Portland stone – The stone at the heart of London
Portland stone is one of the key building materials which has been used to clad some of the most important buildings across the Capital. It comes in several hues, from a warm creamy white, through to a dark grey stone rich in fossils and has become famed for its quality and tight compacted nature which makes it an ideal stone for intricate carving and cladding.
Widely used across London and beyond, with quarrying for this famed material dating as far back as 2nd Century CE, it’s time that we delve deeper into this key construction material. Looking at how best to maintain it and keep it looking its best.
The Geology
We first need to go back in time, to the warm shallow sub-tropical seas of the Late Tithonian Age of the Late Jurassic Period (143.1 to 149.2 million years ago). These seas contained a wide range of marine life, with a range of macro and micro shelled fossils found throughout the 75 metres of the Portland formation.
The stone is made up of tiny (less than 0.5 mm in diameter) calcite (a calcium carbonate mineral) egg-shaped spheres (ooliths) which have been formed on the sea floor from a calcite rich mud (micrite) and a fragment of shell or other debris which then acts as a nucleus for accretion of the mineral around it. Over time these ooliths start to cement themselves together as more calcite is deposited around them. Then through the process of burial and the addition of pressure and heat, turns what was a muddy carbonate rich silt into a durable limestone.
The Portland stone which is most used for masonry comes from quarries located on the Isle of Portland in Devon (it’s named after the location where the rock was first described). These quarries have been worked for many years and have different types of deposits, some of which include significant fossil deposits of gastropod and bivalve shells, such as Aptyxiella Portlandila and Liostrea Expansa.
The History
With a history of use going back to the early 2nd Century CE, the stone has been found in Roman burial sites in Devon. Many well-crafted Roman sarcophagi (intricate stone coffins with matching lids) have been hewn from a single large block of Portland stone.
The first known large-scale building made from Portland stone is Rufus Castle at Church Ope Cove, Devon. First built in 1080, then rebuilt in 1259 and 1450, it is now a Grade I listed building and a scheduled monument. The real growth with the use of Portland stone as a London masonry staple came with the building of the Palace of Westminster in 1349 and the first stone London Bridge in 1350.
However, after the Great Fire of London in 1666, Sir Christopher Wren used almost 1,000,000 cubic feet (or 28,500 m3) of Portland Stone to rebuild St Paul’s Cathedral and other minor churches, and it was this widespread use of Portland stone which cemented it as the building material for London. Other landmarks made using Portland stone include The British Museum, Somerset House, The General Post Office, Bank of England, Mansion House, and even Tower Bridge is partly clad in Portland stone.
But it’s not just in London where you can find imposing Portland stone buildings Manchester has St. James Buildings and Manchester Central Library; Liverpool has the Cunard Building and the Port of Liverpool Building; and Exeter Cathedral and Christchurch Priory are all constructed or clad with this key building material. Outside of the UK, the UN building in New York, Houses of Parliament in Dublin, and the Auckland Museum in New Zealand are all of Portland stone construction.
In 2016 Portland stone was designated by the International Union of Geological Sciences as a Global Heritage Stone Resource.
Maintenance and Cleaning
As with any building façade, a regular, scheduled maintenance and cleaning plan is vital in keeping the building (and the materials which it is made from) looking its best. In the case of Portland stone, due to the chemical make-up of the material and its naturally light colour, it is susceptible to the wear and tear of modern city life and environmental degradation (such as carbon build up, traffic particulates, and rain).
If a biofilm build-up has occurred, a topical treatment to kill off the algae and bacteria will be needed as these organisms can work deep into the surface of the stone. If the period between cleaning is left too long, permanent staining of the stone can occur, this can also result in physical degradation of the stone leading to cracking, chipping, and possibly structural decay.
As the stone is slightly porous, surface dirt and grime can get ‘pushed’ further into the matrix of the stone if unsuitable high-pressure cleaning is used. For best results a low pressure, non-abrasive, chemical free cleaning process should be used to best ensure the continued health of the structure of the Portland stone façade.
Thomann-Hanry® is best placed to offer the care and attention a building with a Portland stone façade requires. Your one-stop-shop for all aspects of façade surveying, cleaning, and maintenance; Thomann-Hanry® has worked on some of the most impressive Portland stone façade buildings in London such as, the Bank of England, Somerset House, and Devonshire House.
Our patented façade gommage® cleaning system which offers a convenient and effective method to gently remove dirt and grime without the need to scaffold. The system uses ultrafine particles of chemically inert Aluminium silicate which are projected under low pressure from a vacuum cabin (which is mounted on a hydraulic boom). The powder gently rubs away the build-up of dirt and grime, to reveal the pristine surface below.
Ensuring that any building using this internationally recognised heritage stone is cleaned and maintained to extend the life of the building’s façade whilst also maintaining its impressive beauty. Call our team TODAY on +44 (0) 20 8453 1494 or contact us to find out how a Thomann-Hanry® can help your building’s Portland stone façade.
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