Wednesday, 16 January 2019

Inorganic Geochemistry in Kenya Part II…by Olivier Humphrey

Job, David, Olivier and Doreen outside the Biotechnology Labs,
University of Eldoret
I was recently involved in field work aimed at assessing the micronutrient status and monitoring exposure rates to potentially harmful elements in western Kenya. Michael Watts, Andy Marriott and I visited the University of Eldoret, Kenya for a 16 day fieldtrip that would consist of us collecting environmental and human biomonitoring samples from over 90 households in western Kenya. For more information see the earlier blog: Geochemistry and Health in the Kenyan Rift Valley.This was my second trip to Kenya for fieldwork, the first being in January earlier this year.
During the sample collection phase of the trip, we were based in Kerichio before moving to Kisumu, located on the edge of Lake Victoria in Winam Gulf; Andy would remain there at the end of our sampling trip for a project of his own investigating aquaculture in Lake Victoria, more to follow in a future blog! Our field teams consisted of 3 vehicles led by Michael, Andy and I alongside our collaborators; Professor Odipo Osano (University of Eldoret), Dr Diana Menya (Moi University), technical lab staff; David Samoei and Doreen Meso (University of Eldoret), drivers and field assistants. In addition to the field teams, we also worked with public health officers (PHO) from each county who assisted with household entry, sample collection and translation. During the fieldwork, we provided training to our partners on how to collect environmental samples and all the necessary information from each site, with particular focus on accurate record keeping. After 5 days in the field, we had collected more than 800 samples and it was time to get back to the laboratory in Eldoret to process them!

Once in the labs at the University of Eldoret, I provided training in sample checking and processing. In addition to David and Doreen, who helped in the field, we were also joined by MSc student Job Isaboke and 6 undergraduate students studying environmental science. Having both David and Doreen with us in the field and laboratory provided them with a greater understanding of the importance of quality assurance and in turn, they can enforce it in their students. In terms of sample preparation, all the fresh fruits and root vegetables had to be peeled, chopped, frozen and vacuum packed, which took 4 people all day to get through! The leafy vegetable material was dried and packed for transport whilst the grains/beans/pulses and nuts were ground, a task that we could not have completed without the help of all the students. Finally, the soils were riffle split, allowing us to bring back a small amount of soil for analysis at BGS and to leave the bulk of the soil in their archive for future student projects at the University of Eldoret. One of the most important tasks I had to complete was ensuring that all the data we collected was handled properly. Showing our partners how to handle data collected from the field is essential to their development and ability to collect samples independently in the future.

From L-R: Preparing grain samples for transport to BGS, Keyworth; David preparing soils for transport to BGS, Keyworth
and their won archive

The partnership with the University of Eldoret has been going for over three years; in addition to multiple sampling trips in Kenya, David visited the UK for two months of lab training under a Commonwealth Professional Fellowship (See previous blog: A model for Quality Assurance, Lab Management and Good Laboratory Practices for Africa). Based off of the success of this trip we are looking forward to welcoming both Odipo and Doreen to BGS for training in 2019.
Overall, the sampling trip was a success, and we now feel confident that we are developing our partner’s skills to the point where they can venture into the field and collect samples in areas that have been missed to strengthen the work. The field work was very enjoyable and productive, I can’t wait to work in Kenya again, keep an eye out for future blogs!

Acknowledgements to the wider team:

University of Eldoret: Jackson Masai, Charles Owano, David Samoie, Prof. Odipo Osano, Doreen Meso. Melvine Anyango, Job Isaboke and all of the undergraduate students for help in the field and lab
Moi University: Dr Diana Menya, Esilaba Anabwani - ESCCAPE, Eldoret (Esophageal Squamous Cell Carcinoma Africa Prevention Effort), Amimo Anabwani- ESCCAPE, Eldoret
British Geological Survey: Dr Michael Watts, Olivier Humphrey – CEG PhD student, Dr Andy Marriott
Public Health Officers: Many thanks to the PHO’s from all the Counties

Monday, 14 January 2019

Setting ‘long tail’ geological data Mike Stephenson, Qiuming Cheng, Junxuan Fan, Chengshan Wang and Roland Oberhänsli

‘Long tail’ data is the difficult-to-get-at data that sits in libraries, institutes and on the computers of individual scientists. Informatics specialists like to contrast it with the smaller number of large, more accessible data sets . The name ‘long tail’ derives from graphs drawn of the size of data sets against their number: there are relatively few large datasets and a lot of smaller ones. Geological science has more long tail data than sciences like physics or meteorology, probably because historically it has been less associated with big science infrastructure and sensors.

Recovering data islands

The fact that ‘long tail’ data is difficult to get at probably holds back progress in geological science. The low discoverability of long tail geoscience data, and its heterogeneity make it difficult to bring it together to gain the benefits of machine learning and artificial intelligence. Informaticians describe ‘data islands’ in geological libraries, in the records of geological surveys and on desktop computers.
Two new projects aim to improve the discoverability of these data islands and the ease of compilation (interoperability) of geoscience data. One looks at the fundamentals of making historical paper data in islands available to cyberspace, the other seeks to bind already-digital data together to answer some of the biggest geoscience questions that still remain.

Collaborative projects

The first project is a unique collaboration between a geological survey and the academics and computer scientists of the GeoBioDiversity database (GBDB). Archipelagos of data islands exist within the geological surveys of the world. An example is British Geological Survey (BGS) biostratigraphical data associated with about 3 million fossils and thousands of localities and stratigraphic sections, gathered over 150 years from all over the country to exacting and consistent standards. The data has great potential for science, but much of it is contained within paper documents or simple document scans and so is inaccessible to big data tools. It needs lifting from the page and into cyberspace. The BGS began working recently with GBDB (the official database of the International Commission on Stratigraphy (ICS)) which is almost unique in being the only large database to hold sequences of fossils tied to sections, rather than just spot collections. To date GBDB and BGS scientists have placed live manipulable data from more than 6000 UK stratigraphic sections on a public access website . The project is also using machine-learning methods to get at biostratigraphical information directly from text.

Deep Time Digital Earth

Another even more ambitious project seeks to make already-digital geoscience data work together better. The Deep Time Digital Earth (DDE) project has just been approved as the first of the International Union of Geological Science (IUGS) ‘Big Science’ programs and its aim is to harmonize deep time geological data. Again the focus is on data islands, but this time the islands are already digital but are not linked or cannot easily be used together. Through DDE, data will be available in easily used ‘hubs’ providing insights into the distribution and value of earth resources and materials, as well as earth hazards. Data brought together in new ways may provide a novel glimpses into the Earth’s geological past and its future.

An example of how DDE will work concerns the evolutionary history of the biosphere. Previous analyses of long-term paleobiodiversity change were mostly at a resolution of ~10 million years, which are too coarse to reveal fine details of past biodiversity changes. Linked databases in DDE could provide high-resolution (10-100 kyr) diversity patterns. In the realm of minerals, DDE could, for example, provide integration of database systems for mapping clusters of porphyry copper deposits (PCDs) by linking georeferenced plate motion and geometric properties of subducted slab data. Linked databases in DDE, including African groundwater and aquifer data, recharge data, meteorological data, sediment flux, subcrop geology, basin subsidence, sequence stratigraphy, compaction, geomechanics and tectonics data could lead to more accurate models for African groundwater storage, underpinning sustainable development in poor countries vulnerable to climate change.

Earth sciences supporting broad-based scientific studies

The DDE is closely consistent with the vision of the IUGS which is to promote development of the Earth sciences through the support of broad-based scientific studies relevant to the entire Earth system. It brings together an almost unique range of partners including the ICS, the International Association of Palaeontology (IAP), the International Association of Sedimentologists (IAS), the Society for Sedimentary Geology (SEPM) and the International Association for Mathematical Geosciences (IAMG). Major geological surveys and institutes including the China Geological Survey (CGS), the BGS and the All Russian Geological Institute (VSEGEI) are also involved.

These institutions are coming together at a time when informatics and computing are evolving fast, but where a wider range of geoscience data were not available until now. In this way DDE may help to solve some of the biggest geoscience questions that still remain.

DDE is being linked with UNESCO, the International Geosphere-Biosphere Programme (IGBP), the Global Sedimentary Geology Program (GSGP), the International Geoscience and Geopark Program (IGGP), the Commission of the Geologic Map of the World (CGMW), the Global Geochemical Baseline (GGB), the International Lithosphere Program (ILP), and OneGeology. DDE will also operate the full FAIR data concept (Findable, Accessible, Interoperable, and Re-usable) and link to desktop systems for geoscientists all over the world as well as to students and teachers in classrooms and on the internet.

Building bridges between data islands

Geology could be said to have lagged behind other physical sciences in capitalizing on its big data, but DDE will enable bridges between data islands to be built and for data to be interrogated using modern tools tackling some of the most important and pressing questions of our time. The project has an ambitious time frame but aims to report its first progress at the 36th International Geological Congress, New Delhi in March 2020.

This article has been written by Prof Mike Stephenson, Director of Science and Technology at the BGS, Qiuming Cheng, President of the IUGS, Chengshan Wang, Chinese Academy of Sciences and Professor at the China University of Geosciences, Junxuan Fan, Director of the GeoBioDiversity database, and Roland Oberhänsli, Past-President of the IUGS.

Friday, 11 January 2019

Updating the World Magnetic Model: From the centre of the Earth, straight to your pocket

What links the centre of the Earth, billions of smartphones, and BGS scientists? The answer is: the recently updated World Magnetic Model (WMM) 

Will Brown of the BGS Geomagnetism Team explains.

The WMM describes the primary component of the geomagnetic field, and is normally produced every five years. It also predicts the Earth’s field for the next five years. Sometimes however, the Earth’s core behaves in an unexpected manner, and so we’ve recently updated the current WMM2015 with a release of WMM2015v2.

At BGS we monitor and map the Earth’s magnetic field using a global network of surface observatories, including our own nine, and satellites in low-Earth orbit such as ESA’s Swarm mission. We use these measurements to build models of the magnetic field that allow us to interpolate between our measurements and estimate the strength and direction of the field at any location.

A model can be thought of like a map of the “topography” of Earth’s magnetic field, but what many people don’t realise is that Earth’s magnetic field isn’t a single fixed feature: it’s a combination of many effects and it changes through time. 

Map of magnetic variation through time from 1900 to 2015 – the magnetic poles are where the strong red and blue contours converge, and the north pole moves very quickly in recent years.

The magnetic poles drift, the field strengthens and weakens, and the immense magnetic field of the Sun, carried by the solar wind, constantly batters at it from the outside. The effect of all these changes vary depending on when and where you are on, under, or above the Earth’s surface. Our models dissect the field we measure into its different parts, or sources, and provide a map of each that varies in time, even predicting the future of some parts.

So where does your phone come in? The WMM is the standard magnetic model used for navigation by organisations such as NATO, the Ministry of Defence, and the US’ Department of Defense, and also by smartphone operating systems such as Android and iOS. When you open your smartphone’s map app, you may see an arrow pointing which way you’re facing, and there’s something quite clever going on underneath. Your phone contains a magnetometer that is measuring the Earth’s magnetic field. In order to make sense of this information a reference model like the WMM is needed to correct the measurements of magnetic north made by your phone to True North. You go through the same procedure if you use a map and compass when out hiking: set your compass for the map’s magnetic variation adjustment (we provide these too for Ordnance Survey maps!), and then convert your compass reading of magnetic North, to give a direction relative to the map’s grid North.

Magnetic variation in degrees – you really need to set your compass correctly when hiking!
The rate-of-change of magnetic variation in degrees per year – the changes are quickest near the north pole.

The WMM is a joint effort from BGS and the US’ NOAA NCEI, on behalf of the UK’s Defence Geographic Centre and the US’ National Geospatial-Intelligence Agency. The WMM is a model of the primary component of the geomagnetic field: that of Earth’s core. The core field, which gives us our familiar magnetic poles and allows us to use a compass, is generated by dynamo action in the swirling iron-rich fluid of the outer core, roughly 3,500 km below out feet. The ever-changing flow of the outer core leads to an ever-changing magnetic field. This is a complex process that we don’t fully understand the physics of yet, and so we have to update our model regularly.

Since late 2014 the core field has varied in an unpredicted, and currently unpredictable, manner. This led to the WMM becoming less accurate, particularly at high northern latitudes, much faster than normal, and so we released an update ahead of the next regularly scheduled WMM release in late 2019. We can map the field changes that have occurred since 2015, and show that they seem to be related to two phenomena, an abrupt unpredictable change called a “geomagnetic jerk” in 2014/2015, and an acceleration of flow in the core in the northern hemisphere. This update to the WMM will be used until the next release in December 2019, when we’ll make our best estimate of the likely change in the core field until 2025.
The change in the vertical component of magnetic field at the core-mantle boundary between 2015 and 2018 – the three intense patches in the northern hemisphere are related to changes like the Livermore et al (2017) “core jet” model 

Wednesday, 9 January 2019

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
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

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
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.
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
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
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