of Lower to Middle Bathonian (Middle Jurassic) age.2 It is thus the stratigraphic equivalent of Bath Stone, from the lower part of the English. Great Oolite Group.
Understanding the Weathering Behaviour of Caen Stone Tim Palmer
Abstract Microscopic
study
shows
that
Caen
Stone
varies
in
its
petrographic
structure between pellet-rich and bioclast-rich forms. The former is much more
microporous
than
the
latter,
and
tends
to
draw
water
into
its
structure if wetted. In historic usage, this has apparently led to more severe decay than is seen in the latter variety, which has generally displayed good durability.
Examples
of
Caen
Stone
that
survive
on
the
exterior
of
medieval buildings have led to the material acquiring a good reputation, but less evident, poorer-quality medieval Caen Stone may also be found internally in remnant blocks, where surface decay was cut out for the insertion of new material in earlier episodes of repair. Both pellet-rich and bioclast-rich market,
and
varieties are
are
likely
encountered
to
be
in
new
distinguishable
stone in
available
standard
in
tests.
the The
appended report on Caen Stone usage at the Palace of Westminster by Tim Yates which follows lends weight to this conclusion.
Keywords: Caen Stone, petrography, microporosity, durability.
Introduction Caen Stone was the only French building stone to have been imported into England in large quantities during the Middle Ages. Its use was promoted by the Normans after the Conquest of 1066 because they were familiar with it in their own building projects in Normandy. It continued to be imported and used widely in southern and south-eastern England until the Reformation, both for internal and external work. Major projects where Caen Stone was used as the sole or principal freestone have been listed in 1
many previous works and recent summaries , and there are also many
Journal of Architectural Conservation November 2008
46
Tim Palmer
minor buildings, particularly across southern England and up into East Anglia, where it appears to have been the freestone of ®rst choice. Much medieval Caen Stone survives to the present day in the original structures where it was used. Consequently, material shipped to England during the Middle Ages has acquired a good reputation for durability amongst
masons
and
conservators.
The
situation
with
all
medieval
stonework, however, is usually more complicated than it appears at ®rst sight, because only the best quality stone is likely to have survived across the
intervening
years.
Some
major
restoration
programmes
of
medi-
eval Caen Stone buildings, such as that currently being undertaken at Canterbury Cathedral, have revealed a mixture of surviving good-quality Caen, mixed with nineteenth-century insertions where decayed medieval stone had been chiselled back to allow insertion of new facing stonework. It seems that not all medieval material was of the same high quality, and that some blocks were more susceptible to weathering decay than others. During modern repairs, it has been possible to take samples from both the good and the poor durability medieval stone so that both can be compared petrographically. Caen
Stone
started
to
be
used
again
in
earnest
in
England
in
the
nineteenth century, partly in restoration projects, but particularly to meet the growing enthusiasm for building new churches that started early in the reign of Queen Victoria. As in medieval times, it was used for both interior and exterior work. Its uniform texture and easy carvability were well± suited to the ornate embellishments of the Gothic style. In contrast to medieval Caen Stone, the material that was used in the nineteenth century has acquired a very bad reputation for durability in external use, and there is no doubt that it has weathered calamitously in some buildings. For example, at St Giles Church Camberwell in south London (1841±43; one of the earliest works by Sir George Gilbert Scott) the ®rst replacements of the Caen Stone dressings in the original structure were required within about 20 years of completion. Since then, various episodes of repair have been necessary. In the mid-1990s, some quoins on the north-eastern corner of the north aisle had decayed completely leaving empty rectangular holes. Similar
stories
of
severe
decay
in
externally-used
Caen
Stone,
often
associated with thick black gypsum crusts, can be heard from architects and
conservators
across
southern
England.
The
sulphur-rich
polluted
atmospheres of urban areas, in particular, wreaked terrible damage on much
external
Caen
stonework.
Furthermore,
it
was
often
used
in
conjunction with impermeable brick, and consequently suffered the effects of damp conduction. It is not clear whether all nineteenth-century Caen Stone was of similarly poor durability, or whether, as with the medieval material, it was of variable quality. At Canterbury, some early nineteenthcentury replacements have weathered badly and some are still in good
Understanding the Weathering Behaviour of Caen Stone
condition. However, it isn't clear whether this is a consequence of their lithology or of their aspect and microenvironment within the building. This
account
considers
the
likely
relationship
between
the
variable
durability of different examples of Caen Stone, and its geology. It draws on observations
made
on
samples
of
Caen
Stone
of
various
dates,
but
particularly on the medieval and nineteenth-century replacements from Canterbury Cathedral. I am particularly grateful for the help that I have received from their conservation staff.
Geological aspects of Caen Stone Age and palaeogeography Caen Stone, or Pierre de Caen (sometimes Pierre de Taille de Caen Caen Freestone), is the commercial name for stone that comes from the upper part of the stratigraphic unit known as the Calcaire de Caen, which is of Lower to Middle Bathonian (Middle Jurassic) age.
2
It
is
thus the
stratigraphic equivalent of Bath Stone, from the lower part of the English Great Oolite Group. It appears to have been deposited on a seabed that sloped downwards from southern Normandy northwards towards the English Channel during Jurassic times.
3
At the southern end of this slope
towards Sarthe, lagoonal sediments and lime muds, fringed locally by oolites, were laid down. To the north, across Calvados towards Bessin (the area inland from the coast northeast of Bayeaux), sponge-rich marls were deposited in deeper water. Between the two, the pelleted and bioclastic ®ne limesands of the Calcaire de Caen were laid down on the slope. These sediments are best developed in the region between Caen and Falaise to the south. They are underlain by thin beds of marly limestone and they pass upwards into shallower, coarser-grained oolitic sediments which are now known as the Calcaire de Rouves south and east of Caen, and the Calcaire de Creully north and west of the city. The thickness of the Pierre de Caen is about 5±8 m in the quarries that underlie the city.
4
Petrology Although the Calcaire de Caen is equivalent in age to Bath Stone, the two are petrographically dissimilar and Caen Stone is not an oolitic limestone. It is a ®ne, buff-coloured lime sand (calcarenite, or grainstone, or biopelsparite in geological terminology) dominated by bioclasts (invertebrate skeletal fragments) and pellets of very uniform texture. Individual grains are not readily distinguishable to the naked eye (though clear in thinsection) and the stone lacks an obvious sedimentary lamination, though larger shell fragments (which are rather uncommon) lie horizontally. The
47
48
Tim Palmer
pellets are faecal or pseudofaecal in origin (formed from the droppings of marine invertebrate organisms), and are uniform in size and shape (oval and about 0.05±0.1 mm across). They are essentially made of compacted limey mud particles that were caught up by the ®lter-feeding currents of the fauna and discarded. The bioclastic component of the sediment is primarily derived from echinoderms. It is likely that these are mainly the disassociated ossicles (skeletal plates) of motile crinoids (sea-lilies). They are not echinoids (sea-urchins), because spine sections are rare and the material is of rather uniform size. Textulariid and other foraminiferans (single-celled microfauna) are quite common in some thin-sections, and locally there are shards of thin calcitic shells. Ooliths are absent. There is a well-developed calcite cement within former intergranular primary porosity and, because of the abundance of echinoderm debris, this is principally in the form of overgrowth cement (a tight, compact form of calcite spar that typically grows on a nucleus of echinoderm debris). The interlocking compound crystals (nucleus plus overgrowth) typically reach about 0.2±0.3 mm across. Minute pyrite (iron sulphide) crystals, often giving rise to a diffuse rusty staining in their oxidation halo, are locally present. In the lower parts of the Calcaire de Caen, particularly in the deeper and muddier facies, there are sponge spicules, which were probably the source for the silica in cherts (both replacive and displacive), which are widely
distributed
in
the
Calcaire
de
Caen,
though
avoided
in
the
commercial stone. Within these petrographic limits, there is a range of variation between beds, or even within the thickness of a single bed. This is based on the relative proportions of the two principal components. At one end of the spectrum is the pellet-rich variety of Caen Stone. In this, the pellets are numerically dominant and the bioclasts are scattered amongst a pellet-rich matrix (Figure 1a). Locally, compaction has pressed the pellets together so that they appear continuous and the rock tends towards a packstone or wackestone texture. There is more scattered pyrite associated with the pellets in this variant of Caen Stone, which has typically oxidized to give it a stronger buff colour. In places there are open pores that are large enough to show under low magni®cation in thin section. In contrast, at the other end of the petrographic range is crinoid-rich Caen Stone. In this variety, the skeletal fragments are numerically dominant and the pellets appear scattered and isolated in a matrix of bioclasts and their dense cement overgrowths (Figure 1b). This fabric is a clear and well-cemented
grainstone
that
is
volumetrically
dominated
by
calcite
spar, with little pore space. This variant of Caen stone is paler in colour because
it
contains
less
pyrite
and
hence
less
rusty
discolouration.
Intermediate textures between typical pellet-rich and typical crinoid-rich occur (Figure 1c), in which neither grain type is particularly dominant over
Understanding the Weathering Behaviour of Caen Stone
Figure 1a±c
Variation in the petrological character of Caen Stone seen in thin
4mm. pellets; cr crinoid plates with a halo of dense crystalline calcite cement.
section through the optical microscope. Width of view of each specimen p
Figure 1a
Variety with pellets
dominant and scattered, light-coloured debris from the skeletons of invertebrate animals, particularly crinoids. Decayed stone used post-1830 on eastern elevation of Canterbury Cathedral, removed in recent repair work.
Figure 1b
Variety with
dominant crinoid debris and natural calcite cement forming a rigid three-dimensional matrix within the stone, with scattered well-separated dark pellets. Well-preserved medieval stone on eastern elevation of Canterbury Cathedral; sample from inside putlog hole.
Figure 1c
Variety intermediate
between A and B shown above. Modern stone sample from suppliers.
49
50
Tim Palmer
Figure 2
Resin-impregnated sample of pellety Caen Stone that has been cut and
etched in weak hydrochloric acid for 30 seconds. The microporosity in the pellets has been ®lled with resin, and stands out from the surface (medium grey); the solid calcite (light grey) has been etched back. This shows the contact between the adjacent pellets and the interconnected character of the microporosity throughout the stone. Scanning electron micrograph, width of view
2.8 mm.
the other one. This range of variation has been noted in material sampled from buildings in southern and eastern England (particularly medieval and nineteenth-century replacement work in Canterbury Cathedral) and also from extant Caen Stone quarries south of Caen. The skeletal and the pelletal components of the stone have different porosity characteristics. The crinoid fragments and their calcite cement overgrowths have a dense structure and a low porosity, whereas the pellets are compacted mud and are full of micrometre-scale pores. This can be demonstrated by impregnating the stone with epoxy resin under vacuum, and
acid-etching
the
surface
after
hardening.
The
resin
that
®lls
the
micropores in the pellets stands proud of the etched surface as a spongy fabric, whereas the pure calcite of the shelly debris is etched back. This is clearly
shown
on
scanning
electron
micrographs,
which
distinguish the two components of the stone (Figure 2).
thus
clearly
Understanding the Weathering Behaviour of Caen Stone
Decay of external stonework General points The
decay
of
limestone
freestones
in
buildings
is
of
great
economic
importance and has been carefully studied over more than a century (for 5
example see review by Price ). Major destructive factors that have been implicated are the freezing and expansion of absorbed water within the porosity of a stone, and the growth of salts within the pore waters. Salts may be picked up from external sources (e.g. NaCl from salt spray or ground
application)
or
generated
within
the
stone
by
reaction
with
polluted water. Gypsum has been found to be particularly destructive, both because growth of gypsum crystals opens up cracks within the stone, and also because calcium sulphate readily changes hydration states (and consequently volume) with changing atmospheric temperature and humidity. From
the
seventeenth
century,
when
sulphur-rich
coal
started
to
be
imported and burned in southern England in large quantities, up until the Clean Air legislation of the mid-twentieth century, reaction with SO 2 polluted rain and fogs was the major cause of chemical decay in limestone buildings ± particularly in cities. In the absence of atmospheric sulphur, gypsum-driven decay may still take place if iron sulphide is present in a limestone and if it breaks down under moist conditions.
6
Both ice formation and sulphate decay are water driven, and hence they occur more readily if a stone absorbs and holds onto water, which is in turn a consequence of its porosity characteristics. Many empirical studies have shown that limestones with a high connected microporosity (pore diameters typically
