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Great British Stone : Bath Stone

4 March 2015
Bath stone comes from underground quarries.

As there are still baths built by the Romans in the city of Bath, the stone used must have some admirable qualities, although a suite of contemporary tests might lead you to believe otherwise. Barry Hunt investigates as he continues his search for Great British stones.

When considering which stones should be regarded as Great British Stones, first impressions are that there appear to be good reasons why Bath Stone should not be included. On the face of it, this is a stone whose properties are at the low end of the quality range for limestones, causing it to struggle in durability tests. Thankfully we have a whole city with up to 300 years of exposure to prove that tests (or the results they provide) can be misleading.

The city of Bath is a World Heritage Site without parallel. Although most of the city seen today is Georgian, there are buildings in the local stone that actually date back 2,000 years to when the Romans made the most of the springs – the same springs that saw the city flourish as a leisure resort in the 18th century.

The Pevsner Architectural Guide for Bath names Ralph Allen, Beau Nash and the elder John Wood as the important protagonists in the development of the British spa town par excellence.

Nash was an interesting character who seemed to fail at everything except gambling and being an opportunist. Becoming Master of Ceremonies in 1704, he embarked on changing Bath from just a resort to a social centre with strict rules on decorum.

Allen’s role as Bath’s postmaster brought him considerable wealth when he reformed the British Postal system. This allowed him to purchase some of the local stone quarries. He built major properties for himself in the stone, which helped him to market it as a building material to the local population.

Allen’s mansion at Widcombe, above Bath, was designed by Wood, who had settled in Bath in 1727 in the belief that Bath presented a great opportunity for planned development to revive the splendour of a Roman city.

During the 17th century the square had become a major feature of city life and Wood designed a square with palatial treatment of the in-vogue Palladian style that was not adopted in London until the latter half of the 18th century.

The larger terraces built in Bath subsequently followed a similar design, where there was a drive towards order and discipline.

This could have proved difficult, given the curve of the River Avon through Bath and the steep slopes, but one novel solution to this was to build crescents that followed the contours. The land in front of the crescents was left without landscaping to allow communication between urban dwelling and nature (known as ‘rus in urbe’).

Wood designed the first ‘circus’ in England, based on his conception almost 30 years earlier of a place for sports. The foundation stone was laid at the time of his death in 1754 and it was Wood’s son, also John, who was left to complete the grand scheme.

Many others were involved in the development of Bath as arguably Europe’s finest city of its time, with so many buildings of note that it is also almost too easy to overlook the many artisan dwellings built in the same period, the surviving Roman and Medieval structures and those structures built after the Georgian period, even up to today.

But what glues all the different facets together is the stone – and there is no other town or city that could lay claim to such consistency of stone use. Indeed, it is why the local stone became known as Bath Stone, even though most of it does not actually come from Bath.

So What is Bath Stone?

To many, Bath Stone is the finest of building stones, not just because of its association with the city from which it takes its name but also because of its golden buff colour that is enriched by the sun, giving it a glow matched by few other building stones.

The combination of an English sunset and the dominating Palladian majesty of form can be quite breathtaking.

Aesthetics rightly win over the properties of the stone in this case, as Bath Stone might otherwise struggle to compete in today’s world. It is also best used with traditional masonry practices, using its bulk under compression for stability and longevity.

Bath Stone is a type of limestone, one of many oolitic limestones formed during the Jurassic Period that outcrop across the UK.

Portland is just one of its famous oolitic limestone cousins and, like Portland, Bath has given its name to a stage of Geological time – the Bathonian. It runs from 168.3million years ago to 166.1million years ago, right in the middle of the Jurassic.

The egg-shaped ooliths in Bath Stone are typically relatively large, imparting a visibly coarse texture to much of the stone. The matrix typically comprises coarsely crystalline calcite or ‘calcite spar’, which results in a closed, dense texture.

Bath Stone suffers from the same nomenclature issues as so many of the UK’s stones. These were highlighted in the article on Yorkstone (NSS July / August 2014).

Derek Hawkins’ book, Bath Stone Quarries, lists a lot of the quarries (see panel on page 30). There are a lot of them, none within Bath itself. They are in an area immediately to the east of Bath that forms an approximate triangle with Bath if you take Bradford-on-Avon to the south-east and Corsham to the north-east as the other two points of the triangle. The area totals about 20 square miles.

Corsham is closer to Chippenham than Bath, so should the stone from here be called Chippenham Stone instead? Or should we simply stick to the precise locality from which it comes?

This conundrum again highlights the fact that human political boundaries do not necessarily have anything to do with geology, and how difficult it might therefore be to police any move to declare boundaries for a particular stone type, as is currently being proposed under the European Geographical Protection scheme (see the previous issue of NSS).

The Changing Fortunes of Bath Stone

The majority of Bath Stone quarries are actually underground workings, even though they are called underground quarries rather than mines. The reason seems to be simply that the people working in them wanted to distinguish themselves from miners of other minerals (such as tin, lead and coal), who they considered less skilled.

Because most of the stone came from underground workings, there is little visible evidence on the landscape of the quarrying history of the region. This was a factor in the decline of the industry in the 20th century.

The building of Georgian Bath came at a time when opportunities for the working classes were limited and the quarries provided a source of income for a significant proportion of the local population.

The construction of the Kennet & Avon Canal between 1794 and 1810 meant that Bath Stone could be shipped to London and elsewhere. This prompted the building of local tramways from the various quarries down to the River Avon. The remains of some of these can still be seen today.

The arrival of the Great Western Railway superseded the canal, while the construction of Box Tunnel revealed extensive resources of Bath Stone that could be readily transported from the area. By 1864, Tunnel Quarry is reported to have been producing 100,000tonnes of stone annually.

As the railways came and opportunities for employment increased, quarrying as an occupation began to fall out of favour.

Increasing labour costs, as well as both competition and the difficulty of winning stone from deeper underground, added to the pressures on Bath stone suppliers.

Competition increased because the improved communications that the railways brought did not only benefit Bath stone. There were many other competing materials to choose from that were being moved around the country.

As World War II approached, Britain’s strategic planners believed the Luftwaffe would bomb the whole of London into oblivion, so a plan was hatched to store munitions out of reach of the Luftwaffe.

The War Office purchased Ridge, Monkton, Farleigh, Tunnel and Eastlays quarries between 1935 and 1937. It was realised the clay layers overlying the Bath Stone prevented moisture percolating down to the quarried areas, creating an unusually dry environment in these tunnels.

A number of the other smaller underground quarries were taken over by various ministries for storage purposes before all of them were eventually requisitioned in November 1940.

Spring Quarry had just over a quarter of a million square metres of space that was adapted as a factory for the Bristol Aeroplane Company, at that time makers of the Beaufighter, a long-range fighter and torpedo bomber.

The Bath Stone industry never really recovered from its wartime closure and the few quarries that have been re-opened since then were never going to re-establish Bath Stone to its former glory.

Properties of Bath Stone

The oolitic structure of Bath stone and its sparry calcite matrix are not strongly bound to one another and when the stone is fresh with a bit of moisture (or ‘quarry sap’) it is relatively soft. Thus it could be cut from the ground using hand picks and saws efficiently enough to make it a viable building material.

During the 1840s, saws known as razzers and frigbobs were introduced that remained in use until well into the 20th Century.

When the stone loses its quarry sap, cutting it becomes more difficult. In service, it appeares the stone surface hardens over time. It is possible that hydrated minerals migrate to outer surfaces as the stone dries and carbonate or alter to less soluble materials that provide added cohesion.

There is little available information on the petrographic properties of the stones from the many different quarries near Bath. The principal differences that cause subtle variations in appearance and properties are the size of the ooliths and the quantity of finer carbonate minerals within the matrix.

Types of Bath Stone described with the suffix ‘ground’ (for example Stoke Ground) sometimes appear to be more resistant to the weather.

It is generally accepted that the ooliths weather out of the matrix of Bath Stone so there is a persistent gradual surface denudation that BRE has estimated to be around 3-4mm every hundred years under relatively mild weathering conditions.

Table 1 (below) lists typical test results for some Bath Stones currently in production. Note: Wet and dry strength values have not been differentiated and results are for materials tested with the loading direction perpendicular to the bedding planes.

It is also apparent that the presence of water does not significantly affect the slip resistance test results, so this has not been differentiated, either. The results of frost testing have not been included as the stone can be made to fail within just a few cycles if it is frozen rapidly while saturated, something that does not easily occur to large block stone in real life unless there is exceptionally poor design or workmanship.

Use of Bath Stone

Bath Stone is suited perfectly to traditional masonry as it can be cut into large blocks with rougher waste materials used for rubble walls or infill.

The stone is perfect for plain walling but only the strongest and most resistant grades would normally be considered for copings, cills, lintels and other architectural features.

For statuary, the stone is easy to carve but fine detail can be rapidly denuded or lost completely. This is a good reason why Bath Stone is not normally used as headstones. Neither is it recommended for plinths.

Bath Stone is suitable for ashlar, but potential risks increase with decreasing thickness and the material should not be used for cladding purposes without great care and consideration and, more than likely, an expensive and difficult to construct support system. Bath Stone might find better favour as a facing to some of the lightweight cladding systems now available, but it cannot be expected to remain pristine.

It does not make good paving or floor tiling because it wears relatively quickly, though the natural maintenance of a rough texture does make it especially slip resistant. Hawkins reports that the stone from the Elm Park quarry was more difficult to quarry, it being harder, and until the 1920s it was used by Wiltshire County Council for almost all of its kerbstones.

Interior wall tiling is a possibility but without regular and extensive maintenance will be susceptible to degradation from scratching, staining, discoloration and all manner of issues.

Limestone is especially good at resisting the effects of heat as exceptionally high temperatures are needed to produce irreversible changes. This has led to Bath Stone (like other limestones) being used for fireplace surrounds.

The trick to the successful use of any Bath Stone in construction is to ensure it does not remain saturated and become vulnerable to frost attack.

Architectural details to throw water away from a building and the use of lead and other materials to guide water away from the stone are essential. Where the stone runs into the ground, the ground should be free-draining. If Bath had not been built on freely draining slopes, Bath Stone might never have had an opportunity to earn its reputation as a building stone.

As with many limestones, grey Portland cement mortars must not be used to set Bath Stone. Close contact with materials that contain grey Portland cement may also be problematic if these can become wet. Alkalis in the cement can strip out organic materials naturally contained by the stone, resulting in strong brown discoloration.

Bath Stone develops a well-defined patina over time. Generally, lower quality Bath Stones that naturally retain a higher level of moisture, attract more airborne dirt and thus turn darker more rapidly.

No stone is irreparable (or irreplacable) but a lot of quarries have supplied Bath Stone and most of them are no longer in production, so a true match cannot be guaranteed by today’s commercially available stones.

Concluding Remarks

Bath Stone rightly deserves its place among the Great British Stones, proving that there is the potential for every building stone out there to be great, although I concede this is a matter of opinion and there is always someone who will beg to differ. One individual described the Royal Crescent in Bath as “a bunch of terrace houses, nothing really special” in their TripAdvisor review.

An opinion of Bath Stone based purely on test results would fly in the face of observed performance. The City of Bath, in my opinion, is a testament to how stone can be used in the most exquisite ways and provide something so special and sublime that is difficult to match.

We live in an age where mediocrity abounds, so we should re-learn past lessons and make the most of what materials offer to create something wonderful. Function over form has been winning the day for far too long.

The author: Barry Hunt

Barry Hunt is a chartered geologist, a surveyor and scientist. He has been awarded the designation of European Geologist and is a Corporate Building, Conservation and Specialist Surveyor. He is also a Member of the Chartered Institute of Building and a Fellow of the Royal Microscopical Society.

Barry has served on a number of professional committees, including the Technical Committee of Stone Federation Great Britain (SFGB), which provides advice on all stone construction issues. He is one of the authors of the SFGB team that has published codes of practice for the installation of stone floors and internal stone finishes.

Barry gained 14 years’ experience working as a consultant for two renowned civil engineering materials consultancies before, in 2001, establishing his own consultancy, IBIS, specialising in the investigation of construction materials.

The specialist knowledge and services provided by Barry have allowed him to be instrumental in the resolution of problems ranging from blast damaged claddings in London’s West End to advice on the quarrying and extraction of stone from abroad for import to the UK. Other areas of experience include the investigation of all types of building finishes, specialist advice on remedial treatments and the preparation of advice for potential and actual use in litigation or arbitration.

Having worked for consultancies that both undertook in-house laboratory investigation to UKAS requirements, Barry is also able to conduct or oversee a wide range of on-site and laboratory techniques and ensure they are carried out to traceable standards.

One speciality in all investigations is Barry’s hands-on approach. Being trained in industrial roped access (abseiling) allows him to get close to the problems with external building envelopes quickly, efficiently and cost-effectively.

Throughout his working life Barry has published findings from the many investigations he has undertaken. He has also authored chapters for two books on building stone and is currently engaged in other book projects in this field. His most important contribution is considered to be the chapter on the repair and maintenance of stone in the landmark Geological Society publication Stone.

Barry is also a regular contributor, covering the full spectrum of natural stone use, for Natural Stone Specialist magazine.
       

 

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