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stone in church and cathedral construction

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index
stone, the cathedral's core material
compression, stress and strength
building stone
limestone
molasse
galuche
volcanic rock
end notes

stone, the cathedral's core material

Church buildings are generally made of stone, the solid and usually stable building material.

Stone is strong in compression, but if it has a grain, this strength is compromised if the weight of the building presses down against the grain of the stone, so creating stress. This is particularly significant when using limestone.

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.

compression forces on Gothic and Roman arches.

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.

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, 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.

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 been 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 Coutance cathedral

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. A page on tracery is in preparation at abelard.org.

Bazas cathedral, west portal

Above: Bazas cathedral west portal, including a tympanum at its centre.

Right: The Smile of Reims, cathédrale de Reimsl

The Smile of Reims, on the right, before and after the bombing and fire on 19th September 1914

délit

This word was originally written as de lit, but the two words have elided over time, acquiring an acute accent. De lit means 'out of bed', in this context not lying parallel to how the sedimentary strata were laid. In architecture, this term refers to the cutting and placing of a stone for construction. Délit contrasts with au lit, 'in the bed'. Traditionally, the stone is cut and laid down in accord with the geological strata of the quarry bed of which the rock is constituted, which strata are substantially horizontal. Laid this way, the stone resists the pressure of the loads that are exerted on it.

In contrast, la pose en délit (posing or placing against the grain) consists in placing the stone so the strata (bed) are vertical. Placing en délit causes a risk of vertical cracking or disintegration. This is why it is prohibited (in France) when using "laminated" materials such as certain limestones and shales; vertical placing/en délit being more suitable for homogeneous materials. However, construction en délit was much used by the Romans, and later by the builders of the flamboyant Gothic cathedrals - for aesthetic reasons (the visual smoothness of the stone, achieving great heights without joints) and, in fact, as demonstrations of technical virtuosity.

Although stone cut en délit is prone to shattering under compression, by cutting en délit, much longer pillars, tracery and other slender structures may be achieved. Compressive shattering can be mostly avoided by ensuring that the stone concerned is not significantly load-bearing.

The architectural use of the word délit is pretty obscure. Nowadays, the word délit in France is much more commonly encountered as meaning a minor crime. The three levels of crime are contravention, délit and crime. A contravention is the least serious offence: irregular parking or light violence, for example. Next is a délit: theft, abuse of public property, discrimination, moral harassment, sexual touching, manslaughter ... Crime is the most serious offence: murder, rape ...

These two usages of délit are not etymologically related in any way. Délit, a minor crime, comes from the same root as delinquent and delinquency.

molasse

Sample of molasse rocks amongst pine needles.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". 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.

church at Mezos
Mezos church

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

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

volcanic rock
(agde cathedral of saint étienne)

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 west facade
Agde cathedral east facade.

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.


 

 

 


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

marker Le Mans and Bourges cathedrals - medieval space technology
marker Lausanne rose window - photo-analysis
marker cathedrals in Lorraine - the Three Bishoprics
marker cathedral giants - Amiens and Beauvais
marker Clermont-Ferrand and Agde - from volcanoes to cathedrals

marker Germans in France - Arras cathedral
marker Germans in France - Reims cathedral
marker Germans in France - St. Quentin cathedral
marker Germans in France - Noyon cathedral
marker Germans in France - Cambrai cathedral
marker Germans in France - Soissons cathedral

marker cathedral plans, and facts
marker stone in church and cathedral construction
marker using metal in gothic cathedral construction
marker cathedral labyrinths and mazes in France
marker cathedrals and cloisters of France by Elise Whitlock Rose

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