Conserving a Cathedral

Oliver Caroe of St Paul's Cathedral discusses how he looks after one of London's most famous landmarks...
02 May 2013

Interview with 

Oliver Caroe, St Paul's Cathedral


The conservation and restoration of great art once relied only on a good eye and talent with a paint brush.  Now though, scientists and art conservationists are working together to develop new techniques to preserve our cultural heritage.

In 2011, one of the most iconic buildings in the UK - that's St. Paul's Cathedral completed a 15-year 40 million-pound restoration project to remove centuries of London grime from its walls. State-of-the-art techniques were required to restore that building to its former glory and to find out more, we were joined by Oliver Caroe who's the current surveyor of the fabric at St. Paul's. 

Ginny - Now Oliver, buildings like St. Paul's seem very permanent, but what actually happens to them over time? An aerial view of St Paul

Oliver - Well, they're made of stuff. They're made of stone and wood, and metal, and all of this stuff in itself intrinsically has a life span to it. But it's in an environment and the environment can intrude on how it behaves rather aggressively and of course, there are many enemies to old buildings. But water is our main enemy, dirt and other. And then there is the sort of the chemical soup of the environment generally in the atmosphere. But of course, we mustn't forget the people are a great enemy of buildings and they get up to mischief, they damage. But they also damage with good intentions and we have to watch out for that too.

Ginny - So, what's St. Paul's actually made of?

Oliver - Primarily, Portland stone, but much else besides and that's a long and complicated story and many books have been written about it.

Chris - What actually is Portland stone?

Oliver - Calcium carbonate, it's sea shells settled on the seabed, compressed and made into a lovely homogenous matrix of a very enduring and very durable stone.

Chris - But susceptible to acid attack.

Oliver - Very much susceptible to acid and a great irony of course that St. Paul's Cathedral was built and funded by the attacks on coal coming in to London. And that coal actually, not only creates acid rain, but soils the cathedral. And Christopher Wren in his own day had the cathedral cleaned because he recognised the damage caused by coal.

Chris - How quickly does grime build up and how thick is the stuff that gets deposited?

Oliver - That's an answer that science has to tell us. Old age is beautiful and I think one mustn't sort of dodge away from that, we love things to look old. But there are encrustations in St. Paul's Cathedral which are literally inches thick and they are quite beautiful. One mustn't ignore that and that they are like frozen black waterfalls of grime even like exquisite folded gossamer fabric. It is exquisite, but...

Chris - Who would've thought that the dirt could be regarded as kind of an added value.

Oliver - Well, it doesn't sort of play on radio, but it is something that you have to see.

Ginny - So, if water is bad for limestone, then how did you go about cleaning it?

Oliver - Well, there are lots of methods and in the past, water has been used aggressively and literally flooding the surface of the cathedral. Back in the 1970s and '80s, that was done extensively and you get some rather nasty unintended consequences. What we do now on the outside of the cathedral and in the last programme that my predecessor Martin Stancliffe led heroically really for 15 to 25 years in fact, they used microabrasives and very, very carefully, just slightly wet abrasives are being used. And then there are lots of other techniques besides.

Chris - But you said that water was bad. Why was just washing it bad? How about every time it rains?

Oliver - Stone has a skin just like you and I. If you sit in the bath and scrub yourself in water - actually, it's not very comfortable if you do that for too long. So, exfoliation has its demerits, but stone doesn't regrow its skin like we do. So, you can actually drive water through the surface of the stone, it loses its hardness and that then leaves sort of soft material behind. But also, what can happen is you get water into the joints of the stone which goes into the core of the wall, and the middle of the wall is lots of rubble held together with lime mortar, and you don't know where it's going then. So, you have to be very, very careful.

Ginny - So, what kind of techniques can you use that don't involve water?

Oliver - Well, on the inside of the cathedral, we used a piece of chemistry - a really quite exotic piece of chemistry and a material called Arte Mundit which is actually rubber, slightly alkaline rubber that was sprayed onto the cathedral and at a very special St. Paul's formulation. And that again, it's very beautiful, the way you do it and it stinks, but you apply it on like a poultice. You leave it for just under 24 hours and then you can peal it off and you can literally see the dirt peel off.

Chris - This is just what you do in a beauty parlour. I mean, not that I'm speaking from experience. Maybe you Ginny, I don't know, but I've heard that people apply these things to their skin and then sort of peel them off and all the kind of gunge comes with it. It's sort of similar there.

Oliver - It's exactly like that and that there is a lovely analogy between sort of makeup and sort of health treatments, and this process, but I think...

Chris - How do you know it would do no harm then? I mean, that's the key thing, isn't it? I mean, you've said here, "It's critical to do no harm." So, how do you know this isn't harming the stone?

Oliver - Well, just a stretch in the analogy with beauty treatments, you wouldn't do some things yourself if you knew it might do you harm.

Chris - Well people have breast implants all the time, I mean, to make a point. I mean, that people do do cosmetic things and actually, they do turn out to have a sting in the tail.

Oliver - And they're not reversible at the end of the day. We have to enter into this world, I think firstly with a philosophical framework and that in a sense is a branch of history of art. But I think most people will have heard of William Morris and the sort of basic intention that you don't go and muck around with all buildings because you lose something every time you do that. And then in the 20th and 21st century, we have lots of international charters - the Burra Charter, the Icomos Charter - these are perhaps a bit obscure, but they sort of try and establish a philosophical framework of why we do things to old buildings.

Chris - But when you buy say, a touch-up kit for your car, it always says, "Try this on a small area first." Do you have a small area of St. Paul's that's your sort of test area when you come along with your latex mask, and think, "I'll just try this here" and if it does make a bit of a mess, it's minimised the damage rather than comprehensively doing a 40 million quid project all at once?

Oliver - Well, I think some misadventures of the past, we strenuously tried to avoid in this latest campaign by doing endless trials and there about 10 sets of trials, all carefully reported and examined by many experts and the people who give permission for these works to establish that actually, we do no harm. I think that has to be our first priority, but any intervention has some sort of long term ramifications, so we keep an eye on things. We also think it's very important to keep records of what we did because there's nothing worse than going back and saying, "Well, we think something happened here, but we don't know." And then you can't track the impact of that.

Chris -  So, is this sort of an on-going project now with the latex cleaning. Is this something that you will then, a bit like the Forth Bridge - you're basically starting as soon as you finish - so you just keep on going doing these peels to keep the surface clean or is this just something you're trying out?

Oliver -  No. This is now being completed and actually, one of those very bold things at the cathedral. It is under Martin Stancliffe is they said, "If we're going to do this clean, we've got to do it all. We can't leave the cathedral half clean." That would've been awful.


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