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A rock in a hard place

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In the second article in our monthly series examining materials in their historic settings, we explore the varieties of stone and the use of stone in a restoration case study

Stone is, of course, no single entity, and no geological museum can ever fully portray its diversity, because it ranges in colour, texture, age, density, matrix, hardness, and permeability throughout the British Isles. The hardest and oldest ingeous stones are in the west (granites in Cornwall and Devon, Cumbria, Wales and Highland Scotland), which usually coexist happily with deposits of slate (metamorphosed clay). Millstone grits and tough sandstones follow. Then come the old and new red sandstone strata, which are not always as ruddy as their names suggest, but these oxidised quartz and silica-bound rocks, formed in a dry desert atmosphere, provide the iron-rich hues of buildings in Hereford, Worcester, Shrewsbury and Chester.

The oolitic limestone belt lies roughly central, after which the youngest sweeps of chalks, occasional greensands and rags, and clays fall away to East Anglia, Sussex and Kent.

The stones of the oolitic (from the Greek for 'fish roe') limestone belt form the bedrock of the rustic living, stretching from Somerset and Dorset through the Cotswolds and Northamptonshire to Rutland, Stamford and Lincoln. These rocks are nearly all calcium carbonate and will fizz in contact with acids, whereas sandstone will not.

The oolites are still much-loved by lichens: Xanthoria Parietina are the splats of deep yellow that encrust rough limestone walls and roofs. Building stones can support plants, insect life, bees and - not least - people who live in stone houses. Lifeless, they are not.

For a focused example, let's take a 10-mile radius drawn around Stamford (Stone ford) in Lincolnshire. The eastern half of the area is fenland, set on brickearth and clay, while the western half is in the Lincolnshire oolites, which form the frequently sudden fen edge. Within this area of rolling pasture and arable land we find Ketton stone (like microfine polystyrene balls, in gold or pink, with rougher beds now quarried for road building and cement). Stamford itself depended on the nearby Casterton quarries, while its roofs form a plateau of Collyweston slate (sandy, dark suede colour; a compact stone that was formed into roofing slates by natural freeze-thaw action as homogeneous as Italian pantiles. Three miles east lie the exhausted quarries of Barnack rag (cream colour, very shelly, but worked out through building the Fenland abbeys and cathedrals); eight miles to the north-west is Clipsham (a fossil-rich, rough and hard golden stone with blue streaks found suitable for the polluted air of London and Oxford). Ten miles southwest we find the more open-grained King's Cliffe and Weldon stones that were chosen to complete King's College Chapel, Cambridge. Stamford's best flagstones were of 'Suties marble' limestone from the Grimsthorpe area of Lincolnshire, rather than Cambridgeshire's 'Alwalton marble', a shelly, dense base-bed stone akin to Purbeck 'marble' but packed with ancient brown oyster shells. Eight-hundred years ago Alwalton marble was turned into shafts and buffed to a shine with goose fat for the great projects of Lincoln Cathedral and Peterborough Abbey, the latter of which owned the quarries.

This small area presents an immense range of beautiful stones.

There are many other abandoned and unexploited quarries that explain why Northamptonshire became the land of 'Spires and Squires', not least because of the technical advantages that strong, fine stone can offer. As a result, some of the region's spires are among the earliest and most influential in England.

Uses of building stone The basic distinction in any historic building stones is - as the Anglo-Saxons put it - 'walstan' (wall stone) or rubble for plain wall masonry and 'werkstan' (work stone) or freestone, for carving.

Stamford is a fortunate area for werkstan, but not all stone is good building stone. It still beggars belief that the early Tudor builders of Launceston church in Cornwall were able to chip Bodmin Moor granite into anything resembling blind tracery panels, let alone the ambitious sculpture of Mary Magdalene beneath the east window. Durham Cathedral is built of a coal-measures sandstone, which is as rich as raspberry ripple for interiors but externally lousy, its marbling spalling and opening up into chasms. In the 18th century it was thought that carving seven inches off the entire cathedral was preferable to piecing in replacement blocks and, as a result, most of the Romanesque exterior detail was recut. The cathedral otherwise survived the episode but, of course, the problem lives on.

Transporting good stone overland usually tripled its initial cost at the least, so in many cases, putting up with the local material was the only option. Where good stone was impossible to find for major projects, such as in London, importation by water was widespread from as far away as Normandy (Caen, Bernay), Isle of Wight (Quarr), Dorset (Portland, Purbeck), Kent (Kentish Rag from Maidstone) Devon (Beer) and Yorkshire (Huddlestone). The question of how much medieval carving and turning took place at the quarries prior to transportation, or on site, remains a point of debate. Once construction began, large blocks were usually laid on wet mortar, spread over oyster shells so that the shells took the weight of the block while the mortar set around them.

Restoration is a curiously modern concept - whereas recycling is a venerably old one. Where derelict stone structures could be cannibalised for more practical ends, they usually were, and many times over. But identifying buildings of reused rubble or flint is usually impossible because of the lack of diagnostic features, while tell-tale carved freestone was often faced into the wall. Take one example among thousands: circa 1070, the limestone blocks and bricks of the ancient Roman fort of Caerleon in Monmouthshire were reassembled upriver in the shape of Chepstow Castle, which itself fell into ruin 600 years later and was heavily robbed for a variety of purposes.

The fact that nobody today would suggest reconstructing, say, Fountains Abbey as something more useful is a quirk of fate.

Conservation is de rigeur: because of this and, somewhat paradoxically, an apparently broader public acceptance of the potential quality of new stone buildings (especially those in the fashionably neutral tones of bleached lime and limestone), stone quarrying and supply is on the up.

Stone Federation Great Britain can be found at www. stone-federationgb. org. uk or telephone 01303 856123

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