Selection at the stone face

Clay Technology magazine
,
14 Aug 2011
Strasbourg Cathedral

Strasbourg cathedral took a
long time to build. Between
the 12th and 15th Centuries,
sandstones piled atop one
another in eastern France. Stones
were extracted from different
regional quarries, and the
weather eroded them at
different rates.

A paucity of exploited regional
Meules sandstones of sufficient
quality led to the use of another
Triassic siliceous sandstone
called Vosgien. However, the use
of this red-coloured stone was
said to modify the perception of
the building’s aspect. Meules
sandstones from Germany
were then used.

Despite coming from different
areas, these sandstones are
composed of similar materials
(quartz, feldspars and a soft
matrix containing clays and iron
oxy/hydroxides), but the reasons
some degrade more quickly than
others is not clear.

To improve stone selection and
boost preventative research, a team of French researchers
from the Université Reims
Champagne-Ardenne, Laboratoire
de Recherche des Monuments
Historiques in Champs/Marne,
and Etude Recherches Matériaux
in Poitiers, has conducted
research into the stone structure
and clay coating of Triassic
siliceous sandstones.

The team assessed compatibility
and durability by studying
macroscopic and mechanical
stone behaviour when subjected
to water, liquid and vapour.

Five Meules and Vosgien
sandstone types from French and
German quarries were tested.
Two exposed Meules sandstones
from the Strasbourg cathedral
were also tested. The first, dating
back to the 1870s showed heavy
damage, while the second, from
the 1420s, was well conserved.

To identify the clay minerals,
they performed an X-ray
diffraction decomposition
procedure. This highlighted the
presence of interstratified illite/smectite (I/S) minerals in
each sandstone.

Lead Researcher Estel Colas
says there is a presence of I/S
minerals in all sandstones but
especially in the Vosgien
sandstone, where it is present
in great quantity (70%). In the
Meules sandstones, kaolinite or
illite are the predominant phases.
‘This particular I/S phase could
promote swelling and plasticity
properties to the stone, which
could be a “drawback” in stone
resistance,’ she claims.

The team assessed the pore
structure of stones using mercury
intrusion porosimetry. Their
analysis of the two cathedral
stones deduced that, as the clay
mineral amounts from the two
cathedral stones presented no
significant differences, ‘the total
clay mineral content could not be
considered as the sole parameter
for stone durability assessment’.

A dilation method was used to
evaluate the behaviour of stones
subjected to hygric dilation under relative humidity
variations. This method found
that, ‘the hygric dilation
properties of sandstones were
controlled by adsorption qualities
of the clay minerals. Although
the Vosgien sandstone had the
lowest swelling coefficient, it
showed a non-reversible
behaviour characterised by a
steady contraction with relative
humidity cycles.’

‘Meules sandstones show
strong grain-to-grain contacts
while Vosgien sandstone is much
weaker, which permits a sliding
of grains,’ Colas adds.