stone in church and cathedral constructionrelated page: |
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franceon first arriving in France - driving Germans
in France France’s western isles: Ile de Ré Marianne - a French national symbol, with French definitive stamps the calendar of the French Revolution the 6th bridge at Rouen: Pont Gustave Flaubert, the French umbrella & Aurillac the forest as seen by francois mauriac, and today after the whirlwind, in les landes the Citroën 2CV: cathedral labyrinths and mazes in France Pic du Midi - observing stars clearly, A64 Carcassonne, A61: world heritage fortified city Marianne - a French national symbol, with French definitive stamps the calendar of the French Revolution mardi gras! carnival in Basque country what a hair cut! m & french pop/rock country life in France: the poultry fair |
compression, stress and strengthWell, 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 stoneHere 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 limestoneLimestone 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.
marbleMarble 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 alabasterAlabaster'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. 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. molasseDuring 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.
volcanic rock
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Seren, 2010, hbk £18.75 [amazon.co.uk]{advert} ISBN-10 : 1854114999 |
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St Margaret's, Bodelwyddan: the Marble Church | |||
St Margaret's, Bodelwyddan, 1997 pbk, 16 pages, illustrated, plan, list of vicars. €2,45 This pamphlet is not recommended to buy. Very poorly written and illustrated. |
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 |
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