stone in church and cathedral construction

related page:
stone tracery in church and cathedral construction

france

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short biography of Pierre Peter) Abelard

compression, stress and strength

Well, I suppose we all know what stone is, it’s that stuff that’s all over the place on this fascinating planet. Well, that’s mostly what I know about it anyway; but I hear tell that there are people who spend their whole lives studying stuff like stones, so I imagine there must be a lot more to it than that. (Maybe eventually, I will find an enthusiastic expert who can write some plain English on the subject for me.)

Just one or two notes relevant to cathedrals. The stone needs to be hard enough to stand up, and to last a thousand or two years, and easy enough for a medieval craftsman to work with rather primitive tools.

Stone is very strong in compression, and only about a tenth of that strength in bending and sheering (tension, stretching), while the medieval mortar is about a tenth of that sheering/bending strength.

Notice the greyed stones in the arches of diagrams A and B. On the arches, you will see a small triangular diagram. If nothing were holding the shaded stone place, it would fall to the earth; but locked in place, weight (force) is coming down on the stone (the stone, of course, also has some weight). That force is directed mainly along the stone.

As you will see, the stone in the pointed arch [A] is at a more vertical angle than the stone in the Roman (rounded) arch [B]. This means that a greater proportion of the load on the stone is transmitted downward and less outward than is the case with the rounded arch. The differing forces are represented by the differing lengths of the sides of the triangle, and these can be calculated. Be aware that in general the wider the span, the greater the forces.

The greater outward force with rounded arches explains why earlier buildings, before the gothic, required thick, heavy walls to stop the pressures bursting them outwards. The more efficient, pointed arch led to much lighter and more open structures.

Those nice vaults under which you are standing can be estimated to weigh towards 300,000 kilograms for the quadripartite vault, and towards 400,000 kilograms for the sexpartite [2] - a nice headache for you if the medieval craftsmen were slacking on the job. Note that the sexpartite tends to cover the equivalent of two quadripartite vaults, thus making the overall weight of sexpartite vaults less and thereby reducing the stresses throughout the structure. (Bourges can be thought of as a very special, high-tech cathedral.)

Moving to diagram C above, you will note that, at the overhang there are forces inclining the stone to bend downwards. Tensile forces are generated at x and compressive forces are generated at y, and thus the pressure is to pull the stone apart at x and tear it apart. In the arch stones, similar forces are being applied. Note that, were the stone at C to be made of butter, it may tend to sheer rather than bend (see also en délit).

building stone

Here discussed are some of the stones used for building churches and cathedrals. For the most part, the stone is quarried, or even mined locally, but when its qualities enable greater ability in shaping and carving, stone is transported to other regions, and even other countries, as in the case of Caen stone

limestone

Limestone is a common sedimentary rock, found all over the world - about 10% of all sedimentary rock is limestone. It is mainly made of skeletal fragments of marine organisms such as molluscs, forams and corals. Its composition is mostly of calcite and aragonite, different crystal forms of calcium carbonate (CaCO₃), and is generally light-coloured. Because it is laid down as sediment, limestone has a generally horizontal grain. When the quarried and shaped stone is laid in a building or wall with its grain parallel to the ground, the stone is termed as being au lit (in the bed), as it is very strong in compression. When the grain is posed (arranged) to be vertical, it is described as being en délit, and is much weaker and liable to shatter.

Limestone comes in different densities, and so different hardnesses and weights. The hardest limestone was used for the structure of the building : towers, buttressing, outer walls and pillars that will hold up the roof. Dense but fine-grained limestone that can be carved in great detail was used for the many statues. Softer and more porous limestone, and therefore light but strong, was used for the wide expanses of vaulting that provides both a decorative ceiling and protection from the elements to the nave below. Limestone's structure is moderated by the way in which it is laid down. The deeper the limestone bed, the more pressure it comes under, thus making it denser and heavier.

A limestone's geographic location can be determined by microanalysis of which microfossils it contains : fossils of plankton, such as orbitolites complanatus, and other very small organisms.

As usual, definitions are never black and white. Limestone is generally accepted to have at least 50% (marine) animal bones or skeletal fragments. Chalk, as used for marking out hopscotch or by teachers at blackboards, approaches 100% skeletal fragments. So you're writing with other people's no longer required bones! I am told that Beavais cathedral is built of stone that is nearer to chalk, which does not help its stability.

When limestone metamorphoses, changing state under the influence of heat and/or pressure, it becomes marble.

Limestone is very common in architecture. Readily available and relatively easy to cut, limestone can be long lasting unless subject to modern pollutions, the chemicals in limestone can be readily dissolved by acidic solutions and water (acid rain).

The Ancient Egyptians used limestone to clad the Great Pyramid of Giza, a wonder of the ancient world. More recently, many medieval churches and castles in Europe are made of limestone. In France, the best limestone was and is Caen stone (pierre de Caen). It is a light creamy-yellow Jurassic limestone quarried near the city of Caen in north-western France. This fine grained oolitic limestone is homogeneous, and so suitable for carving.

Caen stone had been first used for building in the Gallo-Roman period. Production restarted from open cast quarries in the 11th century. This stone was shipped to England by the Normans using it to build parts of Canterbury Cathedral, Westminster Abbey and the Tower of London. More recently, Caen stone was used for parts of the clock tower at the Houses of Parliament, commonly called Big Ben. In recent times, Caen stone has also been exported to America and the Middle East, but since 2004 Caen city council has limited its use and export by only permitting 9,000 tonnes of stone a year.

Portland stone is a similar fine quality limestone found in southern England used there for prestigious buildings such as the Tower of London, St. Paul's Cathedral, the British Museum, Port of Liverpool Building, and many others. The ability to carve limestone was developed to a very high level in medieval France, as illustrated by the development of tracery, of tympani above the west door of most cathedrals, and much else in the cathedral's statuary.

Stone tracery on stained glass windows at Coutances cathedral. [image: placeandsee.com] Similar tracery can be seen in thousands of churches, often as decoration outside the context of windows. See also stone tracery in church and cathedral construction.

Above: Bazas cathedral west portal, including a tympanum at its centre. Right: The Smile of Reims, cathédrale de Reims

marble

Marble usually refers to limestone that has undergone some metamorphic process. Marble comes with many colour streaks and inclusions, and has become texturally more consistent than limestone. Thus, marble is easier to work for statuary, and for achieving smooth surfaces and other such decoration.

Right: Marble pillars at Rion-des-Landes church, Nouvelle Aquitaine

alabaster

Alabaster's name is of ancient Greek origin. It means “a vessel without handles”, because in ancient times, the most frequent use of alabaster was to make perfume vessels without handles. These were called alabastron.

Alabaster is a soft rock with a Mohs classification of 1.5, used for carvings and as a source of plaster powder. Alabaster is so soft that it can be scratched by fingernails and is easily dissolved by water.
There are different definitions and usages for the word alabaster, depending on which expert - archaeologist, geologist, stone industry - is using the word. 

Often a compact fine-textured usually white and translucent gypsum, alabaster is used to carve into statuary, vases and ornaments. It is a 'hard' calcite or aragonite that is translucent and sometimes banded.

Normally snow-white and translucent, alabaster can be artificially dyed, or be made opaque by heat treatment and so of similar appearance to marble. In Italy, Florence, Livorno, and Milan are important centres of the alabaster trade, as is Berlin in Germany. The alabaster used by in ancient times was a brown or yellow onyx marble.

molasse

During the Mid-Miocene epoch, between 11 and 16 million years ago, the Atlantic Ocean washed into the Aquitanian Basin, at least as far as what is now Bas-Armagnac. The sea laid down continental deposits that are now grouped under the general term of “Fawn-coloured sands”. These include molasse, sandstones, shales, or even gravel, and were laid down as shore or foreland layers generally containing fossils of many terrestrial species [see to right].

Bas-Armagnac soil is composed of clay-silicate layers, covered by ochre sands and a fine clay now used for making ceramics. From these deposits are produced elegant brandies with delicate bouquets, particularly with nuances of prune, called armagnacs.

galuche (or galoche)

There is another rock that was found in some parts of Les Landes - galuche (or galoche), a dark lumpy rock that contains large quantities of iron ore. This gives the rock a dark red hue. Because of the iron content, galuche was often quarried and smelted as part of one of Less Landes' former industries. The stone has now been worked out and the forges are now silent, although some towns still hold a memento of this industrial period in their names : Pontenx-Les-Forges, the Les Forges Restaurant at Castets. Another reminder is the use of this brown-red stone for building. Houses and churches have dark stone included in their walls, and sometimes with churches, they are built entirely in galuche.

Now galuche is easily eroded by rain and, in the past, was commonly known as "the wrong stone". This erosion is aggravated by rusting, due to the stone's iron content. Because this stone is so friable, most frequently galuche buildings are hidden in a drab overcoat of concrete or rendering, known as crepi in French, or a heavy lime plaster (chaume). Fortunately, in one town - Mezos, as part of restoring their heritage, the local worthies had the protective disguise removed to reveal an unusual but magnificent church.

Mezos church Right: Close-up of part of the galuche (and mortar) wall at Mezos church

volcanic rock
(agde cathedral of saint étienne)

As at Clermont Ferrand, Agde is distinguished by the dark grey, almost black volcanic rock used in many of its buildings. This rock comes from Mont Saint-Loup, a nearby extinct volcano. The Cathédrale Saint-Étienne d'Agde is a 12th-century fortified church, complete with machicolations, built on the site of a Phoenician temple.

The first cathedral, hardly completed, in the 8th century was rased to the ground by Charles Martel. He did this, as in other towns such as Beziers and Nimes, to prevent the Saracens, whom he had newly ousted, from using the building as "a den". His son, Pepin le Bref, finally conquered the region in the 750s.

The next, Romanesque, cathedral, finished in about 1220, was built by the bishop of Agde who was also its count, its temporal lord. This was common in towns of the Midi at that time. The cathedral's fortified nature was preparation against invasion from the nearby Mediterranean Sea.

The Wars of Religion in the 16th century were a source of damage as the Calvinists burnt sacred relics, and similar profanities occurred during the French Revolution. Then the cathedral became a 'stable' for pigs, while the last bishop of Agde died on the scaffold in Paris, refusing to take the Republican oath and "dishonour his old age".

Agde cathedral east facade.

sarsens - silification, petrification, etc.

Things become a bit complicated now as long-term processes of heat, pressure and submergence in water change the nature of materials. sarsen stones are well known from their use in the Stonehenge monument. These stones are sandstones that have become silificated, that is glued together under anaerobic and wet conditions. Various other materials besides silicates can be involved in these processes. Unlike crumbly sandstone, sarscen stones are very hard, maybe Mohs 7. These processes can occur in limestone and other materials, leading to all the wondrous differences we commonly call mud, earth and rock. Petrified wood, various crystal forms called jewellery, nodules of flint, fossils, agates and so on, these humans use for their delight and decoration.

the variety of stone in cathedrals

Llandaff Cathedral has a long and chequered history, an advantage with buildings that have gone through hard times is that you see views of their innards and bone structures. Llandaff is the cathedral the second-most damaged by Germany during the Second World War after Coventry Cathedral.

For centuries, Llandaff was in a poor area, and was despoiled by Henry VIII, Cromwell's lot, greedy pastors, and sheer poverty. The tide turned with the industrial revolution and the discovery of coal in the valleys. Since when, with the interruption of German vandalism, the cathedral has been on the up. This roller-coaster of history, the cathedral is now a hodgepodge of variety, as shown in the paragraph below. This variety is not uncommon, as the accretions of history for like moss upon a rock.

"Like the entire west front [of Llandaff Cathedral], the bishop is carved from the distinctive creamy-yellow Dundry stone from near Bristol. Many different types of stones were used in the building of Landfall, including some materials not to be found elsewhere in Cardiff. The early Norman parts of the Cathedral use Sutton stone (from Ogmore and Southendon) or Radyre stone with river pebbles. Later the greyish lias limestone from southern Glamorgan was introduced. The nineteenth century reconstruction used Bath stone, oolite from Chipping Camden in Gloucestershire, stone from Wiltshire, Radyr, and the Forest of Dean as well as some Caen stone from Normandy. The upper part of the nineteenth century spire is of a yellowish stone from Rutland. In the David Chapel, Doulting stone from Shepton Mallet was used with river pebbles from the Taff and recycled stone from the demolished cottages in Llandaff. Portland stone was used for the foundation of this chapel and for the panels of inscribed battle honours on the east wall." [Lambert, pp.117-8]

Another church built with a wide variety of stones, in this case marbles, is St Margaret's Church, Bodelwyddan, Gwynedd in North Wales, and nicknamed the Marble Church.

The church has fourteen varieties of marble, including

• red Griotte (sometimes known as Cannes marble) - main chancel piers [dominant interior marble].
• Belgian Red - nave arcades [dominant interior marble].
• Anglesey marble - paired columns in double archway to the nave.
• The church is paved in Sicilian marble (mostly hidden by carpeting).
• green marble shafts - helping support roof trusses.
• bands and inserts of black Kilkenny marble (contrasting with red Griotte) - chancel roof.
• Purbeck marble - base for Languedoc marble shafts.
• Languedoc marble shafts - supporting canopy over altar reredos.
• Povey marble - chancel wall canopies.
• Carrara marble - font.

cosmati mozaic

Cosmati is a form of mozaic flooring (pavement) found in some English cathedrals, such as Westminster Abbey and Canterbury cathedral. It was developed in the 12th and 13th centuries by seven generations of the Cosmati family, a Roman family of architects, craftsmen, and mosaic workers. Cosmati work was fashionable in the 12th and 13th centuries, being used to decorate tombs, pavements, and furniture in churches throughout Italy. The style was used at Westminster Abbey for the flooring on which the royal coronation takes place, and in Canterbury cathedral to mark the location of significant events - the death of Thomas à Becket and Edwar the Confessor.

The original palette of materials employed for both the Romanesque tesserae and the slabs of opus sectile comprised purple porphyry from Egypt, green porphyry from Greece, white Carrara marble from Italy, black Tournai marble from Belgium, and pale grey Thassos marble from Greece. Most unusually, many of the mosaic panels are individually edged with thin strips of latten (brass): some are 19th-century replacements, but others are certainly primary. All are now tarnished and obscured by floor-polish but, like medieval funerary brasses, these fillets would originally have been gilded. The narrow bands of mosaic in the borders were also subtly elaborated by combining latten fillets of three different thicknesses.
[From the-past.com]

Cosmati pavement at Canterbury cathedral
in front of the shrine of St Thomas in the Trinity Chapel
c.1182-1184

At Westminster Abbey, the Cosmati pavement was commissioned by Henry III, and completed in 1268 as an intricate mosaic floor of marble, stone, glass and metal in front of the High Altar. The pavement was hidden under carpet and thus away from public view from the 1870s until a two-year programme of conservation work was completed by the Abbey in 2010. This included coatings of microcrystalline wax. Now but occaisonally, and on paying £15, visitors are allowed to walk on it in their socks (shoes abrade, and the moisture of bare feet is acidic and so damaging).

Cosmati pavement at Westminster Abbey, 1268


Part of the restored Cosmati pavement, Westminister Abbey

bibliography

Llandaff cathedral edited by Nick Lambert
	‎‎ Seren, 2010, hbk £18.75 [amazon.co.uk]{advert} $39.95 [amazon.com] {advert} ISBN-10 ‏ : ‎ 1854114999 ISBN-13 ‏ : ‎ 978-1854114990
St Margaret's, Bodelwyddan: the Marble Church
	St Margaret's, Bodelwyddan, 1997 Jarrold Publishing  pbk, 16 pages, illustrated, plan, list of vicars. €2,45 This pamphlet is not recommended to buy. Very poorly written and illustrated.

end notes

1. There are may types of limestone, how many depending on the definition of type and how niggly are the classifiers, usually geologists and other earth scientists.
   Oolic limestone takes its name from the calcium carbonate "oolites" of which it is mainly composed. Oolites are small spheres formed by the concentric precipitation of calcium carbonate on a sand grain or shell fragment. 
   Other broad types of limestones include fossiliferous, coquina, and chalk.
   
   
2.  Listed Buildings - Full Report - Heritage Bill Cadw Assets - Reports
   Description of a Listed Building
   
   
3. Calcium carbonate forms as both aragonite and calcite. The two minerals only differ in their crystallisation.
   Calcite, the more common mineral, forms in trigonal crystals having three angles or corners, triangular, like a Tobelerone chocolate bar.
   Aragonite forms orthorhombic crystals having three unequal axes at right angles to each other, like a rectangular box.
   
   
4. Mohs scale of mineral hardness

   The Mohs scale of hardness is measured on the basis that a harder material will visibly scratch or mark a softer one.
   
   

   Mohs hardness	mineral	absolute hardness
   1	talc	1
   2	gypsum	3
   3	calcite	9
   4	fluorite	21
   5	apatite	48
   6	orthoclase feldspar	72
   7	quartz	100
   8	topaz	200
   9	corundum	400
   10	diamond	1500

   
   The Mohs scale is an ordinal scale, absolute (real) hardness is measured with a sclerometer.
   
   The Knoops hardness test is a microhardness test, achieved by pressing a pyramidal diamond point in to the previously polished test material. One of the advantages of the Knoop test is only a very small piece of test material is used. The disadvantages include having to use a microscope (the indentation made is measured to an accuracy of 0.5 miccrometre) and the time required to prepare the sample and to make the indentation.

cathedrals – introduction: reading stained glass
gothic cathedral and church construction
cathedrals, an illustrated glossary
Chartres - wonder of the world
Notre Dame de Paris, Paris
lantern towers of Normandy and elsewhere
history of ugly stained glass: Auch, Bazas, Dreux
Auch cathedral choir and stalls
Rouen and Monet
Dax and church iconography photographs, Dax
Bazas - iconography and architectural styles
Poitiers, neglected masterpiece photographs, Poitiers / photos 2
Angers, heart of the Angevin Empire photographs, Angers
Laon, the midst of the gothic transition, with added oxen photographs, Laon
Saint-Jean-Baptiste de Lyon
Notre Dame of Lausanne
Senlis - how a typical cathedral changes through the ages
Saint-Bertrand-de-Comminges - the cathedral of the Pyrenees
Cathedrale Saint-Gatien at Tours

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© abelard, 2017, 15 october


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