Solving the case of the Mercat Cross: conserving one of Edinburgh’s most important monuments... by Luis Albornoz

Mercat crosses were once known as sites of announcements and news, markets and trading posts, but they also have a murkier past involving punishment and executions.  Many now stand weather-beaten on high streets across the country and the Mercat Cross in Edinburgh is one of the most famous of these stone constructions.  Luis Albornoz tells us about the work he has been doing to help preserve these monuments.

Luis Albornoz, Building Stones Scientist at BGS

Hello.  I am Luis Albornoz, a Building Stones Scientist here at the BGS in Edinburgh.  I have been part of the Building Stones Team for over 15 years now.  I came to Edinburgh after finishing my Geology degree at Oviedo University in Spain, where I specialised in Applied Petrology for the Conservation of Monuments.  In search of building stone and rubbish weather (which would contribute to the weathering of such stone!), I came to Scotland and joined BGS in 2001.

The drive behind the creation of the BGS in 1835 was to understand the geology in order to find and extract the geological resources that would contribute to the building of a country.  This would help Britain lead the way into the Industrial age: coal, mineral ore, water and, of course, building stone.  Stone has been used in the British Isles for over 6,000 years and our predecessors at the BGS were very knowledgeable about its sources and qualities.  It’s a nice historical connection that the Building Stones Team helps look after the built-heritage those predecessors found the materials for.

The Mercat Cross

Mercat crosses (‘Market’, in Scots) were built in towns and villages originally as sites for regular markets where merchants could trade.  They also acted as a place where people would gather to hear important public announcements such as royal and parliamentary proclamations, a practice that continues to this day.  

Another wet winter day at the Edinburgh Mercat Cross.

These crosses were also used for a much darker purpose: punishments and executions.  The iconic Mercat Cross in Edinburgh is no exception and people accused of minor crimes had their ears nailed to the door for at least a whole day.  Following public shaming and flogging, their fellow citizens were encouraged to spit and throw rotten food and insults at them.

The current Edinburgh’s Mercat Cross was built in 1885, not far away from the site of the original cross (from 1365), and hopefully saw more enlightened times than the medieval one.  The cross, although damaged by weathering and pollution, as well as by previous human errors in its conservation, it is still in serviceable condition and stands proud.  Over the last few decades, decay seemed to be accelerating and this was of concern to The City of Edinburgh Council.  There was a risk that fragments of decayed stone could fall on the multitude of tourists that gather and rest around the monument throughout the year. 

Nowadays, the witches get to explain the history of the city and the cross itself,
rather than being burnt at the stake at the site

So, who you gonna call when you need advice from people who really understand stone?  You call us, the BGS Building Stones Team! And in early 2017, Edinburgh Council asked us to help with the Mercat Cross. Here is an interesting, multifaceted project, that involves geology, as well as chemistry, philosophy of conservation, history, sociology (public perception is definitely a big one nowadays, isn't it?) and of course, architecture – everything that has to be considered when dealing with such an important monument.  However, to understand its problems, we need to know much more about the monument itself.  I shall try to condense the essentials of a complex project and focus on the most interesting parts only...

First, geographical location: Edinburgh is a very rainy, cold city, which gets frosty winters.  Its location in the Royal Mile means that as well as exposure to a lot of rain water, it also endures funnelled winds coming mainly from the West.  It is in a very public pathway that sees hundreds of thousands of pedestrians around, so in winter there is a lot of salt gritting. In summer, many of those pedestrians stand and sit around it.  Also, it is important to remember that Edinburgh was historically one of the most polluted ‘modern’ cities, as its nickname “Auld Reekie” (The Old Stinky) implies, and the blackened walls of non-cleaned old buildings still testify to this day.  Acidic rain would have been common for most of the history of the cross, until recent years in which we breathe cleaner air.  All this, as you can imagine, has important consequences.  More on it, later.

The monument itself has an octagonal plan.  A wooden door connects the ground floor with the upper platform by stone steps by means of an open hatch, which is exposed to the elements.  Through this (and down those steps), rainwater runs down towards a gutter hole.  At platform level there is a parapet with eight round medallions, which have carved and painted heraldic carvings.  A shaft rises through the middle of the structure to a height of over eight metres, and has a painted unicorn on top (suspected of sandstone, but for now of unknown material).  Some parts of the structure are double walled.  Others, single walled.

The next thing we looked at was the type of stones in the monument.  This one was relatively easy, as there were records that stated their origins in Hermand Quarry (West Lothian), Hailes Quarry (Edinburgh) and Darney Quarry (Northumberland, for repairs), as well as at least a fragment of stone from the original cross from 1365.  It was up to us, though, to figure out which part of the structure was built from which stone (with the constraints of minimal sampling), and prove or disprove the records; but particularly, it was up to us to understand the sandstones, as one type in particular was decaying faster than what was consistent with the age of the monument. 

By visual inspection, it was clear that this accelerated decay was concentrated around mortar joints and more permeable beds in the blocks of stone  .  Furthermore, only one type of stone was affected, and this one, only in the exterior of the blocks.  Around those joints, the stone was disaggregating  noticeably.

Example of a carved rose, completely disaggregated. It was removed due to Health and Safety issues

Decayed mortar joints, shown as disaggregation of the stone along the joints

Decayed mortar joints, shown as disaggregation of the stone along the joints

To start unravelling the mystery, we obtained from an inconspicuous part of the monument a representative sample of the stone that comprises the bulk of the structure, which is Hermand sandstone.

As the other sandstones in the building were not suffering from severe decay, I am going to ignore them for the purpose of this blog.  Each have their own, unique stories, given in first instance by their petrographical characteristics (what the rocks are made of), followed by their function and by their location in the monument. In the stone map of the monument, you will see in grey colour the Hermand sandstone. 

Sample and thin section from Edinburgh's Mercat Cross

Stone map: Grey, Hermand sst. Red: Hailes sst. Purple: probably Darney sst. Yellow: Darney or Hermand (not sampled). Green: fragments from original Cross from 1365. Unicorn is of unknown material. Original drawing by I. Ramsay, stone types overlay by L. Albornoz.

To confirm the records, we created a thin section of the sample from the Mercat Cross and compared both the hand sample and its thin section to the hand samples and thin sections of Hermand Quarry from our own BGS building stone collection. You can see how similar the two are to each other, confirming the origin of the stone in the bulk of the monument as Hermand.

Thin section images of sample ED7583 from Hermand quarry (left) and sample ED11697 from Mercat Cross (right).  The images were taken in plane-polarised light, the field of view in each cases is c.3.3 mm (top to bottom).  The close similarity in minerals and texture leaves little doubt they are the same building stone

When fresh, Hermand sandstone is a strongly cohesive, brownish-grey sandstone.  Its texture, as reflected in the blocks of the monument, varies from uniform to bedded (bedding given by differences in grain-size).  The grain size is in general fine to medium-sand-grade. 

Under the microscope, it is a poorly sorted sandstone, composed of about 40% quartz grains, 25% rock fragments and 7% feldspar, as well as smaller proportions of iron oxides, clay and carbonate minerals.  Its petrological classification is a lithic-arenite meaning it’s a sandstone with a large proportion of rock fragments in it. Total porosity (pore space between grains) is 14%, and its permeability (how well connected those pore spaces are to each other) is moderate-to-high.  These factors control how water can move through the rock.

The puzzle continues

I mentioned before about the city's history of pollution.  Stones, by means of their complicated porous system, behave a bit like sponges.  They absorb water, but you cannot squeeze it out. So very often, what goes in stays in.  Think of the 'cocktail' of chemicals that runs through its porous system: sulphur from coal burning, nitrogen oxides from exhausts, salts of different origins (chemicals left from previous cleaning, salt gritting), fly-ash particles, oils, pigeon poop, reveller's urine, humic acids (caused by biological colonisation) and suddenly, a stone seems more 'frail' and susceptible to decay, doesn't it?  Yet, much of the parapet was in good condition.  Only the joints (as mentioned before) and the carved elements were decayed beyond what is natural (which is to be expected in the latter, due to a combination of the distress caused to the stone while carving it, plus more specific area exposed to the elements).  Other parts were only moderately affected by natural weathering. So yes, surely all those chemical elements mentioned before had a role to play in the alteration of the stone in the monument, but that was not all.  There was something else!

Water management in the monument was poorly designed from the start, with useless water spouts and badly angled gutters that accumulated water, rather than draining it out.  The vertical drains run through the inside of the parapet-blocks through holes in the stones, and at least one was clogged. And let's not forget the open hatch in the platform, leading to the door.  A lot of water would go inside the monument, both inside the stone blocks, and inside the innards of the monument itself. 

In the 1970s, repairs to the monument were carried out.  Part of the shaft was replaced with sandstone from Darney Quarry, and the unicorn was re-painted.  But it seems that realising that water was one of the agents that contributed to the decay of the stone, most of the repairs were to do with fixing the water management.  They tried to do this in two ways:

Cracked bituminous layer, Mercat Cross, Edinburgh

1. by adding a 1 cm bituminous layer covering the platform. This probably did good for as long as it lasted. But when doing the survey, the layer was cracked, with cracks up to 2.5 cm wide in places all around the inner perimeter of the parapet, as well as right through it. If anything, by now this cracked bituminous layer was collecting and focussing the water towards the exterior walls and the east side. This would explain why the walls felt wet to touch. 
2. They also raked out part of the original mortar (about 10cm from the exterior) and filled it in with a new one. This ended up being in great part the key to the mystery.... Keep reading!

When you are a building stone scientist, you need to understand key concepts of architecture.  The concept of 'stonework' as used in architecture refers to the stone blocks, the joints and the mortar, all working together.  The 'pore system' in stonework, through which moisture and air move, includes pore spaces within the stone, within the mortar and in gaps between the stone and the mortar.  

Therefore, at the Mercat Cross, we looked at the existing mortars, in order to understand them and see how they affected the stonework in general and the stone in particular.  The function of a mortar is to be more permeable than the stone and therefore conduce water out of the structure by the more porous and permeable system of the mortar, along the joints, in a controlled manner. 

There were three types of mortar visible in the Mercat Cross.  One other type of mortar, the original one used to build the Cross is not visible, but suspected to exist still in the insides of the monument. We will come to this one later, as it is important.  All the three visible mortars looked 'modern'. Two of them looked 'al-right', and seemed to be performing correctly, so I will ignore them for the blog. But the third one was different, and did not look right: it looked too much like a 'putty', like a 'sealant'. Also, it was the mortar used for the external part of the main structure, made of those Hermand sandstone masonry blocks whose joints were suffering accelerated decay. Something was going on here... 

By doing a bit of historical research, I came across this text: “The bed joints and the 'pier' were routed out to a depth of 4 inches (10cm) and packed firmly with resin-bonded mortar, forming what was effectively a ring of glue round the joint”.

I could not believe they created a net/ring of sealed joints!  What should be the pathway for water to get out of the stonework gets blocked in the last 10 cm; yet the water still has to somehow get out. And here is what happens, and the key to understand the problem of the strong disaggregation around the joints: the water that cannot escape because of this  blockage.  Therefore, it gets forced instead through the porous system of the stone, sometimes concentrating around more permeable beds, but specially forced through the edges of the stone blocks, away from the joints between the blocks where it should naturally flow.    

Clean water alone is enough to cause great harm, helped by frost-thaw cycles. But remember when I told you about the pollution in the environment and the 'cocktail' of chemicals circulating inside the stonework?  That's what's flowing through and coming out of the stone.  And you can see part of it in the form of salts, which crystallise during warm periods as efflorescencences on the surface of the stone blocks.  We took samples of those salts and our friends at Historic Environment Scotland analysed them by X-Ray Diffraction.  This analysis revealed the ultimate surprise: amongst the expected (but still damaging) salts such as halite (from gritting) and gypsum (from reaction between acid rain and carbonate minerals within the stone or from the mortar), there was also a salt called hexahydrite. Hexahydrite is a magnesium salt that is known to be extremely damaging to masonry, due to the high volume increase between its hydrated and dehydrated forms.  Its origin is likely from reactions between the fourth (and non-visible) mortar still inside the monument (the original mortar from when the Mercat Cross was built), and its interaction with acidic rain.  Often old mortars were richer in magnesium than modern ones. 

As you can see, a rather complex case where many different elements (geography, climate, chemistry, petrology and bad conservation practices) come together to cause accelerated damage to a monument… 

As conclusions to our report, we provided a set of technical recommendations on how to deal with all the issues above.  Adding all our recommendations would make this blog even longer (!), but they included desalination (one of the most important steps, in our opinion), how to improve the water management issues and of course, the removal of the sealing 'ring' and replacement of the putty-like mortar with a mortar that would allow the flow of water out of the stonework the correct way. We also offered advice on how to protect the stone (suggesting led flashing, not any kind of 'waterproofing'), on minimal-intervention cleaning techniques and on the use of biocides (better not to!). 

We presented these recommendations as a program of tasks, including long-term monitoring. We suspected that there would be time and economic restrictions that could mean that part of those recommendations were likely not be followed. Still, our guidance was thorough enough that by reading it, anyone involved in the conservation and upkeep of the cross would understand what was best for it and then work within any constraints they had. We advised on the possibility that a change of conditions brought by the conservation repairs could bring unexpected, undesired changes on the behaviour of the decay. Hence monitoring this is important. 

We also provided an interpretation of the geology of the quarry, and the suggestion that being still accessible, new stone could be obtained from it to make any stone repairs completely like-for like. In case they were not able to do so, we provided advice on what stones from currently open UK quarries better matched the Hermand sandstone.  We provided, as appendices, a thorough photographic condition report element by element, with a degree of deterioration attached to each. A further appendix added photographic evidence of points of ingress and egress of water. 

We handed the report to Mr.Ramsay, the Architect of the City, and he took care of the rest.  

The cross during and after its conservation

The Mercat Cross was covered for months under a huge scaffold. I was lucky to visit the site while they were working on it.

I was well impressed to see that the 'plastic' repairs done to the carved parts are looking extremely good, almost unnoticeable from the real stone. 

Example of a ‘plastic’ repair, where the original stone element was restored with conservation mortar.
Would you be able to tell the difference between stone and mortar?  

The coats of arms are looking fabulous, and the stone, although it still looks similar in appearance to how it was (thanks to a well managed, minimal intervention cleaning – respecting the Cross' history) now feels dry. The putty-like mortar has been removed and the new mortar between the blocks appears to be doing its job properly. A new bituminous layer has been laid, hopefully with the correct angles so water will flow away the right way, and not through the stone. 

The new bituminous layer, Mercat Cross, Edinburgh

For now, the Mercat Cross definitely looks like a happier, healthier monument than it did before and I am sure that these repairs will contribute to this important monument lasting much longer, and in better condition than it was for the last few decades. An excellent job by the architect, the stone conservation specialists and the contractors who collaborated in its conservation. 

And to top it all, I was able to look directly into the eyes of the formidable beast itself… 

Facing the unicorn