AP02_3.vp


1 Introduction
The requirement to measure the material moisture

content of a sculptural group within the Prague Loretto
complex was based on its condition in 1991. The sculptural
group was in critical condition with typical symptoms of
deep sandstone degradation in the form of chinks, cracks
and surface spalling of the larger parts.

The objective causes of this condition were directly con-
nected with the properties of the sandstone, which is relatively
heterogeneous, with numerous hydrolytic transformations of
ferrous minerals (limonite). Erosive failures had presumably
also occurred in the sandstone in the more distant past, always
provoked by water, and several attempts had already been
made to conserve the sculptural group.

48 ©  Czech Technical University Publishing House http://ctn.cvut.cz/ap/

Acta Polytechnica Vol. 42  No. 3/2002

Prague Loretto – Measurements of
Moisture Content in Sculptural Group
Material
J. Římal

An innovative method of spatial measurement of the moisture distribution in a sculptural group has been developed and verified. This
method facilitates monitoring of moisture distribution and, therefore, the condition of the sculptural group in a spatial perspective.
The seven-month measurements were in order to take into account all climatic effects influencing the sculpture.
These measurements resulted from the practical requirement to provide source material for future conservation interventions to be conducted
on the sculptural group in the Prague Loretto.

Keywords: Prague Loretto, measurement of moisture fields, mass transport.

Fig. 1: Prague Loretto

Fig. 2: Foutain decorated with the sculpture of the Resurrection
of Our Lord – Jan Michael Brüderle and Richard
Prachner, (1739–1740)



Previous conservation attempts on the sculptural group
are the second, subjective cause of its contemporary di-
lapidated condition. The remains of conservation attempts
can still be identified, namely in the form of fragments of
various fillings and surface crusts, originally meant in
good faith, but which today are the fundamental sources
of secondary erosion and corrosion processes.

These interventions, in particular, have violated the basic
requirements for natural water migration within the porous
structure of the stone. In turn, water tends to accumulate in
various intermediate stages of the natural stone, and in its
artificial forms. This has produced the usual negative effects,
such as frost heaving pressures during winter periods, hydro-
lytic effects, hydration and crystallization pressures, etc.

It is obvious that the condition is not improved by the
increased moisture content in the stone, due to both storm
water and capillary elevation from the subsoil of the sculp-
tural group. There is nowadays heavy air pollution in Prague,
and special in the Hradčany locality. All types of water within
the stone porous system in addition to the above-mentioned
negative effects, develop into sorption or absorption centres
for the cumulation of acid emissions. It is therefore necessary
to know the exact values of the mass moisture content and, on
the basis of the knowledge, to design measures for long-term
protection of the sculptural group.

Based on this critical assessment of the situation and
additional observations of the sculptural group, an ex-
tensive programme was designed in agreement with the
monastery management, with the aim of creating a de-
scriptive model of the condition of the entire volume of the
sculptural group with regard to distribution of moisture
maps inside the structure in relation to climatic conditions,
namely, rainfall intensity and moisture content due to cap-
illary elevation from the subsoil.

2 Measurement technique
In was decided to select a conductometric method for the

measurements. This method involves low input voltage and
the use of copper electrodes fixed at pre-selected points of the
sculptural group that remained stable through out the year.
In the given configuration, the method is highly accurate
(the slight changes in polarization and corrosion potentials
of the copper electrodes in the course of the year may be
neglected), allowing, in particular, an investigation of the
entire structure. By combining the measured points (elec-
trodes) into electrode pairs and making sensitive measure-
ment of the current flowing between them, practically all
vertical and horizontal cross sections of the sculptural group
could be mapped.

The probes consisted of copper wire with a diameter of
2 mm and a length of 60 mm mounted by means of lime dab
into previously prepared boreholes with a diameter of 3 mm.
This type of copper pin anchorage guarantees an alkaline
environment within the immediate vicinity of the electrodes
for a period of at least one year, thus stabilizing the electrolytic
medium in terms of appearance of variable polarization and
corrosion potentials. The points for anchoring the boreholes
and electrodes were selected with due care in order not to
damage the original, valuable relief surface and also to
include the points exposed to rainfall or, by contrast, those

situated in rainfall shades, within all horizontal and verti-
cal sections of the sculptural group. The measurements of
the currents were implemented by a digital system using
apparatus by SYSTEM ULTRAKUST (FRG), MULTIMETER
(UK) and SANWA ELECTRIC (Japan) with an input
working potential of 2 V, which, at currents ranging up to
10 mA, ensured only slight excess values of generated hydro-
gen voltage.

The use of copper electrodes of the first type was pre-
ferred to electrodes of the second type with electrolytic
bridges. The latter type of electrodes have the fundamental
advantage of eliminating polarization voltages, but, over a
longer period of time, they show changes in electrolyte con-
centrations and potentially also in electrolyte volumes, which
may subsequently have a decisive effect on the values of
the read currents. Copper electrodes may be regarded as
the most suitable solution as, when used in a sandstone
medium with the respective moisture contents, they provide
defined coverage with corrosion films, which, secondarily,
render the surface passive and thus provide long-term electric
stabilization.

The used configuration even allowed us to measure the
surface layer of the sculptural group by the neighbouring
electrodes, which was of interest namely during the period
immediately following rainfall.

The distribution of the selected measured points is indi-
cated in Fig. 2. In all, 14 measured points were numbered
from the bottom edge of the sculptural group almost to the
top of it.

Moisture content gauging was performed using the sam-
ples of original sandstone after it has been bored off. The
samples came both from the socle and from the scaled lay-
ers. Simple analyses did not show extreme contents of free
anions, which could, namely in heterogeneous distribution
within the volume of the sculptural group, considerably affect
the conductometric data. Gravimetric gauging using equally
fixed electrodes in the original sandstone samples practically
eliminated all negative effects of electrolytic interfaces. In
fact, it turned out during the measurements that only about
two electrode combinations showed some signs of fluctuations
in the results, namely after heavy moistening, which could be
attributed to electrolytic concentration effects.

3 Results of measurements
The results of the measurements are summed up in a

graphic form (see Graph No. 1, No. 2). The charts treat the
actual values of the mass moisture content in various ways
in relation to the time of measurement and the mutual
positions of the measured points, over a seven-month period
in order to show a nearly cycle, including spring, summer and
autumn. Graph 2 shows the gradients of the moisture field
measured at a certain selected point in relation to other (14)
points measured all over the statue.

The charts make it clear that we managed to collect
complete sets of moisture data for the measured period. The
analysis and interpretation enables us to draw wider conclus-
ions on the moisture distribution inside the sculptural group
and on the movement of the moisture zones inside the struc-
ture during one season of the year. It is also evident how the
sculptural group reacted to heavy rainfall and how the water

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Acta Polytechnica Vol. 42  No. 3/2002



50 ©  Czech Technical University Publishing House http://ctn.cvut.cz/ap/

Acta Polytechnica Vol. 42  No. 3/2002

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moved or was retained inside chinks and caverns, or how it
migrated through the porous sandstone system. A study of
the moisture profiles enabled the water contributions due to
capillary elevation and rainfall to be estimated.

Though some combinations of measured points show
considerable fluctuations of values depending on the actual
moisture contents, it is possible to trace the basic moisten-
ing trends during the different seasons and also during the
period of intensive summer drying up of the surface parts and
their successive wetting after a heavy downpour.

In absolute values, the highest mass contents range
above 10 %, but with regard to the erosion and corrosion
behaviour of the sandstone, the steady average moistening
of roughly the lower half of the sculptural group rang-
ing from 6 to 8 % seems to be more dangerous, as this is
roughly twice the value of the corresponding balanced
moisture content of the respective sandstone. These values
of moistening are probably the basic source of failures in
the intermediate material stages of the structure.

4 Conclusion
The above-described measurements of the mass moisture

content values in the sculptural group Resurrection of Our
Lord, located in the Paradise court of the Prague Loretto
show that the structure is exposed to excessive moistening
practically throughout the seven-month cycle. On average,
the values of sandstone moistening are twice higher than
would correspond to its average balanced contents.

The basic source of the current moistening of the lower
parts of the sculptural group is capillary elevation of water
from the subsoil. Contributions after rainfall cause high-
-level sandstone saturation, reaching up to values of around
12 % of the mass moisture content, which must be regarded
as critical moistening, namely in connection with the exist-
ence of watertight fillings and surface crusts.

It should be emphasised that it is water in its various forms
within the porous sandstone structure that is causing accele-
rating damage to the sculptural group.

To conclude, we recommend that long-term rehabilita-
tion measures on the sculptural group require the installa-

tion of a damp-proofing barrier against moisture capillary
elevation from the subsoil, and at the same time, the instal-
lation of roofing above the structure. Only complete elimi-
nation of excessive moisture sources can guarantee the
success of any preservation or conservation interventions
that will be needed on the sculptural group.

Acknowledgement
This project is being carried out with the participation of

Ph.D. students Ing. Jan Pytel and Ing. Jana Zaoralová and
undergraduate student Jakub Římal.

This research has been supported by the Grant Agency of
the Czech Republic under grant No. 103/00/0776

References
[1] Groot, de S. R., Mazur, J.: Non - equilibrium thermo-

dynamics. Amsterodam: North-Holand Publ. Co., 1962.
[2] Římal, J.: Mass and Heat Transport in Building and Bridge

Structures. Proceedings of Workshop 2000, Vol. B.,
Prague: CTU, 2000.

[3] Římal, J.: Research of Impact of Temperature and Moisture
Fields on the Bridge Structure of Charles Bridge. New Re-
quirements for Materials and Structures, Prague: CTU,
1998, p. 105–110, ISBN 80-01-01838-5.

[4] Šejnoha, J., Bittnar, Z.: Numerical Methods in Structural
Analysis. New York: ASCE Press, / London: Thomas Tel-
ford, 1996.

[5] Vodák, F.: Termodynamika spojitého prostředí. Prague:
CTU, 1992.

Doc. RNDr. Jaroslav Římal, DrSc.
phone: +420 224 354 702
fax: +420 233 333 226
e-mail: rimal@fsv.cvut.cz

Czech Technical University
Faculty of Civil Engineering
Thákurova 7
166 29, Prague 6, Czech Republic

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Acta Polytechnica Vol. 42  No. 3/2002