Portland stone formed in a marine environment, on the floor of a shallow, warm, sub-tropical sea probably near land (as evidenced by fossilised driftwood, which is not uncommon). When seawater is warmed by the sun, its capacity to hold dissolved gas is reduced; consequently, dissolved carbon dioxide (CO2) is released into the atmosphere as a gas. Calcium and bicarbonate ions within the water are then able to combine, to form calcium carbonate (CaCO3) as a precipitate. The process of limescale build up in a kettle in hard-water areas is similar. Calcium carbonate is the principal constituent of most limestones. Billions of minute crystals of precipitated calcium carbonate (called calcite) accumulated forming lime mud (called micrite) which covered the sea floor. Small particles of sand or organic detritus, such as shell fragments, formed a nucleus, which became coated with layers of calcite as they were rolled around in the muddy micrite.[2] Portland stone measures 3.5 on the Mohs scale of mineral hardness.[3]
The calcite gradually accumulated (by accretion) around the fragments of shell in concentric layers, forming small balls (of less than 0.5mm diameter). This process is similar to the way in which a snowball grows in size as it is rolled around in the snow. Over time, countless billions of these balls, known as "ooids" or "ooliths" (from the Greek for "egg-shaped" or "egg-stone"), became partially cemented together (or lithified) by more calcite, to form the oolitic limestone that is called Portland stone. The degree of cementation in Portland stone is such that the stone is sufficiently well cemented to allow it to resist weathering, but not so well cemented that it cannot be readily worked (cut and carved) by masons. This is one of the reasons why Portland stone is so favoured as a monumental and architectural stone. Geoff Townson conducted three years doctoral research on the Portlandian, being the first to describe the patch-reef facies and Dorset-wide sedimentation details. Ian West of the School of Ocean and Earth Sciences at Southampton University completed a detailed geological survey of Withies Croft Quarry before the Portland Beds were quarried by Albion Stone plc.[2]
History of use
Stone has been quarried on Portland since Roman times and was being shipped to London in the 14th century.[4] Extraction as an industry began in the early 17th century, with shipments to London for Inigo Jones' Banqueting House. Wren's choice of Portland for the new St Paul's Cathedral was a great boost for the quarries and established Portland as London's choice of building stone. The island was connected by railway to the rest of the country from 1865. Albion Stone PLC has been quarrying and mining Portland stone since 1984. Portland Stone Firms Ltd have been quarrying Portland stone since 1994.
The tramway incline from the Admiralty Quarry seen from Castletown, Portland
Jordans is part of the Inmosthay Quarry in the centre of the island, which also includes Fancy Beach. The quarry has been worked since the late 19th century. Albion Stone leases the southern section from The Crown Estate and purchased the northern part of the site in 2006. The majority of the southern reserves lie under the grounds of the local cricket club. To avoid disturbing the site at surface level, the company has applied and received permission to extract the stone using mining rather than quarrying techniques. The reserves to the north will be quarried using the diamond bladed cutting machines, hydro bags and wire saws to shape the blocks. This process avoids the use of dusty and noisy blasting as the primary extraction method, thereby protecting the surrounding environment, which has been designated as a Site of Special Scientific Interest (SSSI). Albion Stone PLC now extract all their stone through mining which dramatically reduces the impact on the environment and local residents. Jordan's Mine is currently the biggest mine on Portland.
Bowers Quarry has been operational since the late 18th century. It has been leased from The Crown Estate since 1979, and in 2002 it became the site of the first Portland stone mine by Albion Stone PLC. Extraction from this site is now completely underground, with the original Bowers Mine in the extreme southern end of the quarry and the High Wall Extraction on the eastern and south east boundaries. High Wall Extraction is a series of small mines that extract otherwise wasted stone that sits between the final faces of the quarry and the actual boundary of the site.
Stonehills Mine is the first completely new mine on Portland (not a mine as an extension from an existing quarry). Albion Stone Plc began the process to open this mine in 2015 and reserves are estimated to last for 50 years (2066).
Independent Quarry and Admiralty Quarry are both owned by The Crown Estate and have been leased since 1982.[5] These quarries have been worked since the mid-19th century with the final dimension stones coming out of Independent in 2006, although some stocks of Portland stone block still remain.
Coombefield Quarry, located near Southwell has been open cast quarried over the last 80 years and is one of three privately owned quarries by Portland Stone Firms Ltd, the largest landholder on the island. The quarry is nearing the end of its life and will be regenerated as a holiday caravan park to boost local tourism on the island.
Perryfield Quarry is found towards the middle of the island and being actively open cast quarried. There are over 20 years of reserves left which is privately owned by Portland Stone Firms. Open cast quarrying provides quicker extraction of raw block dimension stone whilst maintaining its integrity. The majority of buildings in London today use Portland which has been quarried using the same methods over the last 60 years.
Broadcroft Quarry is located on the eastern side of the island and is a part of the open cast quarries used for St Paul's Cathedral. Privately owned by Portland Stone Firms Limited there are over 20 years of reserves left and still being actively quarried.
The coastal strip toward the southern tip of the island has permission to be worked by Portland Stone Firms and will provide in excess of 30 years of reserves. Permission has been granted for this to be open cast quarried.
Once quarries have been worked they are then restored. The Portland Sculpture and Quarry Trust was formed in 1983 and is dedicated to preserving a knowledge and understanding of stone and the landscape from which it comes.[5] The main non-working quarry remains Tout Quarry in which the trust is based, where a workshop is held every year. Tout Quarry has been donated to the Portland Sculpture and Quarry Trust by Portland Stone Firms Limited.
Quarrying methods
Traditionally small diameter holes (35mm) were drilled horizontally under each rock and charged with a small quantity of gunpowder, chosen because of its relative non-shattering properties. When fired the gunpowder produced a "heave" which dislodged the rock from its natural bed, hopefully undamaged. Stone was cut using plugs and feathers, where a series of short, small diameter (typically 30mm) holes are drilled in a line where a cut is to be made. One plug and two feathers were inserted into each hole and each plug is hit in turn with a sledgehammer until the stone yields to the extreme tensile stresses produced. Most stone is many times weaker in tension than in compression, plugs and feathers utilise this fact. It is also worth noting that stone tends to split much more easily parallel to bedding planes (called graining) than perpendicular to them (called cutting).
In 1999, Italian stone cutting equipment, originally designed for use in Tuscany's marble quarries was imported by Albion Stone and applied to the extraction of Portland stone. This new technology eliminated the need for blasting, significantly improved the quarries' environmental performance and removed the potential for any possible damage to the stone being quarried through shock. Full account of the local jointing pattern is made when deciding the position and orientation of cuts. Once the quarry faces have been cut, the stone is gently displaced hydraulically. This is done using "hydro-bags", which are thin, flat, steel bags or envelopes that when inflated with water under moderate pressure, are capable of producing the forces necessary to loosen the stone to the point where it can be easily removed using large wheeled loaders. Splitting stone, using pneumatic drills is arduous work and so wire-saws have been introduced into the quarries, replacing much of the plug and feather cutting.
Mining in Portland is done by using a room and pillar method. The mine is advanced by extracting the stone using an abrasive tool chain cutter mounted on a machine. Slots are cut into the top, bottom, sides and middle of the stone. A flat steel pillow is inserted into the middle cut and slowly inflated with water. The stones are gently broken off at the back without placing any stress on the resulting blocks. This method of extraction is significantly more expensive than blasting, but results in a higher yield thereby saving valuable reserves for future generations. The environmental benefits are substantial as mining significantly reduces the impact on wildlife and the local community with reduced noise and dust.
Portland's freestone has almost certainly been used as a building material since Roman times. The many well crafted Roman sarcophagi (stone coffins and matching lids, hewn from single large blocks of Portland stone) that have been unearthed locally over the years, testify to the skill of their makers.
The earliest known building to be constructed using Portland stone is Rufus Castle at Church Ope Cove, Portland. The original structure was probably built around 1080, rebuilt around 1259 and rebuilt yet again around 1450, which is the likely date of the walls seen today. The first known Portland stone quarries were situated on the northeastern coast of the Isle, close to Rufus Castle, where huge landslips made the stone more easily accessible, and the proximity of the sea allowed the quarried stone blocks to be moved over relatively large distances by barge.
Portland stone was used to build the Palace of Westminster in 1347, the Tower of London in 1349 and the first stone London Bridge in 1350. Exeter Cathedral and Christchurch Priory, also constructed during the 14th Century, are built of Portland stone. Its superb characteristics have ensured a popularity amongst masons and architects that has endured ever since. The East side of Buckingham Palace, the official London residence of King Charles III, including the balcony, was faced with Portland stone, first in 1854 and again in 1913. The Victoria Memorial (unveiled 1911) is also made of it.
All of the stone used by Wren was transported by sailing barge from Portland to the centre of London via the sea and then up the Thames. Wren's widespread use of Portland stone firmly established it as London's "local stone" and as one of the best-loved British building stones. Other famous London buildings constructed of Portland stone are The British Museum (1753) with the new WCEC extension in Portland Roach which was short-listed for the Stirling Prize in 2017, Somerset House (1792), the General Post Office (1829), the Bank of England, the Mansion House and the National Gallery. Tower Bridge is partly clad in Portland stone (along with Cornish granite). Portland stone was used in 1923 to build the supporting pillar of the Grace Gates at Lord's Cricket Ground.
The Nottingham Council House, completed in 1929, is also built with Portland stone, as are the public buildings in Cardiff's civic centre. Architect Charles Holden significantly used the stone in his major commissions of the 1920s and 1930s, including Senate House and 55 Broadway, the home of the London Underground. The 1929 steel framed building (one of the first erected in the United Kingdom) of the London School of Hygiene & Tropical Medicine, based in Keppel Street, has a Portland stone façade to complete its stripped Classical design.[8] After the Second World War (1939–1945), the bombed out centres of many English towns and cities, such as Plymouth, Bristol, Coventry and London were reconstructed using vast facades of Portland stone.
Oxford typically uses oolitic limestone in its buildings, and the Ashmolean Museum has been refurbished using a large amount of Portland stone.
Portland stone has also been used across the world. Examples include the UN building in New York, the Casino Kursaal in Belgium and the Auckland War Memorial Museum.[9] Most of the important civic and administrative buildings which survive from 18th and 19th century Dublin, Ireland, known then as "the second city of the Empire", are of Portland stone, including City Hall (1779), the Houses of Parliament (1767), the Custom House (1791), the National Gallery (1864) and the General Post Office (1818). More recent projects include the BBCBroadcasting House in London, which won the "New Build (Modern Non-Load-bearing Stone) Award" in the 2006 Natural Stone Awards.
Following the First World War, Sir Edwin Lutyens used Portland stone (quarried from the bottom of Wakeham) to construct the Cenotaph in London's Whitehall. Erected in 1920, The Cenotaph commemorates the millions of people killed in this and subsequent conflicts. The RAF Bomber Command Memorial in London's Green Park commemorates the 55,573 crew members of the RAF's Bomber Command who were killed between 1939 and 1945.
Portland stone was used for the Armed Forces Memorial in Staffordshire, England. Designed by Liam O'Connor Architects and Planning Consultants, it was completed in 2007 at a cost of over £6 million and bears the names of over 16,000 service personnel of the British Armed Forces killed since the Second World War.[12]
Substitution
Portland stone is recognised to be of a high quality, but is somewhat expensive. In the case of the refurbishment of the British Museum's central court, the planned Portland stone was controversially substituted with Anstrude Roche Claire stone from France, another similar (but cheaper) oolitic limestone.[13]
The term, "Portland cement", was coined by Joseph Aspdin who in 1824 patented a hydraulic binder created by burning a mixture of limestone and clay, resembling the previously existing Roman cement and presenting a material which he hoped looked like the famous Portland building stone.[14]
Limestone is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of CaCO3. Limestone forms when these minerals precipitate out of water containing dissolved calcium. This can take place through both biological and nonbiological processes, though biological processes, such as the accumulation of corals and shells in the sea, have likely been more important for the last 540 million years. Limestone often contains fossils which provide scientists with information on ancient environments and on the evolution of life.
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratch hardness comparison. Large calcite crystals are used in optical equipment, and limestone composed mostly of calcite has numerous uses.
Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. Chalk is common throughout Western Europe, where deposits underlie parts of France, and steep cliffs are often seen where they meet the sea in places such as the Dover cliffs on the Kent coast of the English Channel.
Oolite or oölite is a sedimentary rock formed from ooids, spherical grains composed of concentric layers. Strictly, oolites consist of ooids of diameter 0.25–2 millimetres; rocks composed of ooids larger than 2 mm are called pisolites. The term oolith can refer to oolite or individual ooids.
Clunch is a traditional building material of chalky limestone rock used mainly in eastern England and Normandy. Clunch distinguishes itself from archetypal forms of limestone by being softer in character when cut, such as resembling chalk in lower density, or with minor clay-like components.
The Wren's Nest is a geological Site of Special Scientific Interest in the Dudley Metropolitan Borough, north west of the town centre of Dudley, in the West Midlands of England. It is one of the most important geological locations in Britain. It is also a Local Nature Reserve, a national nature reserve (NNR) and Scheduled Ancient Monument. The site is home to a number of species of birds and locally rare flora, such as Scabiosa columbaria, milkwort and quaking grass. The caverns are also a nationally important hibernation site for seven different species of bat.
Lime is an inorganic material composed primarily of calcium oxides and hydroxides. It is also the name for calcium oxide which occurs as a product of coal-seam fires and in altered limestone xenoliths in volcanic ejecta. The International Mineralogical Association recognizes lime as a mineral with the chemical formula of CaO. The word lime originates with its earliest use as building mortar and has the sense of sticking or adhering.
Bath Stone is an oolitic limestone comprising granular fragments of calcium carbonate originally obtained from the Middle Jurassic aged Great Oolite Group of the Combe Down and Bathampton Down Mines under Combe Down, Somerset, England. Its honey colouring gives the World Heritage City of Bath, England, its distinctive appearance. An important feature of Bath Stone is that it is a 'freestone', so-called because it can be sawn or 'squared up' in any direction, unlike other rocks such as slate, which form distinct layers.
Ketton stone is a Jurassic oolitic limestone, cream to pale yellow or pink in colour, used as a building stone since the 16th century. It is named after the village of Ketton in Rutland, England. It was used as freestone, mostly in buildings outside Rutland, particularly in Cambridge, where it was used in many of the colleges.
Gloucestershire is one of the most geologically and scenically diverse counties in England, with rocks from the Precambrian through to the Jurassic represented. These varying rock-types are responsible for the three major areas of the county, each with its own distinctive scenery and land-use - the Forest of Dean in the west, bordering Wales, the Cotswolds in the east, and in between, the Severn Vale.
The Michigan Limestone and Chemical Company operated the world's largest limestone quarry, which is located near Rogers City in Presque Isle County, Michigan. It was formed and organized in 1910; however, production did not begin until 1912. Ownership of the quarry has changed a number of times, but it is still one of the largest producers of limestone in the United States. The quarry was inextricably interlinked with lake shipping and railroad transportation.
Bowers Quarry/Mine is an active stone quarry on the west side of the Isle of Portland, Dorset, England. The quarry is operated by Albion Stone and became the site of the first Portland Stone mine. In October 2002 the firm successfully initiated Portland's first ever underground mining operation, which was a precursor to Jordans Mine, which would start in 2008. It has also been the selected site for the planned Mass Extinction Memorial Observatory.
Mining in the United Kingdom produces a wide variety of fossil fuels, metals, and industrial minerals due to its complex geology. In 2013, there were over 2,000 active mines, quarries, and offshore drilling sites on the continental land mass of the United Kingdom producing £34bn of minerals and employing 36,000 people.
Indiana state stone is a public sculpture at the Indiana Statehouse in downtown Indianapolis. It is an 85-pound (39 kg) cube of Indiana limestone that is mounted on a wooden, rotating dolly. It was carved from limestone quarried from the P. M. & B. limestone quarry located in southern Indiana. This sculpture commemorates limestone becoming Indiana's official state stone on March 1, 1971.
Perryfield Quarry is an operational stone quarry and part butterfly nature reserve located on the Isle of Portland, Dorset, England. It is situated towards the middle of the island, east of the village of Weston and south of the hamlet of Wakeham. The reserve section is now a valued home for a number of butterfly species, while the working quarry area is one of the largest active quarries on Portland. The quarry is owned by Portland Stone Firms Ltd, along with Broadcroft and Coombefield Quarries. The firm is the largest landholder on the island.
Stonehills Mine is the first completely new mine on Portland. Albion Stone Plc began the process to open this mine began in 2015 and reserves are estimated to last for 50 years (2066). The planning application to begin work on Stonehills was submitted to the Dorset County Council in February 2001 and was finally approved in February 2005. The site is owned by the Crown Estate and the leased by Albion Stone plc who extract Portland Stone Whitbed and Portland Stone Basebed. Albion Stone Plc use room and pillar mining method of extraction which is already being carried out in their Jordans mine and Bowers Mine.
Reigate Stone is a freestone that was mined from the Upper Greensand in north east Surrey. It was used in building work throughout the Middle Ages and early modern period. It is sometimes classified as a calcareous sandstone, although very little of the silica content is in the form of detrital sand grains. In addition to silicon dioxide, the stone also includes clay, fine-grained calcite, mica flakes and glauconite.
↑ Gerhartz, W., ed. (1986). "Cement and concrete: see p. 492". In: Ullmann's Encyclopedia of Industrial Chemistry. Fifth completely revised edition. Vol.A5. VCH Publisher, New York. ISBN0-89573-155-X.
Further reading
Hackman, Gill (2014). Stone to Build London: Portland's Legacy. Monkton Farleigh, Wilts: Folly Books. ISBN978-0-9564405-9-4. OCLC910854593.
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