Webinar on Purbeck Marble: Conservation and Repair

Speakers: Clara Willett, David Odges, Angus Lawrence, Matt

Clara: Well, hello everybody and welcome. Yes, this is our “Hard to treat stones” webinar on Purbeck Marble and just to explain, “hard to treat stones” is the term, and it’s a topic that we refer to, but it isn’t only an issue for Historic England, but for the wider conservation sector. And by “hard to treat stones”, we mean the building stones we don’t really understand how they deteriorate or they’re difficult to repair and in some cases, both. So, it’s not a finite list, but currently on our list of “hard to treat stones” are Magnesium Limestone, Blue Lias, Clunch, Reigate and of course, Purbeck Marble. So, I’d like to… there we go. I was wondering whether I had any control over the slides then!

But, yeah, obviously I’d like to start by introducing myself and then introduce our two other presenters. My name’s Clara Willett, I’m a Senior Historic Building Advisor at Historic England and before joining Historic England in 2003 I worked as a stone conservator in private practice. And as a Senior Historic Building Advisor, I focus primarily on masonry conservation and including memorials, terracotta and one of our long-term running projects, the Strategic Stone Study. And I’m delighted that we’ve got two experts delivering this seminar today, David Odges and Angus Lawrence. So, I’d just like to ask them both to introduce themselves, so, David, please.

David: Thank you, Clara. I’ve just notice what a youthful picture it is of me. My name’s David Odges, and I was an apprentice conservator at Wells Cathedral way back and then founder of Nimbus Conservation and I left them in 2005 and have since then been acting as an independent consultant on the conservation of stone plaster and I’m an Accredited Conservator.

Clara: And Angus… if you could introduce yourself.

Angus: Yes. Afternoon, everybody. Angus Lawrence. I’m also an Icon Accredited Conservator. I started my career as an archaeologist at the Museum of London. I subsequently trained in conservation at City and Guilds of London Arts School and worked at the British Museum and then in the private sector for Holden Conservation and Nimbus Conservation. I’m currently a Director of Taylor Pearce Limited, specialising in the conservation of architecture, monuments and sculpture.

Clara: Thanks, both. So, moving on and before we make a proper start, for those of you who are new to Technical Tuesday, I’d just like to let you know a bit about the Technical Conservation Team. We are group of technical specialists, including conservators, scientists and engineers and we undertake research and provide advice and guidance on technical conservation issues. I think, well there’s links there, but the first link I think Matt will put into the chatroom for you takes you to the main Technical Advice page on the Historic England website where you can find all of our technical advice. So, that includes our research reports and PDF guidance documents which are all free to download and I think a second link that Matt will put in there will also take you to the catalogue which you can see on the left, which you can download. And that includes the whole list of our technical conservation guidance and research. I also thought you might like to know that we’ve introduced a technical conservation newsletter which is emailed out on the first Tuesday of every month. And this will let you know about new technical conservation guidance and publications and forthcoming technical conservation training events.

And here’s a good plug to highlight that we have a new technical advice note coming out on Purbeck Marble which you can see on the right there. So, it will be a good time to join up because hopefully you’ll get to hear about when this is published which I’m hoping will be in the next few days. So, if you’d like to add your name to the mailing list and you haven’t already signed up to a previous webinar, just click “Yes”. But, if you’re not sure whether you have already signed up, don’t worry. Even if you click “Yes” again, we’ll only send you one email. And just to be clear, this is for technical conservation guidance and events. It’s not for all Historic England publications or training. So, I think Matt’s publishing that for you, so… I’ll give you a bit of time to respond.

Matt: Well, we’ve got 58 people who want to join the mailing list. I think that’s pretty good going. Let’s move on.

Clara: That sounds good. Brilliant. Thank you very much.

So, just to give you an overview of this presentation so you can see where we’re going. I’m going to begin with an overview of Purbeck Marble, about the geology and how it’s been used. And then Angus will talk about the decay and causes of deterioration of the stone. And then David will look at the past treatments of it. And after a brief look at ways of recording Purbeck Marble, David will then summarise some of the current treatment options and then Angus will outline a case study from some conservation works to Purbeck Marble in the cloisters at Norwich Cathedral. And just to remind you, you can post questions in the chatroom as we go along, and then we’ll aim to have a discussion about these at the end. But, before we start, we’ve got a couple of questions for you that we’d like you to give us a little bit of information. So, I think Matt is going to post one of those up there. We just wanted to know about your involvement with Purbeck Marble. What’s your experience?

Matt: So, we’re getting a lot of responses coming in. So, as we expect, a lot of Local Authority Advisors, Heritage Sector Advisors, and quite a lot general interest as well.

Clara: That’s great.

Matt: I can broadcast those results. There you go.

Clara: That’s lovely. Yeah, well that just gives us an idea of where you’re coming from as well, so that’s really helpful for us. And I think we’ve just got a second question at this point as well, Matt.

Matt: We do. So, Question Two. Here we go. This is actually a text answer, so you’re actually going to have to write something now. So, yes. What issues have you had with Purbeck Marble?

Clara: Yeah, so that’s just sort of, what’s your involvement with it? Just again, so we know where you’re coming from as well. It’s really helpful for us to know that. Oh lovely. So, yes, some people haven’t had any experience, so that’s fine. Oh, floors, yes, very good. Damp. Damp always crops up doesn’t it? Yeah. Cleaning. Yep, we will discuss all these things. Diagnosis and specification for repair. Yep, we’ll be covering that. Quite a few people saying they’ve never knowingly come across it. So, that’s really interesting. So, hopefully, by the end of this seminar, you’ll have a better eye for it. That’s great.

Matt: That’s really interesting. A really good cross-section of experiences there. OK, let’s move on.

Clara: So, I’m just gonna talk about, give you an overview of the material. So, for those of you who just aren’t familiar with it at all, I can tell you all about it. So, Purbeck Marble. Here you can see there’s a geological map. So, Purbeck Marble is one of the stones of the Purbeck stone group, which is the general name given to the limestones quarried from the Isle of Purbeck in Dorset which you can see here. And as you can see, hopefully you can see all the stratigraphic details. But basically what you need to take away from this is that the Purbeck stone group were laid down in the Jurassic and Early Cretaceous age. And the climate during this period was relatively warm and this area was covered with fresh and brackish or mildly salty water which supported marine and land flora and fauna. And you can see the red lines, the red arrows there point to the thin band of Purbeck Marble which runs from Peveril Point in the east across to Tyneham Beach in the west. So, you can see it’s a very thing band. And if you’re really into your geology and building stones, which I hope most of you are, then you can read more about the building stones of Dorset in our county atlas which is part of the Strategic Stone Study and that will be shortly completed as the Building Stone Database for England. Matt will probably put up a link for you at some point as well for that.

So, as I said, Purbeck Marble is found near the top of the Purbeck Group and typically occurs as thin beds or as outcrops at or near the ground surface. So, you have individual marble beds which range from about 300 to 600 millimetres thick, so they’re very thin. And these are interspersed with layers of shale and clay. You can see this drawing on the left by Trev Haysom and you can see the various beds including green and grey and blue marble. And on the right, you can see a picture of slices of this bed, which are on the walls of the quarry at Quarr Farm.

So, the popularity of Purbeck Marble was partly due to its distinctive appearance as a polished, high-end stone, but it has a number of particular physical and chemical properties that have contributed to its particular usage. And despite its name, Purbeck Marble isn’t a true metamorphic marble, but a sedimentary rock. And it’s classified as a biomicrite limestone. So, thinking back to the conditions the rock was formed in, Purbeck Marble contains abundant fossils, particularly gastropods, freshwater snails, the Latin name, Viviparus carinifer, you can tell I never learned that at school! And these are packed into a fine grained mud and limestone matrix. So, you can the image on the right there, you can see some of those fossilized shells. And Purbeck Marble is essentially 90% calcite, mineral form of calcium carbonate with significant clay in the form of smectite content. It also contains mineral feldspar and pyrite. And Purbeck Marble’s distinctive colours, varying from red to a green and blue, depend on the content of minerals such as hematite and glauconite. Purbeck Marble is a dense stone with a low porosity of less than 5%, low absorption, but a high compressive strength of over 100 megapascals. And remember, this is perpendicular to the natural bedding surface. So, these properties make it hard to work, but it’s a material that can be polished.

And Purbeck Marble has been used since Roman times, for plaques and mouldings and it was available as overburden from the outcrops, but as these became more sparse, it was quarried from mines.

Purbeck Marble was mainly popular between the 12th and 16th centuries for architectural details, columns, ledger slabs, floors, fonts and tombs, and it’s primarily found in cathedrals and churches and sometimes secular buildings. It’s an attractive and characterful material and it’s hard, colourful and interestingly textured, easily polished, and it was cheaper than real marble. In its hay day, during the 12th and 13th centuries, when centres were established, initially in Corfe, which was close to the quarries, and then in London, principally for the construction of Westminster Abbey.

And you can see here, remember it’s got a very small bed height, so it was generally used in its natural bed for long, thin elements such as string courses, capitals, ledgers and floors. However, it was also very expensively used for columns, but this required the natural bed to be vertical and we’ll see the effects of this later on. So, in this image you can see all these elements here. The Purbeck Marble is in its natural bedding setting. And although it’s very hard to carve, the skills of the masons were such that you could produce very delicate carvings. So, here we have an effigy of King John in Worcester Cathedral and another label stop at Salisbury Cathedral.

This map is from Rosemary Leach’s book on the use of Purbeck Marble, and it shows the distribution of the stone in the late 12th century through to the mid-13th century. And you can see that there was an extensive use of Purbeck Marble. It was mainly in southern England and places which were accessible to see in river transport. And that was a key. It’s also found in Brittany and Normandy as well. And again, the Strategic Stone Study is a good resource if you’re looking for examples of Purbeck Marble throughout England.

I think we have another question just to ask you about whether you have Purbeck Marble. If you work in a specific geographical area, do you have Purbeck Marble on your patch?

Matt: This is going to be quite interesting, I think, because hopefully we’ll find people who say they have Purbeck Marble on their patch, which will be a long, long way away from where the actual marble comes from or where we think it’s going to be. So, that’s going to be really interesting. So, there you go. 60% do have Purbeck Marble on their patch. Who would have thought?

Clara: Yeah. And because we kind of know where some of you are… We won’t be coming to find you, don’t worry. But, you know, that will give a key… As Matt said, we’ll maybe find some interesting Purbeck Marble that we didn’t know existed. So, that’s really great. Thank you very much. So, now I’m going to hand over to Angus and he’s going to talk to you about the decay and causes of deterioration of Purbeck Marble. So, over to you, Angus.

Angus: Thanks, Clara. Well, as with all stones, Purbeck Marble is subject to both physical and chemical processes which bring about decay. Often these processes are facilitated by both the location of the stone and the manner of its use. These can be summarised as follows. It’s important to bear in mind that these processes often overlap or indeed work in tandem. They’re in no particular order. So, we have mineralogical changes due to expansion and contraction of clays in the stone. Oxidation of iron pyrites with the formation of ferrous sulphate and sulphuric acid which then reacts to form calcium sulphate at RH of above 60%. We have laminar structure and bedding planes leading to cleavage. We have the formation of soluble calcium sulphate from reaction with sulphur dioxide. We have salt crystallisation on surfaces and the deterioration is accelerated by environmental changes and condensation. We also have to take into consideration previous interventions which may also enhance decay mechanisms.

As Clara previously described, the petrographical characteristics of Purbeck Marble, having a predominantly calcite matrix at approximately 90% with a number of other clays and minerals. The smectite clay, that Clara mentioned in particular, exhibits a common characteristic of hydrational swelling when exposed to fluctuation in relative humidity and surface condensation. And repeated expansion separation of layers caused by this swelling leads to detachment of flakes or larger spores, long bedding planes.

Relative humidity again plays a role in another chemical process which accelerates decay in Purbeck Marble. Above an RH of 60 degrees, oxidation of pyrite occurs to form ferrous sulphate and sulphuric acid. This oxidation causes some of the characteristic browning seen on Purbeck Marble while the formation of gypsum, as the calcite binder reacts with the sulfuric acid, can disrupt the surface and make it more prone to dissolution.

As we’ve heard, the Purbeck Marble occurs in a shallow bed and elements such as shafts and columns are often by necessity laid with the bedding planes in a vertical orientation. Expansion of the stone frequently results in cleavage along bedding planes, as here in Salisbury. You can see the vertical crack which is running along the bedding planes at Salisbury and Lincoln. The use of lead between sections of marble can also exacerbate the problem. There’s a tendency to use little pads of lead rather than a soft lime mortar which didn’t allow the expansion of the material. Where compressive forces might be at play, for example, where a heavy ledger stone rests on stop of a monumental chest tomb, vertical elements such as the side or corner plasters can split along the bedding planes.

Now, Clara mentioned the low porosity of Purbeck Marble, but the movements and crystallisation of soluble salts can still contribute to decay. Salts can occur naturally in the stone itself, derived from ground water, restoration materials such as cements, or in fact the use of de-icing salts. The image on the top right shows parts of the remaining Chapter House entrance at Norwich where deterioration of this Purbeck Marble stooling has been exacerbated by a number of causes, including the use of de-icing salts.

Purbeck Marble in an open environment is also susceptible to general air pollution, leading to the formation of soluble sulphate crusts. These Purbeck elements at Netley Abbey have suffered extensive decay due to increased levels of pollution from a nearby oil refinery. Sulphur dioxide deposited on the surface of this stonework has reacted with water to form sulphurous acid, which in turn has reacted with the calcite to form gypsum. This gypsum has then dissolved in rainwater run-off from the arch above. Other causes of decay are, of course, human intervention. You often see people touching rather characteristic Purbeck in cloisters around the country. David, over to you.

David: Thank you, Angus. Just worth mentioning about that last slide at Netley Abbey is that there are pictures taken in the 1920s that show that capital and columns pretty much entirely intact. So, this is something that’s happened quite fast and dramatically.

[they discuss an issue with the recording of the webinar].

David: OK. Yeah, so, we’re just going to look at some of the past treatments because Purbeck, perhaps more than many other stones has had more than its fair share of different treatments and these very much affect its current appearance and behaviour. So, in order to mitigate the causes of decay, preventive methods have always been at the heart of the continued preservation of Purbeck as this extract from Eric Benfield’s book robustly describes. And I think he was optimistic in thinking that all of the Purbeck would be regularly covered in wax given if you go to somewhere like York Minster where there are thousands of Purbeck columns. But the underlying element of protection is very true. Of course, if protection gets applied again and again and again, then it builds up a significant layer and that is indeed what has happened in some places as we will see.

So, examples of these past treatments. We’ve got surface consolidation using shellac, oils and waxes. Surface repair using a multitude of different binders and mortars, some of which were applied obviously with the intention of being subsequently covered over because they then try to match the Purbeck in any way. The structural repair of splits in the columns. There are examples of repositioning of Purbeck, particularly columns and indeed floors to hide decayed areas. Replacement and that certainly, in the 17th, 18th, 19th century, was often done with a different stone because of the lack of availability of Purbeck Marble. And the removal of decay and restoring surface polish. So, we have some slides to illustrate that.

So, traditional treatment. On the left, we’ve got the covering of Purbeck Marble with shellac which… actually I’ll wait until the next slide to describe that in greater detail. But that is a shellac covering, and then on the right-hand side you’ve got a traditional wax covering. So, the traditional surface coating such as linseed oil, beeswax and goose grease were often applied to Purbeck. And these surface coatings acted as barriers to the ingress of moisture. But in time, they tended to crosslink or deteriorate and often discolour. So, all of these traditional historic treatment have in common a sort of hydrophobic characteristic to try and exclude moisture as a driver of decay, but of course, continued application built up layers that were then prone to cracking and discolouration. And certainly I haven’t come across any historical evidence of removal of these coatings prior to reapplication.

So, if we now have a look at this process called induration which found great favour in the mid-19th century, driven by George Gilbert Scott’s use of it at Westminster Abbey. I can read that out if you like but it’s basically about his process which was to use shellac. Now shellac is one of those materials you wonder how people ever started using it because it’s actually the sap from a tree which is then digested by the lac beetle and then excreted and what comes out the back end is then crushed up and dissolved in either alcohol or methylated spirits. So, it’s interesting wonder how the first person came across this stuff but it was used extensively as an adhesive in medieval times. And Scott thought this was a very good material to use for consolidating decaying Purbeck Marble, so he applied it using the techniques that he describes. The problem is it looks fine when it was applied but then it goes dark brown. So, all of the nice green, blue tones of Purbeck have now been rendered very dark brown. And also it breaks down and you get little cracks and when you get little cracks then you get moisture getting in behind leading to the decay processes that Angus has described.

Some examples of the repair mortars. Obviously, for those of you who know Purbeck or have seen some of the slides that we’ve already shown, it’s a very complex and interesting surface in that it has all the shells, so, it’s a very difficult material to try and simulate with a mortar and people have had mixed results. And here you can see on the left-hand side some rather grey cement. In the middle, somebody has gone to quite serious lengths to try and get these textual characteristics but the colour isn’t quite right. And then obviously the white plaster of paris on the right-hand side. And many of these repairs, particular the one of the right actually were done with a view to then covering the whole thing or painting it so that it wouldn’t actually be that visible.

An extreme example of this is the scagliola repairs of Purbeck at Canterbury Cathedral and you might be surprised to know that all of the dark elements you can see in that picture are actually the repair and it’s only the light grey which is the original Purbeck. So, that and the scagliola repair actually matched the darkened Purbeck when it was applied, but then when they cleaned the Purbeck, obviously the contrast is now very evident.

A couple of examples from Norwich Cathedral cloisters of the use of Roman cement repairs to Purbeck capitals and bases done with I’m sure good intentions, but perhaps not as artistically talented as we would like. But the Roman cement is very hard, brittle material which has cracked in time. John Bacon, who was the Clerk of the Works at Ely Cathedral in the late 19th century, he did a very extensive series of repairs to the columns at Ely. Again, shellac was at the heart of it, but resin, but this process which again, I won’t read out, but it’s very, very complicated and it finishes off with the paste of spermaceti which only comes from the sperm whale. So, one wonders just how easy this whole process was but it has been actually very successful. You can see very clearly where there is no deterioration of them and they remain intact. But, of course, they have the advantage of being in a stable and dry environment.

There are many examples of repairs to these splitting columns. Angus showed us earlier a couple of examples. And these range from the formal band from Ely Cathedral on the left, to the, I imagine temporary fixing using cable ties to prevent the splitting from getting any further. Of course, splits in Purbeck shafts are visually alarming but the loads are usually shared by a number of shafts. So, the splitting of one may not be a matter of structural instability. But, obviously, further splitting is best avoided and so various techniques have been used in the past and in some cases, the split might just be filled with mortar but often that cracks because the column continues to move, to shear.

The replacement of Purbeck has a long history. As I mentioned earlier, it wasn’t easily available in the 17th, 18th, 19th centuries, so it was often replaced with a different stone. And the bay of Norwich Cathedral cloisters that you can see on the left there had Purbeck shafts in the middle which have now been replaced with Bath stone, which of course, changes the aesthetic appearance. Purbeck was used to create a difference in colour and texture from the rest whereas that’s now slightly lacking. In York Minster, they’ve gone one step further and they’ve replaced some of the columns, actually quite a lot of the columns, with a different stone and then somebody has, with great talent, has painted the Bath stone to resemble Purbeck Marble. Those ones pictured there are in the Zouche Chapel in York Minster and if you’re up there, they’re well worth looking at.

Just a couple of grander buildings… Excuse me, I’ve gone one too far. Wells Cathedral West Front, remarkably given that it’s not very far from Purbeck, they never actually used Purbeck Marble shafts. They were Blue Lias shafts which were replaced with Kilkenny marble in the 19th century. But, not far down the road at Salisbury Cathedral West Front, they were Purbeck Marble shafts, which were replaced with Ashburton marble in the 19th century. So, these have still got this aesthetic contrast of the blue against the creamy colour of the stone.

One other method just to show you is the removal of decayed material and the re-polishing of the surface. These are taken from Westminster and Canterbury Cathedral but you can see, obviously by removing decayed material, you change the shape of the column or in the case of Canterbury Cathedral, the base of the shaft. And then, re-polishing it to try and re-establish that shininess, that polish, but of course it shows up the typography of the surface which is now not as it was originally intended. So, from all of these you can see that there is a long history of different types of repair to Purbeck. And we wanted to make sure you understood those so you can recognise them, but also, it gives us a sort of background to what we then propose in terms of current practice.

And one last example, I think it’s the Walter de Grey monument in York Minster where a number of different methods were brought together by Bernard Fielden. Again, stripping coatings with “Nitromors”, columns were drilled and grouted, they rubbed down the surface with cream grit, rub it down again with snake stone and then polish it with putty powder and oxalic acid crystals . Again, this coherent program of repair has been largely successful but again, we need to acknowledge that the environment in which it stands is generally benign and comparative stable and the greatest challenges for Purbeck occur when it’s in damp conditions as in cloisters.

Clara: Thanks, David. So, just to us an idea, we’ve got another question to ask you about what have you come across in terms of deterioration. We’ve given you an overview, but it would be interesting to know what you have come across.

Matt: And this is a multiple answer question, so you can tick as many boxes as you like.

It’s really interesting to see, environmental conditions are obviously in the forefront there. Let’s broadcast those results, so people can see actually what’s going on. So, environmental conditions is the big leader there definitely.

Clara: That’s really interesting to see, and as we said, it’s the environment that plays a big part in how Purbeck Marble survives really. So, that’s good to know. Thank you very much for that.

So, moving on, I’m just going to briefly talk about how we can look and evaluate the condition of this material, because that is an essential part of the conservation process. Purbeck Marble, as you may begin to gather, occurs in large quantities within buildings as architectural details. Therefore, it is important to have a system to identify and record these elements. So, creating an inventory is a really good starting point. And, so before any attempt is made at repair, it’s really important to understand the condition of the Purbeck Marble which can then be used as a benchmark for understanding any subsequent rate of decay. And the images you can see here, these are from the then English Heritage Polishable Limestone Project which was carried out in the 1990s and this established a protocol for recording the types of decay associated with these types of stone, including Purbeck Marble. And the different structural and surface deterioration patterns can be annotated onto drawings or images. Fear not, you can read more about this in the forthcoming technical advice note on Purbeck Marble.

So, by understanding and mapping the condition, this may highlight the need for further detailed assessment of specific issues and that can help determine conservation options. For example, undertaking analysis of surface coatings will help determine a way of removing it, if that’s what’s required. And as we’ve seen, the environment is a really important factor in this and monitoring the environment can help us understand if these conditions can or should be adapted to reduce the rate of deterioration.

And you can categorise the condition status and this helps prioritise the repairs. So, here, quite simply putting these all into different categories. So, if some Purbeck Marble is in a poor condition or missing, then these are going to have to be addressed quite imminently. And then it allows you to look at all the conditions. They will need to be considered, scheduled and inevitably budgeted for.

I’m just going to hand over now to David who’s going to talk about treatment options for Purbeck Marble.

David: Yeah, thank you Clara. As I hope you will have realised, Purbeck Marble suffers from a number of different problems and there are a lot of different treatment options, but as with most things, we must always start from attempts to mitigate the causes of decay by initiating preventive measures. So, this might be by stabilising the environment but also, if the surface coatings are contributing to decay, then there must be a case for their removal, although, in many cases this may reveal underlying conditions or previous repairs that have actually been obscured by the coatings. So, we also need to stabilise fractures and splits. We need to make sure we’re grouting cracks and fissures, perhaps consider consolidation of the surface, certainly surface repair, possibly replacement and, as Eric Benfield said in the earlier slide, surface protection. So, we’ll just have a look at those in a little bit more detail.

This is an example of environmental monitoring to establish the likely cause of deterioration of the Purbeck base of the monument to Edward II in Gloucester Cathedral. It had been treated in the past but there was a lot of blistering in that treatment and moisture getting in behind and causing deterioration. There’s actually a heating pipe, which you can’t really see in the slide, but it actually runs behind the monument. And you can see the change, the fluctuations in humidity you have during the winter and in mid-April when the heating gets turned off, the humidity changes and starts to fluctuate less, but at a higher level. So, it’s understanding these sort of things that are happening will help to try and mitigate problems. Of course, it’s very difficult for Purbeck that’s in cloisters which are notoriously prone to condensation, but we can only do what we can.

So, cleaning of surface coatings is tricky because there are so many different types of coatings. So, of course, trials are essential and these ones at York Minster, you can see a column before cleaning and then with the trials in place and then after it had actually been cleaned where you can again see the blue/green colour of the Purbeck and subsequent repairs. But, these would have involved a number of different poultices, mainly carbonate, latex, EDTA. In general, water based methods would be avoided, as we’ve seen how much moisture can cause damage to Purbeck Marble, so we need to make sure we’re using as little water as possible in order to avoid it initiating other decay processes. And this was the case in Salisbury Cathedral where ammonium carbonate poultice was used and then a V&A mix, which is a mixture of water and white spirit, in a sepiolite poultice. And that was specifically targeted for the type of coatings that’s on the surface. But the split in one column opened up when water was applied to remove the poultice, so even quite small amounts of water can be a problem. So, as a result, the cleaning then adopted was a method using no water which was micro air abrasive.

So, water based cleaning should be avoided and therefore actually, the choice of other methods are restricted because of this water issue. So, even a poultice that requires a lot of rinsing will potentially be a problem. So, we try and use minimum water, and that can involve microporous sponges which are surprisingly effective. Dry steam cleaning, which is pressurised steam at a temperature above 150 degrees. This is not doff or thermatech cleaning which is superheated water, this is superheated steam. Very small amounts of water involved. Micro air abrasive, of course. EDTA poultice usually in the form of a natural or synthetic latex, again to reduce the amount of water required. And if you’ve got wax, of course, hot air guns and solvents can be used.

Securing fractures and splits. This continues to be a difficult one to do because you might say why we don’t just drill through them and pin across the split. The problem with that is the marble is very brittle and so the actual drilling process can cause damage. And also, if you have any subsequent movement in the shaft, by having a rigid pin in the middle, it can actually cause further fracturing. So, apart from grouting a fracture, a useful method for securing the fracture is the use of very high tensile strength of carbon fibre tape, which is what you can see here. It’s secured to itself using resin and actually it can then be disguised by applying an acrylic resin to the surface and applying stone dust while the resin is still tacky. So, certainly from a couple of metres a way it can be almost indistinguishable and yet is very strong and will hold a split for certainly in the short and medium term.

Small scale grouting of surface fissures and voids, of which there are usually quite a number in decayed Purbeck. And again, materials like Paraloid, which is an acrylic resin, have been sued in the past, but we are now advocating a better method, we think, which is the use of dispersed lime injection mortar mixed with a fine pozzolan such as trass. This is specially designed to carbonate even when applied very thin. It’s pure putty lime, with a small amount of [INADIBLE] to encourage the flow of the material. And because the dispersed lime is so reactive, it is able to carbonate even if applied 1mm thick. So, this seems to be a good way and now there are quite a number of places where this has been used very effectively to secure small spores on the surface and fill little surface voids.

Surface consolidation. This is a very complex area. Many consolidants have been trialled, and to be honest, none of them have really shone out as being the answer and a lot of them, the trials have gone so far and then they’ve stopped being reviewed. There were a lot of trials being put on the roof of Ely Cathedral but they then disappeared. They were unable to follow that up. But you can see on the left, there’s a whole list of trialled consolidants and the sharp-eyed amongst you will spot that nanolime appears both on the left-hand side as a trialled consolidant but also as another material requiring further investigation because we don’t think we’ve really understood what it can do. So, surface consolidation is a difficult one. There aren’t any easy answers. Nanolime seems to be effective at near-surface consolidation. Ammonium oxalate has been used on other types of marble very effectively, particularly surfaces suffering from sulphate deterioration. Ethyl silicate has very good penetration but maybe has the advantage of residual hydrophobic qualities, but there are possible problems at the interface between the treated and the untreated.
Mortar repair is principally intended to protect decayed areas and their secondary purpose is to build up missing detail. In both cases, a close behavioural and visual match to the Purbeck Marble is desirable. And so, most recent works have been using lime binders mixed with an aggregate of marble dust, crushed Purbeck, sand, sometimes with pigments. But it is very difficult to simulate the complex characteristics of Purbeck Marble. The lime binders tend to give a mortar colour which is lighter than the darker tones. When you crush up Purbeck Marble, it tends to go paler and lose its colour. There are examples of resin binders which can be very accurately colour matched but then you have a weakness at the interface between the repair and the original. Mortars, also with casein additive which can actually be polished, have been used, particularly at Salisbury Cathedral, and these are hard to apply because they are viscous when wet but actually can make a very [INAUDIBLE].

So, we can see there, just a detail of a string course at York Minster which has been protected and repaired. And here you have a range of different mortars and we’re not saying particularly any one of them is the right mortar but it shows they all have a similarity but try and get to the end result in different ways.

And so finally, we look at surface treatments and they are largely, again, subject to trials, as you can see in the middle there, from Canterbury Cathedral, where those trials have been made. There is still traditional use of beeswax. There are casein coatings that have been used. Again, not perfect in damp conditions because you tend to get microbiology growth and mould. Microcrystalline wax, I think, is the one that is most widely used. And that, of course, is a generic term, and many of you may have come across it as cosmolloid or renaissance wax. Again, important to know what’s in these things. Renaissance wax is actually a mixture of microcrystalline and paraffin waxes. But these are applied, and as Eric Benfield said all those slides ago, if we can keep the moisture away and out of the Purbeck, then we are halfway to stopping it decaying. I think that’s me done. Thank you.

Clara: Thank you, David. We’ve just got one more question for you just to ask you a little bit about what’s your experience of repairing Purbeck Marble. If you’ve had that opportunity, what have you used? I think you can tick as many which are applicable.

Matt: Yep, this is multiple answers again. Also, we are at 2pm now. So, I know we are going to run over, for which I can only apologise to those of you who have to leave us. Do remember that this session is recorded and obviously that recording will be made available to you via our website after the session. Hopefully we can finish up at about quarter past I think, the way we’re going at the moment. But, for those of you can stay with us, we very much appreciate your company.

Clara: Of course. That’s really interesting. So, yeah, a lot of people have actually dealt with Purbeck Marble, consolidating it. So, that’s really interesting and quite surprising actually. I hadn’t expected that. And then, cleaning and mortar repairs. That’s very interesting and surprising either. But the consolidation is very interesting. So, perhaps if we get time, it would be really useful to know what sort of consolidation methods maybe people have been using. But anyway, we’ll press on and try to get to the end so that people get that opportunity. So, I think now Angus is just going to give us an overview of the project at the cloisters in Norwich Cathedral.

Angus: Thanks, Clara. Yes. Built over a 130 year period between the 13th and 15th century, a total of 49 bays form the cloister at Norwich Cathedral and the architectural style ranges from the geometric to the perpendicular. As you can see from this slide, a variety of different stones were used in the construction, with the shafts, capitals and stoolings along with the benching floor slabs all originally of polished Purbeck Marbles. But, over the centuries, these have progressively weathered and failed, often leading, as David mentioned, to replacing them with Bath and other limestones. Where Purbeck elements have been retained, these have often been restored using Roman cement and other mortars with detail remodelled to varying degrees and with varying degrees of accuracy.

Now, due to concerns over the overall appearance and rapid deterioration of surfaces within the cloisters, trials were carried out in 2006 focused on one bay which displayed typical deterioration traits, not only of the Purbeck but other materials too. What these trials included were cleaning using the Torc abrasive machine in conjunction with poultices of ammonium bicarbonate and sodium dithionite to try and stabilise the decay of the iron pyrites. We have to always bear in mind that the cloisters at Norwich are open, so they have that special, challenging environment that David mentioned. Consolidation using Paraloid B72. Again, Paraloid being an effective consolidant but not necessarily is going to stand the test of time in a damp environment. And repairs using lime putty mixed with various aggregates and pigments. And then an applied coating of cosmolloid wax. What these trials did achieve was that cleaning, retention and conservation of surfaces including the Purbeck elements was both desirable and achievable.

Now, with the development of a clear written conservation policy, the Cathedrals Fabric Commission of England agreed to a new set of trials which would then set the benchmark for future conservation works and which could be applied to the cloisters as a whole. And these works, carried out between 2015 and 2018, focused on five bays in the southwest corner and included cleaning, consolidation and conservative repair of all surfaces. Cleaning to remove layers of discoloured surface coatings and soiling, to restore some of the qualities and character of the Purbeck elements, were subject to trials including the use of Monumentique paste. So, that’s one of the EDTA based latex treatments that David mentioned. Monumentique has a PH of 7. And this was applied by brush to a pre-wetted surface and left to dry. It was then agitated with soft nylon brushes and then any residue was rinsed off with the dry steam.

Repairs included the use of dispersed limes to support detaching material and putty lines to fill voids and fissures. Extensive trials were carried out to create mortars which could sit well with the deteriorated Purbeck surfaces. And you can see here that you have the mortar applied and then subsequently, the surface is finished using dried pigments and I’ll show you how that’s done in a slide in a minute.

Where Roman cement repairs were failing or causing accelerated decay of the Purbeck, these were mechanically removed. Ceramic armatures were inserted, as you can see there, and the repairs built up in layers. The overall form of the element was reinstated with clear reference to the surviving adjacent Purbeck Marble and the top surface textured using hand tools. So, you’d wait for the top layer of the lime to set up, to harden slightly, and then you’d very dexterously attack it with your hand tools to create that weathered surface.

Now, in order to try and simulate the complex surface colouring of the original Purbeck, small amounts of mixed pigments, ochres, umbers, were applied to the wet surface, and we were blowing them from a spatula. And as the mortar carbonates, the pigments become fixed to the surface, somewhat akin to the fresco process. You can see my colleague there blowing these pigments from his spatula onto the damp mortar surface.

So, this shows the completed repair, and if I could just use my pointer, you can see that this is the original, surviving Purbeck and this is the mortar repair. And this is some of the only surviving, original top surface of the original Purbeck. Over to you.

David: Ok. Thanks, Angus. So, finally, and I appreciate we’re running a bit late here, but finally, options for replacement, and this has been a difficult issue for a long time. There’s a lot of Purbeck stone still be quarried which is used for new flooring but there’s only one source of good quality Purbeck Marble run by Haysom Quarries. It was due to open again in the spring this year but I think probably that was put off for obvious reasons. It has a nice greeny-blue Purbeck Marble with quite a limited bed height. The problem with a replacement is obviously, you want to get a similar colour but also, although with all stones, there’s quite a contrast between new stone adjacent to decayed stone. Purbeck seems to be starker than other stones and of course, the image you can see there is, I think we’d agree, not terribly successful and these ones are perhaps better. So, you can see a newly turned capital and column on the left but with a worn base and then on the right, a very successful piecing-in of Purbeck Marble. So, it’s definitely an option but the supply is quite limited and of course, when you have new Purbeck Marble, again, to repeat myself about Eric Benfield, it needs to be waxed and then looked after. So, I think that’s… We’ve put together a few reference books which you might like to look at.

Clara: I think there’s a few you your favourites there aren’t there, David?

David: There are, yes. I should also mention that the Trev Haysom. No-one in the world knows more about Purbeck Marble than Trev Haysom and his book, which I think has been in preparation for quite a few years now. It was due to come out this summer, but I suspect again, it might have been pushed forward a bit. Eric Benfield’s book is a fascinating personal account and social history too, so it’s very interesting. The life of a Purbeck stone worker in the 1930s.

Clara: And there is also some online information there, isn’t there? I plug again, the Strategic Stone Study. But there are a couple of others specific to Purbeck Marble there which are of great interest.

David: Good. OK.

Clara: Lovely. OK. And just to finally mention… Just to remind you again that our Technical Conservation Guidance and Research catalogue is available and, as I mentioned, the Purbeck Marble Technical Advice note, which includes much of what we said here, and more, will be available in the next few weeks we hope. And if you’ve signed up for the Technical Conservation email list, you will be informed of that. So, thank you for listening. Thanks to David and Angus, our fantastic presenters. It is nearly quarter past the hour, if anybody has any burning questions. Matt, do we have time for any questions?

Matt: Oh, we’ve always got time, but it’s a matter of whether or not our guests have time. So, feel free if you have questions, do type them into that little window there so we can have a look. I did notice a question in the chatroom here. It says, “Isn’t the post-repair weathering of replacement sections contrary to normal practices for replacement stone?” What say you to that?

David: Could you say that again?

Matt: Yes, of course. I’m probably getting the inference wrong but, “Isn’t the post-repair weathering of replacement sections contrary to normal practices for replacement stone?”

Clara: I’m trying to understand what that question actually means.

Matt: Joe, if you could maybe elucidate slightly in the chat if you’re still with us and we’ll come back to that one. We’ve got some questions here. “Could Derbyshire fossil marbles be used as a substitute/replacement for Purbeck Marble – i.e. Sheldon Moor or Mandale Limestone?”

David: I think the simple answer is yes, but Purbeck has a pretty unique appearance so you are going to obviously have a different aesthetic value perhaps. I don’t know those stones well enough to be able to say whether they’re similar in physical and chemical characteristics. But I think it’s important we always go through the process of considering whether it’s something that needs to be replaced and then if it is we then have to try and find what is the best replacement stone. And for flooring, I mean, there are Purbeck stones that are quite shelly and might make an adequate match for Purbeck Marble, but the simple answer is yes, I would have thought so, but you need to go through the usual processes of just looking at it, seeing what its characteristics are etc.

Clara: Yeah, I think the key is identifying what you’ve got and then you can make your selection of what would be suitable replacement. And it is very important to identify your stone and Purbeck Marble is a case in point, because there are other polishable limestones around the country and it is very important to make sure that you are identifying those correctly. Somebody has asked about recognising Purbeck Marble. I think, as we’ve sort of shown haven’t we, that it’s got a very distinctive patterning and these gastropods that you see very clearly within that is quite a key part, isn’t it?

David: The problem is that so much of it is obscured behind these coatings. So, sometimes you have to have a little, I don’t want to encourage people to go scrapping away, but it’s important to try and see what the stone is behind the coatings because it can be completely disguised, whether it’s shellac or waxes or whatever. So, often it’s not completely obvious it’s Purbeck, but once you see the stone’s surface, then it is completely unique.

Matt: Interesting comment here: “I could not understand why the experience from Norwich Cathedral is not transferable to a larger scale project?

David: Again, I think those initial trials that Angus talked about, and forgive me Angus, I’m just going to answer this because I was involved at the time. But the slide that Angus put up was a very simplified version of what the trials involved and it was actually a very complex process and a very slow process. And it was basically, although technically it would have be transferable to a larger in terms of time and cost, it was just not sustainable. The methodology and detail was more akin… it would have perfect if you’d had a museum object that you were working on, but when you’ve got 49 bays of cloisters to do, it wasn’t going to be possible to do that.

Matt: OK.

Angus: I think also the 2015 to 2018 trails have set a benchmark for transferring it to all 49 bays if possible. But again, it’s all time and [INAUDIBLE].

Matt: OK. Acutely aware of time and I know that we’ve got some more questions coming in. If we’re not able to answer them, I will make sure that we answer them outside of this session. So, apologies for the shoddy timekeeping on my part. One question here: “Do you have any experience in using other surface treatments such as polysiloxane for Purbeck Marble to protect surfaces exposed outdoors? These coatings are more durable than waxes”.

David: Again, yes, they are more durable than waxes. I’m not aware of any trials that have been carried out with the polysiloxanes on external Purbeck. The problem is that most external Purbeck already has a rough texture and I think the problem with any coating is making sure that you actually coat it effectively with all that roughness to the surface. But, as I mentioned, the consolidation and indeed the protection is something which we’re sort of…We’re somehow guided by the experiences of the past and trying to introduce now new materials runs the risk of is it going to work in the long-term. So, it’s a difficult path to tread, and what we’ve try to do in the advice note and in our advice here is to sort of steer a path which is…We can as far as possible know it’s going do no harm. And I think that’s the crucial issue.

Matt: OK. Right. I’m going to , with great sadness, bring this session to a close. I’d have been quite happy to listen to you guys on this topic all afternoon.

Angus: You say the sweetest things, Matt.

Clara: Be careful what you wish for!

Matt: What can I say? Purbeck Marble Number 2, go on then! Why not? So, to Clara, Angus and David, thank you so much for your time this afternoon. Absolutely fascinating presentation.