muschelkalk has a bimodal distribution and two peaks at
Table 2 Bulk and matrix density, porosity, mean and mode pore radius, capillary water uptake, saturation coefficient, vapor diffusion resistance,
and sorption
Rock type Bulk
density
(g cm-3)
Matrix
density
(g cm-3)
Effective
porosity
(vol.%)
Mean pore
radius
(lm)
Mode pore
radius
(lm)
Capillary
water uptake
(w value)
(kg/m2Hh)
Saturation
degree
(s value)
Vapor
diffusion
resistance
factor (l)
Sorption
(max.
moisture
content)
(wt%)
z x
Drachenfels trachyte 2.33 2.64 11.92 0.414 1.334 0.55 0.74 37.38 17.91 1.88
Montemerlo trachyte 2.35 2.66 11.76 0.108 0.211 0.99 0.71 40.49 31.82 1.11
Stenzelberg latite 2.46 2.69 8.53 0.017 0.013 0.30 0.76 56.39 50.20 2.78
Obernkirchner sandstone 2.16 2.65 18.58 0.821 3.350 1.26 0.64 15.89 14.95 0.72
Schlaitdorfer sandstone 2.10 2.63 19.91 2.891 33.497 6.68 0.64 20.56 16.93 0.38
Bozanov sandstone 2.17 2.63 17.79 5.953 21.135 6.90 0.65 16.51 19.69 0.75
Krensheimer Muschelkalk 2.25 2.68 16.03 – 0.64/8.2* 1.30 0.59 69.45 51.25 0.29
Londorfer basalt lava 2.52 2.92 13.13 – 0.0082/28* 0.39 0.59 37.78 38.60 1.62
*Bimodal PSD
1808 Environ Earth Sci (2011) 63:1799–1822
123
0.64 and 8.2 lm (Fig. 7g; Table 2). Although 84% of the
porosity belongs to the capillary active pores, the w value is
not high, because of the low connectivity of the pore space
(Kraus 1985a). The Stenzelberg latite and the Londorfer
basalt lava show extremely slow water suction with low
w values. Ninety-five percent of the pores in the latite are
micropores (\0.1 lm) (Fig. 7c). Londorfer basalt lava
consists of about 34% micropores (Fig. 7h).
Based on the measured data, the stones can be divided
into three groups (Snethlage 2005): (1) Stenzelberg latite,
Londorfer basalt lava, and Drachenfels trachyte have low
mean pore radii and low capillary water absorptions
(w values); (2) Montemerlo trachyte, Krensheimer
Muschelkalk, and Obernkirchner sandstone have mean
pore radii in the lower to medium range of capillary active
pore sizes and medium capillary water absorptions;
Fig. 7 Pore size distribution of
the investigated natural building
stones (A = porosity)
determined by means of
mercury porosimetry. The red
bar indicates the percentage of
micropores and capillary pores
Environ Earth Sci (2011) 63:1799–1822 1809
123
(3) Schlaitdorfer and Bozanov sandstones with high mean
pore radii have high water absorbing coefficients
(w values).
Water saturation coefficient
The values for water uptake under vacuum and atmo-
spheric pressure were also determined as well as the degree
of saturation (s = Watm/Wvac). Porosity is equivalent to the
water uptake under vacuum in percentage of volume rela-
tive to the total volume of the rock. The saturation coef-
ficient (s value) was measured according to the standard,
DIN 52103 (1987). It represents the rate of the pore space,
which fills up with water under normal atmospheric pres-
sure conditions. The closer the water saturation coefficient
comes to 1, the higher the proportion of pore spaces filled
with water under atmospheric pressure. The values for the
water saturation of the investigated stones range from 0.59
to 0.76 (Table 2). The Krensheimer Muschelkalk and the
Londorfer basalt lava show the lowest s values. The
Schlaitdorfer, Obernkirchner, and Bozanov sandstones are
in a medium range. The Drachenfels and Montemerlo
trachytes as well as the Stenzelberg latite are rocks with
higher s values.
Sorption/desorption
For measuring the hygroscopic water adsorption, the
equilibrated sample weight was measured with ascending
and descending relative humidity in steps of 10% from 15
to 95%, and from 95 to 15%, at 30C on cylindrical sam-
ples (Ø 20, 50 mm in length) according to the standard,
DIN 66138 (2008). At the hygroscopic level, the water
adsorption of a rock is regulated by the humidity of the air,
and it is separated into sorption (moisture adsorption) and
desorption (moisture release). In the hygroscopic range of
0–95% relative humidity, moisture content of the rocks
increases with rising humidity along so-called sorption
isotherms.
The highest mass increase (at 95% RH) is shown by
the Stenzelberg latite with a value of 2.78 wt%, whereas
the lowest value was determined for the Krensheimer
Muschelkalk (0.29 wt%) (Table 2; Fig. 9a). The Dra-
chenfels trachyte and the Londorfer basalt lava also
show a relatively high water adsorption, whereas the
Montemerlo trachyte, the Obernkirchner and Bozanov
sandstones, have a medium water adsorption. The Sch-
laitdorfer sandstone only shows a small mass increase.
The Stenzelberg latite, Londorfer basalt lava, and Dra-
chenfels trachyte show a hysteresis in their sorption–
desorption behavior: the decrease of mass is less than the
increase, indicating that the stone material dries slower
with descending relative humidity and still contains a
residue of moisture as a possible indication of capillary
condensation (Fig. 9b). The Stenzelberg latite probably
Fig. 8 The investigated natural building stones show a tendency of
higher porosity correlated to a higher mean pore radius
Fig. 9 a The diagram shows the moisture content of the stones by
moisture adsorption (sorption) at 95% relative humidity. b Equilib-
rium moisture sorption isotherms, showing a significant increase at
relative humidity levels [85% reflecting capillary condensation.
Drachenfels trachyte (DT), Montemerlo trachyte (MT), Stenzelberg
latite (SL), Obernkirchner sandstone (OS), Schlaitdorfer sandstone
(SS), Bozanov sandstone (BS), Krensheimer Muschelkalk (KM) and
Londorfer basalt lava
1810 Environ Earth Sci (2011) 63:1799–1822
123
shows the effect of capillary condensation, whereas the
Montemerlo trachyte and Londorfer basalt lava may
possibly show little effect as well.
Water vapor diffusion resistance
The water vapor diffusion resistance is defined by the
vapor diffusion resistance coefficient (l value). It was
measured at 20C by using the wet-cup method on disk-
shaped stone samples (Ø 50, 10 mm in thickness) accord-
ing to standard DIN EN ISO 12572 (2001). The water
vapor diffusion resistance value indicates to what extent
the transport resistance of the water vapor is higher in rock
than in air. With the help of the wet-cup method, the l
value is determined at 50 and 95% relative humidity (RH).
This range represents the central European climate. Of the
investigated stones, the Krensheimer Muschelkalk and
Stenzelberg latite have a high resistance to water vapor
diffusion. Drachenfels and Montemerlo trachyte as well as
the basalt lava show a medium resistance, respectively. The
sandstones have the highest permeability of the investi-
gated stones (Table 2; Fig. 10a). The Drachenfels trachyte
shows a remarkable directional dependence of water vapor
diffusion resistance which could mainly be controlled by
the flow fabric (Table 2). A higher resistance correlates
with a higher amount of micropores (Fig. 10b): capillary
condensation takes place in micropores, which holds back
water due to solvent water diffusion. This leads to capillary
suction (retention), which is much slower than water
vapor diffusion (Snethlage 1984). Only the Krensheimer
Muschelkalk does not fit this correlation.
Moisture expansion––hydric dilatation and hygric
dilatation
The length and volume increase and decrease of rocks with
changes of moisture is well known as hygric (in the range
between 0 and 95% RH) and hydric (water saturated)
expansion and contraction (Delgado Rodrigues and Cha-
rola 1996; Weiss et al. 2004; Ruedrich et al. 2010a). Hydric
dilatation is measured on cylindrical stone samples (Ø 20,
50 mm in length) completely immersed in water. An
overview of the hydric dilatation on the different stone
materials is given in Table 3. In general, hydric dilatation
is low. The highest dilatation is determined at the
Fig. 10 a Water vapor diffusion resistance factor (l value) of the
investigated stones, perpendicular (z) and parallel (x) to the bedding
of the stones; b average water vapor diffusion resistance factor in
correlation with the percentage of micropores of the investigated
stones. The diagrams indicate the diversity of the stone properties and
show a correlation of higher water vapor diffusion resistance due to a
higher amount of micropores. Drachenfels trachyte (DT), Montemerlo
trachyte (MT), Stenzelberg latite (SL), Obernkirchner sandstone (OS),
Schlaitdorfer sandstone (SS), Bozanov sandstone (BS), Krensheimer
Muschelkalk (KM) and Londorfer basalt lava
Table 3 Thermal expansion
coefficient and hygric dilatation
Rock type Thermal dilatation coefficient (expansion) Hydric dilatation
x (10-6K-1) z (10-6K-1) Anisotropy (%) x (mm/m) z (mm/m)
Drachenfels trachyte 5.32 6.05 12.0 0.253 0.236
Montemerlo trachyte 6.25 4.65 25.5 0.291 0.316
Stenzelberg latite 9.41 7.36 21.7 0.196 0.230
Obernkirchner sandstone 11.60 12.17 4.6 0.089 0.060
Schlaitdorfer sandstone 9.65 11.96 19.3 0.025 0.025
Bozanov sandstone 8.78 8.65 1.5 0.027 0.013
Krensheimer Muschelkalk 4.75 6.82 30.3 0.000 0.005
Londorfer basalt lava 5.32 5.78 8.0 0.226 0.186
Environ Earth Sci (2011) 63:1799–1822 1811
123
Montemerlo trachyte with a value of 0.316 mm/m per-
pendicular to the bedding. High hydric swelling was
measured in the Drachenfels trachyte, Stenzelberg latite,
and Londorfer basalt lava. The Obernkirchner sandstone
has medium hydric swelling, whereas the other two sand-
stones, Schlaitdorfer and Bozanov, have low hydric dila-
tation. The length change of the Krensheimer Muschelkalk
is within the accuracy of measuring (Table 3).
Generally, hydric dilatation is anisotropic in nature, and
values for anisotropy of about 50% are reported in the
literature (Ru¨drich et al. 2005). With respect to the rocks
from the Cologne Cathedral, only the Bozanov and
Obernkirchner sandstones show a medium anisotropy.
Furthermore, the expansion of the different stones is time-
dependent: Londorfer basalt lava, Drachenfels trachyte,
Bozanov and Schlaitdorfer sandstones, have already
reached over 50% of their maximum expansion perpen-
dicular to the bedding in the first 30 min. Montemerlo
trachyte attained 88% of the total expansion in that time.
Obernkirchner sandstone already expanded to its whole
extent after 5 min. Stenzelberg latite has only reached 13%
in the first 30 min. This time dependence is ascribed to
different pore space properties. Where stones with well-
interconnected pores and relatively high porosities show a
fast expansion, stones with a less well-interconnected pore
space have a slower expansion (Ru¨drich et al. 2005).
Hygric dilatation processes occur with changes of the
relative humidity. The measured hygric expansion differs
from hydric expansion. The sandstones show low hygric
dilatation values, Schlaitdorfer (0.063 mm/m) and Bozanov
(0.010 mm/m). Moisture expansion in the Krensheimer
Muschelkalk is negligible (0.001 mm/m). Obernkirchner
sandstone has a somewhat higher expansion result
(0.065 mm/m). Stenzelberg latite shows high hygric
expansion with dilatation in the z direction (perpendicular to
bedding) of 0.231 mm/m. The Londorfer basalt lava also
has a high hygric expansion (0.185 mm/m). Hygric dilata-
tion in the Montemerlo trachyte (0.142 mm/m) is slightly
higher than that of the Drachenfels trachyte (0.110 mm/m).
With increasing relative humidity, a sharper increase of
hygric expansion can be observed at around 80–85% rela-
tive humidity (Fig. 11).
Thermal dilatation
Thermal expansion was measured on cylindrical samples
(Ø 15, 50 mm in length) within five consecutive heating
cycles from 20 to 95C. The length variation (resolution
\1 lm) was determined as a function of temperature and
was measured in two directions: perpendicular (z direction)
and parallel (x direction) to the bedding. The thermal
expansion coefficient was calculated (a = Dl/l 9 DT) as
well as the residual strain (ers = Dlrt/l) after one heating
cycle (Zeisig et al. 2002; Koch and Siegesmund 2001).
Anisotropy was determined by the difference of the
expansion coefficients in both directions.
Drachenfels trachyte and Londorfer basalt lava show
low thermal expansion coefficients and only little anisot-
ropy. Montemerlo trachyte and Krensheimer Muschelkalk
display low expansion values and pronounced anisotropy.
Bozanov sandstone has a medium coefficient but no
anisotropy. Stenzelberg latite has a medium thermal
expansion coefficient and anisotropy of about 21%. Obe-
rnkirchner and Schlaitdorfer sandstones show high thermal
expansion coefficients, only the latter displays an anisot-
ropy of about 19% (Table 3). No residual strain was
observed for the investigated stones.
Strength properties
The uniaxial compressive strength was measured in two
directions (z direction: perpendicular and x direction: par-
allel to the bedding of the rock) on planar cylindrical
samples (Ø 50, 50 mm in length; r/d = 1) according to the
standard DIN EN 1926 (1999). Based on the scale 3 effect,
the compressive strength ratio may differ up to 20% higher
by measuring samples with r/d-ratio of 1 instead of r/d-
ratio of 2 (Peschel 1983). The load was applied with a
strain rate of 1,000 N/s until failure. The compressive
strength varies between 45.1 and 126.4 N/mm2 (Table 4).
The Stenzelberg latite has the highest compressive strength
(126.4 N/mm2), while the strength values for the Krens-
heimer Muschelkalk, Schlaitdorfer, and Bozanov sand-
stones are less than 50 N/mm2. Only the Stenzelberg latite
belongs to the high strength rocks, while rocks with com-
pressive strength values between 55 and 70 N/mm2 belong
to the low strength rocks (e.g., Drachenfels trachyte and
Londorfer basalt lava) (Mosch 2008). Obernkirchner
sandstone and Montemerlo trachyte display higher uniaxial
compressive strength values. Except for the Obernkirchner
Fig. 11 Hygric dilatation perpendicular to the bedding of the stones
with relative humidity ranging 15–95%
1812 Environ Earth Sci (2011) 63:1799–1822
123
sandstone and the Montemerlo trachyte, a directional
dependence of the compressive strength is less pronounced
in the other samples (Table 4).
Flexural strength is an important mechanical property
for natural building stones. Failures due to bending stress
are more common than those caused by compressive or
shear stresses. The flexural strength values are usually
lower than the compressive strength values for a rock.
Mosch (2008) mentions a correlation of 10:1 for the
compressive to the flexural strength. Flexural strength was
measured according to the standard DIN EN 12372 (1999)
on oblong samples (150, 40, 25 mm) in two direction
(perpendicular and parallel to the bedding of the rock). The
flexural strength values of the investigated rocks cover the
range between 4.0 (Bozanov sandstone) and 15.7 N/mm2
(Stenzelberg latite) (Table 4). Londorfer basalt lava has a
high flexural strength, whereas the Schlaitdorfer sandstone,
the Drachenfels and Montemerlo trachytes show medium
flexural strength values. The Obernkirchner sandstone and
the Krensheimer Muschelkalk display somewhat higher
flexural strength values. The values are listed in Table 4.
In order to obtain the values of the tensile strength,
measurements were made using the Brazil Test according
to the standard DIN 22024 (1989). Investigations were
performed on disk-shaped specimens (Ø 40, 20 mm in
thickness). The load was applied with a strain rate of
30 N/s. Measurements were applied in two directions
(perpendicular and parallel to the bedding of the rock).
The tensile strength varies between 3.1 (Drachenfels
trachyte) and 9.7 N/mm2 (Stenzelberg latite), depending
on the sample and direction of load with respect to the
rock fabric. Of the investigated stones, Londorfer basalt
lava has a medium tensile strength. Obernkirchner,
Schlaitdorfer and Bozanov sandstones, Krensheimer
Muschelkalk, and Montemerlo trachyte have a lower
tensile strength (Table 4). Many rocks show the smallest
tensile strength perpendicular to the bedding. In the case
of the samples investigated, the heterogeneity may also
play an important role in explaining the observed data
(Table 4). However, the anisotropy of all the rocks is not
very well pronounced.
Discussion
Building stones of the Cologne Cathedral: deterioration
phenomena and decay processes
Due to its building history, many different building stones
were implemented at the Cologne Cathedral, which show
different deterioration behavior. These stones differ not
only in their genesis, but also in their visual appearance,
their mineralogical composition as well as in their porosity
features and rock fabric, and therefore in their petrophys-
ical properties, which again determine the deterioration
behavior. Furthermore, exposition, climatic situation,
industrial-based pollution, and building physics play a
major role (for further discussions see Siegesmund and
Snethlage 2011).
A high porosity in connection with a high water uptake
is considered as having a high damage potential. High
water uptake values (w value) combined with a high sat-
uration coefficient (s value) are the first indicators for a
possible susceptibility to weathering, or in other words,
pollutant transport, hygric and hydric expansion, frost
damage and salt crystallization in the pore spaces, etc.
Along with the capillary water uptake, an important role is
also played by the sorption (water derived from the
absorbed humidity) and desorption (water released in
relation to the relative humidity). This determines, among
others the drying behavior, which is influenced by the
capillary transport, the water vapor diffusion and the crit-
ical moisture of the stone.
The capillary absorption capacity of a porous stone is
defined by its water uptake coefficient (w value). This is a
process driven by the capillary forces that originate in the
Table 4 Uniaxial compressive strength, tensile strength and flexural strength of the investigated rocks in non-weathered condition
Rock type Uniaxial compressive strength (N/mm2) Tensile strength (N/mm2) Flexural strength (N/mm2)
z x z x z x
Drachenfels trachyte 65.54 66.59 3.087 3.674 5.999 6.100
Montemerlo trachyte 75.53 84.75 3.439 3.680 6.725 8.195
Stenzelberg latite 126.41 120.02 9.735 8.621 15.707 9.881
Obernkirchner sandstone 86.72 76.29 4.594 4.669 7.992 6.825
Schlaitdorfer sandstone 47.59 51.44 3.256 3.343 6.492 5.733
Bozanov sandstone 45.10 52.08 3.462 3.343 3.975 4.410
Krensheimer Muschelkalk 48.35 52.74 4.498 4.544 8.467 6.763
Londorfer basalt lava 63.10 72.18 5.099 5.921 12.567 12.749
Environ Earth Sci (2011) 63:1799–1822 1813
123
micropores and capillary pores (Klopfer 1985). Rocks with
a high amount of capillary pores are expected to have a
high w value, which means they have the capacity to rap-
idly absorb water by capillary uptake in the pore spaces.
Stones with low capillary absorption (suction) have a
w value of \0.5 kg/m2Hh, those with medium absorption
range from 0.5 to 3.0 kg/m2Hh and stones showing strong
water suction have w values [3.0 kg/m2 Hh (Snethlage
2005). A w value of [3.0 kg/m2Hh suggests a sufficient
uptake of water in the pore spaces to keep the stone moist
for a long time and to mobilize any salts present. The
importance of this parameter cannot be underestimated,
since a strong absorption capacity simultaneously means
that a high pollutant uptake and distribution occurs in the
pore spaces. This is the reason why dense building stones
will sometimes weather on the surface, whereas those with
a good absorption capacity will deteriorate at depth.
The water saturation coefficient (s value) gives an
approximate value for the frost resistance of natural
building stones. Hirschwald (1912) proposed the following
guideline values using the saturation coefficient: when
s \ 0.80 the rock is weathering and frost resistant; for
values ranging between 0.80 and 0.90 it is uncertain and
further investigations are necessary; and when s [ 0.90 the
rock is not frost resistant. Similar limitations are given by
the standard DIN 52103: a rock with s \ 0.75 is considered
weathering resistant and susceptible to weathering when
s [ 0.9. A s value [ 0.75 indicates that if the water supply
is high enough, the pore space is filled with water to a
higher degree and frost action could happen.
Sorption is the adsorptive addition of water from the air.
This occurs under isothermic conditions in two steps: 1.
Adsorption of molecular water films on the inner surface of
the stone material, and 2. Capillary condensation in pores
less than 0.1 lm in size (Kraus 1985a). The pore size
distribution gives a clue to the sorptive water uptake: with
an increasing amount of micropores the sorption increases
as well due to capillary condensation, assuming the pore
space communicates well (Fig. 12).
Pore size distribution and porosity of a rock are
responsible for water and moisture uptakes as well as water
transport. Generally, pores are divided due to their size into
different classes: micropores (\0.1 lm), capillary pores
(0.1 lm–1 mm), and macropores ([1 mm) (Klopfer 1985).
When capillary pores are present, water can be taken up
and rises by capillary action. Fluid and capillary transport
mechanisms are the main driving factor. On the other hand,
micropores adsorptively accumulate water from the air at
their inner surface (capillary condensation). Surface and
solution diffusion are the main transport mechanisms
(Siegesmund and Du¨rrast 2011).
The drying of natural building stones is a function of the
capillary transport, the water vapor diffusion and the crit-
ical moisture of the stone. According to Kraus (1985a),
when a water-saturated rock dries, the relative rapidly
absorbed water from precipitation is by comparison
released at a slower rate. This lengthy process is due to the
capillary absorbed water being released to a large extent by
vapor diffusion. The first stage in the drying of a stone
occurs over the rock surface, as an evaporation surface,
provided that the capillary water is replenished from the
deeper parts of the rock. When this capillary thread tears
off (critical moisture), water vapor diffusion transport
starts. Thus, a natural stone with a low critical moisture can
dry much faster. In this case, the capillary transport forces
are much stronger. At a value less than the critical mois-
ture, the significant determining factor for the drying pro-
cess is the water vapor conductivity. When a stone has a
high water vapor diffusion resistance, the water release
becomes progressively slower. This can be correlated to the
pore size distribution, whereby a high resistance can be
expected as a result of a large proportion of micropores.
Since the capillary water is condensed and bound, the
expulsion of any water from the stone material is very
difficult (Snethlage 1984). Furthermore, the communica-
bility of the pore space plays a certain role. The process of
drying in this second phase decelerates more and more,
because the distances of water vapor diffusion transporta-
tion become greater until moisture reaches equilibrium
with the surrounding air. Besides water vapor diffusion
also surface and solution diffusion takes place (Kraus
1985a). Therefore, the length of the first drying phase is
mainly determined by the percentage of capillary active
pores, besides external climatic factors, e.g., wind, tem-
perature, and insulation, etc., the second phase is deter-
mined by water vapor diffusion properties; considering the
general water uptake and saturation properties (w and
Fig. 12 The investigated stones of the Cologne Cathedral show a
tendency towards a higher percentage of micropores correlated to a
higher water adsorption due to capillary condensation. However, the
ratio of micropores in the Drachenfels trachyte would suggest a lower
sorption
1814 Environ Earth Sci (2011) 63:1799–1822
123
s values) and the connectivity of the pore space. Further-
more, Kraus (1985a) mentions, that with a capillary active
and well communicating pore system, water may even be
transported further into deeper zones of the stone, although
the active water supply already ended.
Moisture expansion (hydric and hygric dilatation)
describes the length or volume change which most natural
building stones undergo by wetting in correspondence to
climatic factors of the environment. The processes
responsible are not yet ultimately defined; the volume
change may be attributed to the swelling of clay minerals
as well as to disjoining pressure. This latter effect is rele-
vant for all minerals, and significant moisture expansion is
correlated to a large amount of micropores (\0.1 lm)
(Ruedrich et al. 2010a). According to Ru¨drich et al. (2005),
hygric dilatation is essentially a reversible process, i.e., no
residual strain is ascertained after reducing relative
humidity back to the starting value. This only applies for
demineralized water, which means by the presence of
damaging salts in building stones, these processes might be
affected remarkably.
A number of deterioration phenomena can be traced
back to the volume increase of natural building stones by
moisture expansion, e.g., scaling, flaking, and granular
disintegration. In most cases, building stones show an
irregular moisture distribution, whereby moisture gradi-
ents diverge leading to a build-up of strain and resulting
decay.
Rocks show volume changes due to moisture content, as
well as they undergo length or volume changes due to
changes of temperature. This process is determined by the
individual properties of the mineral content and composi-
tion but also by the structure and the rock fabric of the
natural building stone. The volume change does not nec-
essarily increase linearly to the temperature, which means
that the linear thermal expansion coefficient is often only
valid for a certain temperature interval (Siegesmund and
Du¨rrast 2011). The residual strain is of pronounced rele-
vance in terms of deterioration resistance. A permanent
length change of building stones after returning to the
initial temperature can be traced to microcracking and thus
indicates potential decay (Ruedrich et al. 2010b).
Strength properties such as compressive, flexural, and
tensile strengths are rock parameters, which also limit the
durability of dimension stones. Material failure occurs,
when stresses induced by mechanical weathering processes
exceed the strength of the material. In respect to frost and
salt deterioration resistance, damage occurs when the
stresses due to salt and ice crystallization exceed the tensile
strength (Ru¨drich et al. 2005). The strength properties
correlate to the grain fabric cohesion. Important fabric
parameters for the strength are the porosity, the pore size
distribution, the grain size, the grain contacts, the type and
state of cementation as well as a preferred grain boundary
orientation.
Drachenfels trachyte has a medium porosity, a low
capillary water uptake and a high s value, which might
suggest a certain sensitivity to frost-related weathering.
Luckat (1973b) demonstrated that the Drachenfels trachyte
is especially sensitive to salt weathering processes. Flaking
and scaling can be pronounced, especially in the direct
neighborhood of carbonate replacement stones (Kraus
1985a; von Plehwe-Leisen et al. 2007). Flaking and scaling
are often noticeable as a predecessor for further accelerated
decay by fissures, cracks, crumbling and material loss.
The sorptive water uptake of the building stone is in a
medium range, moisture expansion is relatively high, and
with 84% the percentage of capillary active pores is quite
high. In terms of desiccation of the Drachenfels trachyte,
water vapor diffusion transport mechanisms already start at
relatively high water content ([3 wt%) of the stone (Kraus
1985a). In respect to its water vapor diffusion resistance, a
long drying time is observed, showing after 15 days no
complete drying (Kraus 1985a). Furthermore, Kraus
(1985a) mentioned, that building stones exposed to the
natural environment experience a rewetting before they
might dry out completely. While uniaxial compressive
strength is in the medium range of the investigated stones,
tensile and flexural strength are low. For the Drachenfels
Trachyte a continuous high water yield, and therefore
sufficient water supply as ‘‘support’’ for deterioration
mechanisms exists, which presumably are due to the high
s-value, sorptive water uptake, vapor diffusion resistance
and retarded drying. Here different moisture gradients are
assumed, whereby hydric dilatation has an effect to a
certain extent. In the context of electrolytes i.e., ions dis-
solved from other carbonate stones nearby for example, salt
deterioration processes might be enhanced in addition
to pollution. The low strength values, especially for the
tensile strength, indicate a modest resistance against
weathering.
Montemerlo trachyte has been used at the Cologne
Cathedral since 2005 and still does not exhibit any struc-
tural damage. However, Lazzarini et al. (2008) reported on
exfoliation and flaking, powdering and alveolic weathering
for the Montemerlo trachyte in Venice (Italy) mainly
related to salt deterioration. The relatively low water
uptake only allows a certain uptake of pollutants but
porosity and pore size distribution assume a prolonged
drying time. Therefore, crystallization of salts can occur
and due to the low tensile strength, damage is possible.
The typical deterioration phenomenon of the Stenzel-
berg latite is a scaling of 2–3 mm thick scales (Fig. 5e).
Stenzelberg latite may have a low capillary water uptake,
but also a high water saturation (76%), which indicates a
certain liability to frost-related decay. Sorption is slow, but
Environ Earth Sci (2011) 63:1799–1822 1815
123
shows high values and hysteresis, which implies a decel-
erated moisture release. The high percentage of micropores
underlines the slow moisture uptake and also release. A
further retardation and especially zoning of these processes
is to be expected due to the technical surface treatment and
the material compaction involved. Thereby gradients of
moisture, material consistency and strength are evolved,
which could lead to surface parallel detached material
layers of a few millimeter in thickness due to frost shat-
tering. Hydric dilatation might have a certain but minor
impact.
Obernkirchner sandstone is mentioned as a building
stone with a high resistance against weathering. It mainly
shows superficial deterioration phenomena, which does not
have a severe structural impact, except of the gypsum
crusts (Fig. 5b). The formation of these crusts indicates a
strong pollution emission at the Cologne Cathedral in the
past. Due to the application of a coat of paint, to color
adjust to the Krensheimer Muschelkalk, applied in the
1930s, surface parallel scaling of approximately 1–2 mm
occurs. Spalling along edges near to joints (Fig. 5c) is
presumably due to mechanical impact of strain, caused by
the joint fill material. However, Morales Demarco et al.
(2007) determined a strength loss due to water saturation of
about 14% for the Obernkirchner sandstone.
Schlaitdorfer sandstone shows characteristic deteriora-
tion in the form of rounding and notching in context with
scaling and granular disintegration to sand (Fig. 5d). Kraus
(1985a) describes the decay of Schlaitdorfer sandstone due
to loss of cementation through the formation of damaging
salts, i.e., gypsum due to high SO2-immision. The Sch-
laitdorfer sandstone has a high porosity and a high w value,
which determines the high water uptake. The formation of
gypsum in the pores leads to accumulation of damaging
salts and thereby to scaling and back-weathering. Efes
(1980) observed an increase of smaller pores near the
surface and a reduction of water vapor diffusion up to 50%,
leading to retarded moisture release. With a saturation
coefficient of 0.64, the Schlaitdorfer sandstone is not vul-
nerable to frost attack. On the Schlaitdorfer sandstone, a
second desiccation phase by water vapor diffusion starts at
a water content of \2 wt%. During this drying phase by
water vapor diffusion, a pronounced accumulation of not
readily soluble gypsum salts exists in the pore space (Kraus
1985a).
At present, the Bozanov Sandstone shows little evidence
of decay except spalling along the edges caused by the
mounting of the building stones. These stones have only
been implemented at the Cologne Cathedral since 2001.
Prˇikryl et al. (2010) have reported on granular disintegra-
tion, scaling, flaking, crust formation, blistering, fracturing,
salt efflorescence, alveoli formation for the medium-
grained Bozanov sandstone. They detected a high amount
of water soluble salts responsible for blistering, granular
disintegration, scaling and flaking. Other weathering pro-
cesses described are: cyclical wetting and drying, freeze–
thaw cycles, a different thermal expansion of insulated
stone surfaces and less heated interior areas of the stone.
The petrophysical data of the present investigations support
these observations. Although Bozanov sandstone does not
have a very high water saturation degree, the capillary
water uptake is high. Thus, water supply is high, promoting
frost-related weathering as well as possible high loads of
damaging salts leading to salt deterioration phenomena.
Since strength values are low, these decay processes may
propagate to a vast extent. The thermal dilatation coeffi-
cient supports the assumption of structural deterioration
due to different temperature gradients.
Krensheimer Muschelkalk and Londorfer basalt lava are
building stones with a high resistance against weathering.
The main deterioration phenomenon of the Krensheimer
Muschelkalk at the Cologne Cathedral is black surface
crusts, which occur solely in rain-protected areas (Fig. 5f).
Due to the decrease of SO2-emission over the last several
years, this decay probably will regress as well (Siegesmund
et al. 2007; To¨ro¨k et al. 2010). The accumulated pollution
of the past certainly affects historical monuments in the
present but also in the future. Krensheimer Muschelkalk
shows microkarst phenomena, which are typical for car-
bonate stones and less problematic.
Londorfer basalt lava is only affected by pronounced
microbiological growth, which is associated with a great
number of large pores (Grimm 1990).
Requirements for replacement stones
At the Cologne Cathedral exists a material mix of sand-
stones and carbonates as well as volcanic rocks. This mixed
diversity of material on one building is an exception and a
great challenge. Owing to the long building history and
the continuous repair works, Drachenfels trachyte is
found in masonry bonds together with Stenzelberg latite,
Obernkirchner sandstone, Krensheimer Muschelkalk and
Londorfer basalt lava. Due to actual restoration works,
Montemerlo trachyte is integrated into the masonry bond.
Occasionally the Schlaitdorfer sandstone is also found
together with the medieval Drachenfels trachyte. This
indicates, that a potential replacement stone for Drachen-
fels trachyte has to be compatible with all other stone
materials, i.e., its basic properties have to be suitable with
all the other stone materials. In addition to it, Bozanov
sandstone is used as a replacement material for the
Schlaitdorfer sandstone.
To assess the compatibility of weathered and fresh
stones, Snethlage (2005) suggests that the properties of a
replacement stone should be in a similar range as the
1816 Environ Earth Sci (2011) 63:1799–1822
123
original stone. The mineralogical composition and the
optical appearance are important criteria for replacement
stones. Furthermore, the porosity, the pore size distribution,
the capillary and sorptive water uptake, the degree of sat-
uration, the water vapor diffusion value and the strengths as
well as the Young’s modulus of elasticity (E-modulus) are
necessary to determine. Many of the parameters required,
according to Snethlage (2005) were determined in the
present study.
Mineralogy
In terms of the mineralogical phase composition, the
implemented stones at the Cologne Cathedral cover a wide
range. Since the different building stones (sandstones,
carbonates, and volcanic rocks) exist together in the
masonry bonds, it is not possible to find the particular
replacement material of the same mineralogical classifi-
cation. Therefore it is of crucial importance, that the pe-
trophysical data, especially porosity and moisture behavior
are compatible.
Optical properties
In general, all building stones of the Cologne Cathedral
show gray to black surfaces when weathered or are origi-
nally grayish-black, except for the Krensheimer Muschel-
kalk in rain-washed areas. Quarry-fresh stones show
varying optical appearances. Stenzelberg latite and Lon-
dorfer basalt lava are both dark gray stones. Drachenfels
trachyte and Krensheimer Muschelkalk are originally light
grayish to beige stones. While the weathered Drachenfels
trachyte at the Cologne Cathedral shows black surface
crusts, the carbonate rock also forms black gypsum crusts
in rain-protected wet areas. However, where the stone is
washed, the light beige-gray color may even bleach
somewhat. Schlaitdorfer and Obernkirchner sandstones
originally show a similar light beige to yellow and orange
color, but weather differently. While the Obernkirchner
sandstone develops a grayish-black surface layer, the
Schlaitdorfer sandstone forms dark brown gypsum crusts,
but also tends to show some greenish microbiological
growth. Besides the color matching, also structural features
i.e., grain sizes are critical for the optical properties. The
employed sandstones have different appearances in respect
to their grain sizes. The Obernkirchner sandstone is a fine-
grained sandstone, while Schlaitdorfer sandstone is coarse-
grained. The actual replacement stone for the Schlaitdorfer
sandstone is the coarse-grained sandstone from Bozanov.
For the recently employed Bozanov sandstone, weathering
proceeds slower and only minor formation of dark crusts
can be anticipated, since the period of high pollution
impact has changed since the 1970s and 1980s. Moreover,
the Bozanov is free of carbonate minerals, and is therefore
less sensitive to the formation of gypsum crusts.
The actual replacement material for Drachenfels tra-
chyte, the Italian trachyte from Montemerlo has a similar
trachytic matrix, but is slightly more brownish in color
intensity. From an aesthetical point of view, the stone is
visually compatible with the slightly weathered Drachen-
fels trachyte. However, in areas where the medieval
building stone is weathered and shows intense black crusts,
the newly inserted Italian trachyte stands out quite signif-
icantly. Even stronger is the difference of weathered Sch-
laitdorfer to Bozanov sandstone. To cope with the situation
of these different appearances, the Obernkirchner sand-
stone and Drachenfels trachyte were painted to color adjust
to the light gray Krensheimer Muschelkalk in the begin-
ning of the twentieth century (Schumacher 2004). The
actual discussion on matching optical properties is whether
to apply an ‘‘aqua sporka’’ stain onto the new stones or to
employ laser cleaning to the old weathered stones.
Petrophysical criteria
In terms of porosity, all investigated stones except the
Stenzelberg latite belong to a medium porosity class
(Snethlage 2005, Siegesmund and Du¨rrast 2011). More-
over, both the trachytes and Londorfer basalt lava are in the
lower range of medium porosity (see also Stu¨ck et al. 2008).
In view of the pore size classes, clear distinctions are
registered. Figure 7 shows that the stones from the Cologne
Cathedral diverge in terms of their pore size distribution.
The Drachenfels trachyte, Schlaitdorfer, Obernkirchner and
Bozanov sandstones, as well as the Krensheimer Mu-
schelkalk have 83.3–89.6% capillary active pores. Lon-
dorfer basalt lava and Montemerlo trachyte show 65.8 and
62.8% capillary pores. Stenzelberg latite is the other
extreme with 95.4% micropores and only 4.6% capillary
pores. Due to its high percentage of micropores, Stenzel-
berg latite probably shows the effect of capillary conden-
sation. Montemerlo trachyte and Londorfer basalt lava
might possibly show little effect as well.
Comparing the pore size distribution of the Drachen-
fels trachyte and the other stones used at the Cologne
Cathedral, it is obvious that the Drachenfels trachyte
does not have a close match (Fig. 7). The Obernkirchner
sandstone has a wider distribution than the Drachenfels
trachyte with 67.6% of pores [1 lm, whereas the Dra-
chenfels trachyte only has 35.1% of that range. Sten-
zelberg latite and Montemerlo trachyte have relatively
high percentages of micropores. Londorfer basalt lava
and Krensheimer Muschelkalk have an even but very
unsorted pore size distribution from 0.0064 to 64 lm,
respectively 0.0064 to 82 lm. The Londorfer basalt lava
has 65.8% capillary pores, whereas the Krensheimer
Environ Earth Sci (2011) 63:1799–1822 1817
123
Muschelkalk shows 85.0%. Only the Schlaitdorfer and
Bozanov sandstones may be grouped as ‘‘heavy soak-
ers’’––stones with a high water absorption––with 83.3
and 89.6% capillary active pores and 47.8%, respectively
68.7% pores [10 lm.
Snethlage (2005) suggests using a replacement stone
with a low to medium water suction value when replacing
damaged parts, which originally consisted of a stone with a
higher water suction value.
All the w-values of the samples are in the range from 0.3
to 6.9 kg/m2Hh. The original building stone, the Dra-
chenfels trachyte, shows a w value of 0.6 kg/m2Hh. The
Schlaitdorfer and Bozanov sandstones can be classified as
belonging to the group of strongly absorbing stones.
Montemerlo, Obernkirchner and Krensheimer are medium
absorbing stones. The maximum water content attainable
by capillary water uptake was not determined, since it does
not play an essential role in nature. In general, the pene-
tration depth of rain is smaller than the thickness of
building components. The stones used in the Cathedral and
the evaluated replacement stones show a strong divergence
when considering the capillary water uptake, and thus do
not meet the necessary suitability requirements (Fig. 13).
The s value is a factor for the determination of the frost
resistance of natural building stones. The degrees of satu-
ration (s value) of the investigated stones are between 0.59
and 0.76 (Table 2). According to the limit value of
s \ 0.75 (DIN 52103), only the Stenzelberg latite has a
higher water saturation coefficient (Fig. 13).
The sorptive water uptake plays an important role in the
deterioration of natural building stones due to the central
European climate. The Stenzelberg latite has a very high
sorptive water uptake, whereas Drachenfels and Monte-
merlo trachyte as well as the Londorfer basalt lava have a
medium sorptive water uptake. The three sandstones and
the Krensheimer Muschelkalk adsorb water only by a small
degree due to their pore size distribution and the lack of a
well communicating pore space. Moreover, salt contami-
nation, e.g., due to pollution and deterioration may cause
significant increases of the sorptive water uptake in
exposed building stones (Kraus 1985a).
Until now, no guidelines are present to evaluate the
sorptive water uptake in respect to replacement materials.
Analogous to the guideline for the capillary water uptake,
sorptive water uptake of the replacement stone should be
the same or less as the original stone. Thus, only Stenzel-
berg latite does not fit into this scheme (Table 2, Fig. 14).
Since water is a driving factor for deterioration, drying
processes and their length play an important role for the
decay resistance. Kraus (1985a) ascertained drying durations
for Schlaitdorfer sandstone of 11 days, for the Krensheimer
Muschelkalk and Obernkirchner sandstone of 13 days and
for the Drachenfels trachyte and the Londorfer basalt lava
longer than 15 days. Krensheimer Muschelkalk shows
moderate drying despite a high water vapor diffusion resis-
tance (Kraus 1985a). The latter is determined by the low
connectivity of the pores. With respect to drying, Kraus
(1985a) determined that salt contamination and dirt deposi-
tions on the stone surfaces decelerate the drying process. As a
requirement for replacement stones, these should dry within
a moderate period. The newly inserted stone should not stay
humid longer than the neighboring original one, not func-
tioning as water supply to the latter one.
The investigated stones from the Cologne Cathedral
show a broad distribution in terms of the water vapor
Fig. 13 Capillary water uptake (w value) and saturation degree
(s value) of the stones from the Cologne Cathedral show much
divergence. If the proposed requirements on these two parameters are
set in comparison to the Drachenfels trachyte (marked area), only the
Londorfer basalt lava would show appropriate s and w values
Fig. 14 Diagram showing the sorption and water vapor diffusion
resistance of the investigated stones. Setting the proposed require-
ments in correlation to the Drachenfels trachyte (marked area), makes
it clear that for one parameter, e.g., sorption, all building stones
except the Stenzelberg latite might be suitable. A second parameter,
the water vapor diffusion, shows the insufficient compatibility of the
different stones
1818 Environ Earth Sci (2011) 63:1799–1822
123
diffusion resistance. Following the outline of a maximum
divergence of 10% for the water vapor diffusion value
(Snethlage 2005), it becomes obvious that the building
stones at the Cologne Cathedral are not compatible with
each other (Table 2; Figs. 10a, 14).
Moisture and thermal expansion are volume changes of
natural building stones induced by extrinsic factors
(exposition, climatic situation, and building physics). A
critical hydric swelling is observed at the Montemerlo and
the Drachenfels trachyte as well as the Stenzelberg latite
and the Londorfer basalt lava. On the building stones from
the Cologne Cathedral it can be observed, that with
increasing amount of micropores hygric swelling increases
as well (Fig. 15a), which indicates, that the main driving
factor for moisture related length changes could be
addressed to disjoining pressure in small pores. Further-
more, a smaller mean pore radius can be correlated to a
higher hydric dilatation. The stones from the Cologne
Cathedral with a high capillary water uptake do not nec-
essarily show high hydric dilatation, however high sorption
values can be correlated to higher hygric and hydric
expansion, indicating a damage potential; as it may be
interpreted for the Stenzelberg latite, Londorfer basalt lava,
Montemerlo and Drachenfels trachyte (Fig. 15b).
For both moisture and thermal dilatation, lower expansion
should be the aim for the replacement stones being employed.
Since the investigated stones show no residual strain in terms
of thermal dilatation, it is of minor importance.
In respect of strength properties, the following evaluation
of the investigated stones is based on the proposed criterion
of 80–120% of the strength value of the original building
stone (Snethlage 2005). In respect of the uniaxial compres-
sive strength, only the Londorfer basalt lava would be
compatible in terms of the strength values to the Drachenfels
Fig. 15 a The building stones from the Cologne Cathedral show that a
higher percentage of micropores indicates stronger hygric dilatation,
which again might be due to disjoining pressure in small pores. b Higher
water sorption values can be correlated to higher hydric dilatation,
indicating a potential damage source for Drachenfels trachyte, Sten-
zelberg latite, Londorfer basalt lava, and Montemerlo trachyte
Fig. 16 Diagrams showing the
strength values of the
investigated stones from the
Cologne Cathedral (a) uniaxial
compressive strength,
(b) flexural strength and
(c) tensile strength. Each have
the proposed constraint of
80–120% of the original
strength value (Snethlage 2001)
in correlation to the Drachenfels
trachyte (dark blue marked
area), and widened limitations
of 70–130% (light blue marked
area). Drachenfels trachyte
(DT), Montemerlo trachyte
(MT), Stenzelberg latite (SL),
Obernkirchner sandstone (OS),
Schlaitdorfer sandstone (SS),
Bozanov sandstone (BS),
Krensheimer Muschelkalk
(KM) and Londorfer basalt lava
Environ Earth Sci (2011) 63:1799–1822 1819
123
trachyte (Fig. 16a). A compatible flexural strength to the
Drachenfels trachyte is shown by the Montemerlo trachyte
and Schlaitdorfer sandstone (Fig. 16b). Assuming that
80–120% of the tensile strength of the original building
stone would be suitable, the Drachenfels and Montemerlo
trachytes as well as Schlaitdorfer and Bozanov sandstones
are of one comparable range (Fig. 16c).
If the limitations are extended to 70–130%, for uniaxial
compressive strength all investigated stones except the
Stenzelberg latite would be suitable, for flexural strength
the Obernkirchner sandstone could be added. In terms of
tensile strength it would be the same grouping as at
80–120% limitation (Fig. 16).
Conclusion
The different building stones employed at Cologne Cathe-
dral show a diverse petrography and mineralogical compo-
sition as well as a broad variety of petrophysical properties.
To understand possible interactive deterioration processes it
is important to determine the basic petrophysical data. The
investigations described here indicate possible decay sce-
nario and incompatibilities. The comparison with valid
guidelines reveals the broad spectrum of the used materials.
Moisture properties, which in turn are determined by pe-
trophysical features, become the center of focus in terms of
an evaluation of stone compatibility. A huge water exchange
in the microstructure of the stone is generally correlated to a
huge rise of pollutants, which not only supports a decay
potential for salt and frost deterioration but also provides the
necessary elements for chemical deterioration. Based on the
data, a first characterization scheme for further selections of
adapted stone materials for restoration purposes at the
Cologne Cathedral can be predicted.
Acknowledgment This work is supported by Deutsche Bundesstif-
tung Umwelt. Special thanks go the colleagues of the cathedral
maintenance department (Cologne Cathedral) as well as to Thomas
Schumacher and master builder Barbara Schock-Werner for support-
ing our work. We would also like to thank Christian Gross for his help
with English proofreading as well as Jo¨rg Ru¨drich, Stephan Mosch and
Helmut Du¨rrast for their helpful comments on an earlier version.
Open Access This article is distributed under the terms of the
Creative Commons Attribution Noncommercial License which per-
mits any noncommercial use, distribution, and reproduction in any
medium, provided the original author(s) and source are credited.
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