Having examined the broad categories of building stones from around the world, consultant geologist Barry Hunt now turns his attention to the great building stones of the British Isles, beginning with what he argues could be considered the greatest of them all.
Christopher Wren was a great polymath and quite probably one of our first great ‘modern’ geologists. He is not immediately recognised by today’s geologists as such as there is little written about his skills, while geology in the 17th century was a science still cowed by religious dogma. It was not util the 19th century that geology emerged from the darkness.
But Christopher Wren was a great geologist because he recognised the role Portland limestone could play in the rebuilding of London after the Great Fire of 1666, culminating in St Paul’s Cathedral. This was almost 150 years before the science of geology truly began in Britain.
Using Portland stone was a gamble, because in the 17th century no-one could really have predicted how it would perform in the London environment. It turned out to be an almost perfect choice.
These days it might not even have been selected because tests carried out on building stones now would have you believe Portland is a mediocre stone with moderate strength that hates frost and frets when dunked in salt solutions.
But Portland limestone is one of the best examples of why tests are never able to tell you the full story and how long-term observation of existing buildings provides more information than any test ever can.
However, we cannot avoid tests these ‘enlightened’ times and they are an essential part of the story of Portland stone.
What is Portland Stone?
Portland stone is a type of limestone, mostly calcium carbonate (CaCO3), which is the same mineral that forms the bulk of chalk, many marbles, travertine and limescale in kettles.
Calcium carbonate loves to precipitate out of solution, a feature that allowed early organisms in Earth’s seas to make shells that have left an amazing fossil record – a record that is the backbone of geological science.
When you peer into the heart of Portland stone you see that a great proportion of the rock consists of fragments of the shells of organisms from a part of the Jurassic epoch some 150-145million years ago.
Picture a huge reef, not unlike the Great Barrier Reef in Australia today, teeming with life, some of it protecting itself with shells. This reef is battered by waves. Although some shells survive largely intact, most are broken down and smashed to smaller pieces that are eventually laid to rest. These act as nucleii for a process known as ooid formation.
To understand Portland stone you have to know about ooids, which are mesoscopic (not quite microscopic) concretions of roughly circular or oval shape that you might say look like little eggs (the word ‘ooid’ derives from the Ancient Greek word for egg-shaped).
Cut one of these ooids open and you will see a series of concentric rings around a central nucleus.
What has happened is that the original fragment of shell (or other material) has lain in a warm tropical sea and calcium carbonate has precipitated while currents have moved the juvenile grain. Coatings build up over time depending on seasonal and tidal factors (among others) to become ooids.
Now take all those ooids, which in Portland stone average around 0.6mm in diameter, dump them in a huge pile that builds up over millennia, cover them with later sediments that put the mix under pressure and wait.
Slowly the grains recrystallise at points of contact with each other. We now have stone formation. Pore water is moving more calcium carbonate through this formation and allowing it to precipitate, further cementing the ooids together.
At this stage geologists term the ooids ‘ooliths’ and the stone formed is ‘oolitic’ limestone. The story of Portland stone is a little more complicated than that because there are different beds, each requiring a modicum of explanation. But, in essence, this is Portland stone.
Types of Portland Stone
There are three main beds of the Portland stone sequence that have been traditionally recognised as providing blocks large enough to be cut for masonry: Whit Bed, Base Bed and Roach.
The Whit Bed might be regarded as the purest of the three as it tends to be ooliths and little else. The Base Bed is ooliths mixed with a proportion of finer calcium carbonate materials, or muds (although not the thick sticky clay muds you might be thinking of). The Roach is typically a dramatic stone due to the incorporation of numerous large fossils set within the ooliths and some mud, but also with a significant proportion of later calcium carbonate cement.
These subtle differences help to explain why each type of Portland stone performs quite differently.
Other types of Portland stone you may hear about include the Curf, which is traditionally thrown away as the bed was too small in height to be commercially viable, although there are locations where the Curf is thicker and can be used as a building stone. The Curf appears much like a mixture of both Whit Bed and Base Bed and sometimes is more of one than the other.
Generally the full Portland stone sequence from top down is described as follows:
- Hard Cap – a very hard limestone
- Roach – an oolitic limestone
- Whit Bed Freestone – an oolitic limestone, and the best freestone
- Flinty Bed – limestone full of chert
- Curf – soft, chalky limestone
- Base Bed Roach – shelly, oolitic limestone
- Base Bed Freestone – a good freestone with few shells, soft, white and oolitic
- Cherty Series – soft limestone with bands of very hard flint-like chert
For masonry work, the Whit Bed is prized for its consistency and durability.
The Base Bed is considered to be less durable in exposed locations than the Whit Bed and is prized for its relative softness and ease of carving, making it perfect for statuary and more intricate working.
The Roach is sufficiently strong to act as sea defences and is the stone of choice around Portland itself for many of the buildings that are subjected to high exposure to the elements, including salt spray from the sea.
Properties of Portland Stone
I feel test results on Portland stone are almost an irrelevance as they do it a considerable disservice. If you compare the test results with those from other limestones you will find many stronger materials. The density and porosity are also unremarkable. So what makes Portland such a good building stone?
The colour provides a clue. Most limestones are blue or grey when fresh but Portland stone is much lighter due to geological weathering. Sometimes geological weathering is beneficial because it removes potentially problematic materials. This is the case with Portland stone and is why it can outperform stronger, harder, denser and unweathered limestones in use. Test results completely miss the point of how the stone is used and how it actually behaves in the built environment.
How could Portland stone stand up so well to the London environment for the past 300 years when we have much harder limestones and even granites suffering problems over much shorter periods of exposure?
The answer lies in the ooliths… or rather how the ooliths lie.
The ooliths were overlain and compacted, which pressed them together in one overall direction vertically, so they are mostly joined at the top and bottom and less so at their sides. This connection of the ooliths means there is more interconnected space running vertically through the stone than there is horizontally, so as long as the stone is used on-bed, as it should be, any water entering the stone tends to run vertically down through it rather than into it. Basically the stone is able to drain off water quickly and consequently is less likely to suffer frost attack because there is little water to freeze and cause damaging expansion.
Leave Portland stone where it cannot drain (in a laboratory test, for example) and freeze it and it will fall apart because it cannot resist the internal bursting forces created by ice formation.
I have surveyed numerous Portland stone buildings around London and many in other parts of the country, too, and it is clear Portland stone of all types will last 100 years almost without any loss of material.
When losses do occur it is more often than not due to poor design detailing that has prevented the migration of moisture and allowed frosts to attack moisture-laden stone. This is especially prevalent underneath lead flashings where the stone is less able to dry, which is ironic as the lead flasing was intended to protect the stone.
Looking at copings, cornices and other exposed locations devoid of lead covers shows that over 200 years or so Portland stone might lose between 1mm and 5mm from the surface, which is an exceptionally minimal level of weathering. And even then the stone that remains is blissfully unaffected by the weather. Typically there is no deeper zone of weathering taking hold.
Copings are often interesting because where they are placed horizontally – and are, therefore, less able to drain – they can lose 15-30mm over about 100 years.
It is clear that Base Bed suffers more than the Whit Bed and this is due to the carbonate matrix that, while apparently increasing the density of the stone, actually results in the moisture flow through the stone being disrupted, resulting in the matrix clinging on to a higher volume of moisture.
With limestone it is not water absorption that is important, it is the ability of the stone to expel moisture quickly. Looking at buildings where both Whit Bed and Base Bed Portland stones have been used together this often becomes all too apparent. Because Base Bed takes longer than Whit Bed to dry out, it is more susceptible to dirt sticking to it and over time will appear dirtier than adjacent Whit Bed. This also means it is taking on board more salts and pollutants than the drier Whit Bed and will suffer a higher degree of attack from these as well as frost. There really should be a test to determine the drying rates of stone for masonry, but no-one has thought of it yet.
By the way, you might wonder how you can determine how much material has been lost over time. With Portland stone this is simple – just look at the shells. The shells are made up of dense calcium carbonate that is far more resistant to weathering than the ooliths. Over time the ooliths are removed from around the shells, leaving the shells standing proud and indicating the original surface level.
There are other ways of looking at the geometry of blocks but this is not usually necessary. Another indicator is the use of cramps, lead joints and other fixings, which can eventually stand proud of the surrounding stone as it weathers away.
The interconnected voids around ooliths also means that when testing the stone to determine its flexural strength, anomalous results may be obtained when tested across the bed, with the stone sometimes appearing weaker than when tested with the bed. This is contrary to how most stones perform and is mostly a function of the relative thinness of the specimens taken for testing.
Use of Portland Stone
The principal use of Portland stone is for traditional masonry, from massive blocks to hand-set ashlar. Used in this way it is essentially the perfect building material.
At this point I could reel off a ridiculously long list of esteemed buildings where you can visit to see the stone. I would rather just say go to the City of London and marvel. Christchurch Priory is worth a mention as it was built by the Normans and is one of the oldest surviving examples of the use of Portland stone.
There is a belief that the Romans used some Portland stone, but the Dorset island was too remote from their main cities, most of which had much more convenient sources of local stones. In London the Romans used the Kent Ragstone that outcropped along the southern end of Watling Street.
Portland stone is used for paving as well as walls and has fared well in many cases. The paving in my own back garden is Portland. It sits on soil – and has done so now for more than 110 years and remains sound. Again, its success is down to having good drainage and accepting that there will be some degree of deterioration of the surface over time.
My main problem with using it for paving is that it is a waste of a stone so well suited to masonry construction. Also, York stone is better suited for this, as we will discover in a future edition of Natural Stone Specialist.
In interiors, Portland makes excellent flooring. The British Library is testament to the pleasing effect that Portland can be used to achieve. It will scratch. It will fill with dirt. But this brings out some of the natural features of the stone that are often lost on masonry structures where the stone’s sparkling whiteness can be almost blinding on a bright day.
In recent years the traditional use of Portland has given way to using thinner cladding and looking to test results to tell the story rather than the history and geology. Thinner stone is less able to resist the forces of attack and there is often a tipping point at which the stone can fail under changing environmental conditions.
Presently, 50mm thick appears to be the lower safe limit before there is a sharp and exponential increase in the risk of failure. Care is required to design and build with this thinner material.
The supposed cost benefits from using thinner material are far outweighed by the benefits of longevity of thicker material over the lifetime of a building intended to last centuries rather than decades, but this can be difficult to explain to accountants.
For a while from the 19th century, building methods changed and Portland cements started to be used for all aspects of masonry building rather than lime mortars. However, it was soon identified that Portland stone did not much care for being used in conjunction with the cement that was trying to be its doppelganger.
This may be apocryphal, but it is said that Portland cement is so called because at the Great Exhibition of 1851 a block of concrete cast using the then relatively new cement compound was described as looking like Portland stone, and the name stuck. If this is true it was a major slur on Portland stone. The materials are different in so many ways.
Remember that pile of shell fragments that went on to form the ooid nuclei of Portland stone? These were part of living organisms and the dead organic matter also had to end up somewhere. Often it collected within rock reservoirs to became oil and natural gas, but often a little of it is retained by the stone. Portland stone is no exception. Grey Portland cement has the unfortunate ability to attack the natural organic materials contained by limestones, leading to nasty brown staining. Only limes or white cements should be used with Portland (and other) limestone.
Conclusion
I wish I could talk to Christopher Wren and ask him what he saw in Portland stone that made him select it over the many others that were available to him – stones that seemed harder, stronger, more decorative.
Was it a fluke? Purely down to colour? Did he enjoy the subtleties of the bedding? Did he know something more from his observations of the outcrops and local buildings on Portland? Had he seen the use Inigo Jones had made of Portland stone when he introduced it to London in a limited way? Surely it could not have been simply that it was easy to ship round from the Dorset island and up the Thames?
I would like to believe Wren was a great geologist. We may never know, but his legacy will be with us in the buildings he left behind for hundreds more years yet.
There are many articles and books on Portland stone showing it off in all its glory, but none seems to have ever really tried to understand it. How could they? So many of the tests are inappropriate and have not been able to tell the whole story. It is the little things that count – the ooids and how they got together and what joins them.
There is still more of the story waiting to be told but there is no doubt the island of Portland proffers no ordinary stone, but a stone that deserves to be rated as possibly the greatest of all the British stones, both for what it is and for its contribution to the character of the nation’s capital city.