Inside the Bartlett Brick

The new building at 22 Gordon Street houses The Bartlett School of Architecture. Completed in 2017 and designed by Hawkins\Brown Architects, the building has recently been named on the shortlist for The Architect’s Journal AJ100 Building of the Year prize and the RIBA London Awards. The structure is a retrofit of the former Wates House, the ghost of which still exists in the core of the building. What replaces it is a spacious, fit-for-purpose building for the UCL School of Architecture.

Now those of you that know me and know that my interests in the fabric of UCL are mainly biased towards the use of stone, may have assumed that I was disappointed to see a brick and concrete structure appear on the corner of Gordon Street and Endsleigh Gardens. Not so. I declare a fondness for both materials, though I confess to prefer my concrete to be around 2000 years old, the bricks used to clad 22 Gordon Street are rather special.

I am very grateful to the Bartlett’s Kevin Jones for supplying me with a specimen of ‘The Gordon Street Klinker’ a brick made specially for the construction of the building by the German brick makers Janinhoff.

Brick

Janinhoff make a great deal of bricks of different compositions, but mainly to a standard size. The Gordon Street Klinker is slimmer and longer than the standard bricks in their repertoire. 140,000 of these bricks, measuring 290 x 52 x 70 mm were used on the façade.

The brick is water-struck and twice fired. The water-struck brick making process works well for high moisture content, high plasticity clays. The clay is ‘struck’ in a wooden mould, and the large amount of water present allows for the mould to be easily removed, without the clay sticking to it. Evidence of this process can be seen in the puckered, ‘troweled’ surface of these bricks and the lip from clay overhanging the top of the mould. Lower water content clays need to be ‘sand-struck’; i.e. the mould is coated with sand which stops the brick from sticking to it.

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Evidence that these bricks are hand made can be seen in this example from the Endsleigh Gardens façade of the Bartlett’s 22 Gordon Street Building …

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Look closely and you can see the brick maker’s fingerprints …

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Janinhoff fire their bricks in a circular, so-called Hoffman Kiln which can operate continuously. Patented by Friedrich Hoffmann in 1858, these are the standard brick kilns used worldwide today. They are large circular or oval structures often with a central chimney. The interior has firing chambers radiating out from the central space (below the chimney). The kilns are fired by a movable ‘fire wagon’ which can travel on rails around the interior, firing each chamber consecutively.

The Gordon Street Klinker is coal fired at maximum temperatures of 1200°C. Looking at the surfaces of the bricks, ‘firing ghosts’ are seen patterning the surface of the bricks. This tells us how they were stacked in the kiln, subtly changing the oxidation of the brick surfaces where they touch each other.

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Above: Janinhoff’s Hoffmann kiln, showing bricks being stacked ready for firing and also note the use of spacers and other kiln furniture.

The grey colour of the bricks indicates a low-iron content clay. Such materials are available in the northern Germany and Denmark deposited as Quaternary glacial clays. The Jutland Peninsula lay at the southern edge of the ice sheet during the last glaciation and the clays were deposited as the ice sheet retreated between 20 – 10 thousand years ago.

To be workable, reduce shrinkage and cracking on drying and firing and be strong, brick clays need to contain inclusions or temper. Inclusions are natural mineral grains that occur naturally in the clay whereas a temper is added by the brick makers and can include both mineral and organic particles.

To look inside the Bartlett bricks it is necessary to produce a thin section, a slice 30 µm thick for observation using polarising light microscopy. This technique is routine in geology for examining rocks and identify their component minerals. As ‘synthetic stones’, ceramics may also be analysed in this way under the discipline of ceramic petrology.

Under the microscope, the Bartlett bricks have an almost isotropic (opaque) clay matrix indicating that the clay minerals have broken down and have begun to melt. This indicates that the brick has been fire in excess of 1000°C (and indeed we are aware from the manufacturers that temperatures of 1200°C were attained). It is not possible to distinguish whether or not the brick clay contained inclusions or it has been tempered, or perhaps both! Mineral particles present are dominantly quartz, but chert, feldspar and zircon are also present, indicating granitic rocks in their source. Grains are poorly sorted ranging from very fine (a few 10s of microns diameter) to particles of around 0.5 mm, the latter just visible to the naked eye. It is reasonable to expect that the finest portion are naturally occurring inclusions in these glacial clays.

The photomicrographs below show the brick photographed in plane polarised light and under crossed polars, x 40 magnification.

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The mineral grains are sub-rounded to sub-angular, indicating a natural sand source (i.e. not mechanically crushed). The composition is clearly granitic as indicated by the feldspar, zircon (this in typically tiny grains) and the large quartz grains with fluid inclusion trails. Some of these features are seen in the photomicrographs below. Left, the large grain in the centre is a feldspar; x 40 magnification, the field of view is ~ 3.5 mm. Middle and Right, quartz grains with inclusion trails, x 100, the field of view is ~ 1.5 mm.

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Citation

Siddall, R., 2017, Inside the Bartlett Brick, Blog: Orpiment https://orpiment.wordpress.com/2017/06/02/inside-the-bartlett-brick/

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Posted in Bartlett School of Architecture, Brick, Ceramic Petrology, London, Materiality, Minerals, UCL, Urban Geology | Leave a comment

Geology on Postage Stamps: #1 Energy Resources, Great Britain 1978

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This set of GB commemorative stamps was issued on 25th January 1978, the first issue of commemoratives for that year. The make the link between energy and geology in a world when alternative energies were a mere twinkle in most peoples eyes.

They were designed by Peter Murdoch (b. 1940) and depict symbols representing energy; oil, coal, gas and (nuclear) electricity coupled with stylised depictions of geological structures and strata associated with hydrocarbon reserves.

The stamps were printed by Harrison & Sons for the Post Office.

Posted in Geology, GeoStamps, Philately, SciArt, Science, Stamps | Leave a comment

First I was afraid, I was petrified … A short history of scary silicified log cabins

This is urban geology at the extreme; petrified wood is not exactly a common building stone, though it is becoming more frequently seen as a decorative stone, used for interior feature walls and even chopping boards and coasters. However it does have a place as a building stone, albeit mostly for novelty purposes … and only in America. Or at least this is the case as far as I am aware.

Silicified or petrified wood is relatively common and always an attractive fossil, with excellent preservation of the wood structure down to cell level. Identification of wood down to species level is often possible from microscopic observation of preserved cell structure. Trees have been around since the Carboniferous but are often much ignored in earth history. They are the background to what are perceived to be more interesting events. Here is an Upper Cretaceous scene whereby a right porker of a Tyrannosaurus is bringing down a green skinny lizard as an amuse bouche, whilst being eyed by circling Pterosaurs. These trees in the background are probably Araucaria sp.

7TyranosaurusRex

The process of silicification of wood is frequently closely associated with local volcanism. Silica is leached from decaying volcanic ash and carried in groundwaters and stream waters where it can permeate logs and branches. This works particularly well if the logs are buried in ash. Many fossil forest deposits represent preserved log jams in river systems and tree debris becomes silicified in anoxic environments (see Sigleo , 1979). Colours are imparted by trace elements; iron, copper, manganese, even uranium. Black is produced by carbon or finely disseminated pyrite.

Silicified wood deposits are known from Chemnitz in Germany from the Permian Leukersdorf Formation (Luthard et al., 2016) where the coloured stones were used as decorative inlay and as semi-precious gemstones. Also in Europe, deposits occur in Lesvos in Greece (Miocene; see Vasileiadou & Zouros, 2012). Worldwide, silicified wood is known from China (i.e. the Cretaceous ‘Jehol Biota’; see Ding et al., 2016) and exceptionally well-preserved, Pliocene trees are found in Java, Indonesia and Triassic trees occur in Madagascar, and these regions are the origin of most silicified woods on the decorative stone market today (Mandang & Kagemori, 2004; Yoon & Kim 2008). Recent discoveries petrified trees have been made in Brazil (Lower Permian tree ferns; see Maria da Conceiçao et al., 2016) and Turkey (Miocene willows, junipers and oaks; see Akkemik et al., 2016).

IMG_9263Silicified wood panelling supplied by the Emperors of Stone Bling, Maer Charme.          Photo by Ruth Siddall.

There are a large number of deposits of various ages in the USA and it is here that fossil trees have largely been used for building. Deposits are known from the Dakotas, Colorado, Oklahoma, Texas etc. The best known deposit is in the Upper Triassic Chinle Formation represents continental facies and outcrops throughout the southwestern USA. These famous fossil conifer deposits occur in the Lower and Upper Petrified Forest Members of Carnian-Norian age. Conifer logs are also found in the intervening Sonsela Member, representative of an alluvial floodplain (see Ash, 1992; Trendell, 2013). However, there are many other examples through the West and Midwest. Many of these deposits have been raided for garden ornaments and other small-scale structures.

A well-known and rather American Gothic building constructed from petrified wood is the (former) gas station in Lamar, Colorado, described by David Williams in his book ‘Stories in Stone’ and also illustrated in his blog. The gas station was built by one William Brown in the 1930s. The Colorado trees are of Cretaceous age but the precise age of strata of origin is unknown. The ‘logs’ were found in fluvial outwash in farmland around 25 miles south of Lamar.

DSC00947Lamar Gas Station; photo from Geologywriter.com

And then there is Petrified Wood Park in Lemmon, South Dakota, also constructed in the 1930s. This is a truly terrifying place; cones, pinnacles, pyramids and creepy-looking grotto-like buildings are built from petrified wood as well as dinosaur and mammoth bones. The park occupies a whole block of the town and was built by amateur geologist and ‘visonary’ Ole S. Quammen. To be fair his intention was altruistic, it was to provide work for 50 or so otherwise unemployed men during the depression era West. Quammen’s heirs donated the park to the grateful town of Lemmon in 1954 where it remains ‘the world’s largest petrified wood park of its kind’. Obviously. An on-site museum once house a collection of stuffed animals playing musical instruments. Sadly, these are no longer on display. The petrified wood was sourced locally, potentially from several strata. The Early Cretaceous Lakota Sandstone Formation outcrops in the eastern Black Hills and has fossilised logs of cypress, palm and cycads. The Hell Creek Formation of South Dakota also has petrified Late Cretaceous cypress (and dinosaurs). The Palaeocene petrified wood is found in the Badlands of South Dakota where conifers and broadleaf tree trunks and branches are preserved (Teachout, 1995).

Petrified-Wood-Park-Lemmon-SD-800x500_c-1Lemmon Petrified Wood Park sign; photo from SouthDakota.com

According to Snider (2012), Texas is the state for construction in petrified wood, and she cites Austin, Huntville, Decatur (which also has a petrified wood gas station) and Stephenville as all having buildings incorporating this unexpected stone. However, the town of Glen Rose in Somerville County has over 40 buildings and other structures constructed of silicified wood. It was once known as ‘The Petrified City’. The post office, several houses, fountains, a (now disused and ruined) speakeasy and the bandstand are all built from petrified wood. The bandstand also incorporates slabs with spectacular dinosaur footprints too.

glen-rose-tx-petrified-houseThe ruins of Glen Rose’s speakeasy; photo by Tui Snider (2013)

glen rose bandstandBandstand, Glen Rose, Texas; photo by Tui Snider (2012)

texas-dinosaur-tui-sniderDetail of dinosaur footprint in Glen Rose’s bandstand; photo by Tui Snider (2012)

The origin of this Texan petrified wood building spree, which occurred in the 1920s and 30s was a unexpected consequence of the mechanisation of agricultural machinery. Farmers were able to dig deeper into their soils than before and they hit a petrified wood motherlode in the local fields. The logs were hauled out and used to build the town.

Finally, the most attractive structure built from petrified wood is not all scary. It is Agate House, located in the Petrified Forest National Park, Arizona. It was originally constructed between 1050 and 1300 AD by Ancestral Puebloans from Triassic Chinle Formation petrified trees. Other examples of structures built from petrified wood have also been excavated in the surrounding region, since the 1930s. The excavation of Agate House and its subsequent reconstruction was overseen by archaeologist Cornelius Burton Cosgrove Jr. (1906 – 1999). Petrified wood was also used for arrowheads and similar artefacts by the Ancestral Puebloans.

agate-house-1[6]Above, Agate House; photo by Amusing Planet

agate-house-wall-of-petrified-woodAbove, the wall of Agate House; photo by NotsofastinBoulder’s Blog.

More petrified wood buildings in the USA …

Gas Station, Decatur, Texas

Carter County Museum, Montana – does anyone have a photo of this? It is partially built of Hell Creek Formation wood.

Petrified wood and petrified wood buildings in Oklahoma

Outside the USA …

The town of Mata – “Cidade da Pedra que foi madeira” (“The city of rocks that once were wood.”) – in Rio Brande do Sul, Brazil. Petrified wood from the Upper Triassic Caturrita Formation is used to build several buildings and can be seen in the Palaeobotanical Garden. The fossils were collected by priest and palaeontologist Daniel Cargnin.

Thanks to Christian Kammerer  for this information and photos.

 

If anyone knows of any more structures built of petrified wood outside the US, do let me know …

 

How to cite this blog:

Siddall, R., 2017, First I was afraid, I was petrified … A short history of scary silicified log cabins., Orpiment Blog https://orpiment.wordpress.com/2017/04/13/first-i-was-afraid-i-was-petrified-a-short-history-of-scary-silicified-log-cabins/

 

References

Akkemik, Ü., Arslan, M., Poole, I., Tosun, S., Köse, N., Kiliç, N. K. & Aydin, A., 2016, Silicified woods from two previously undescribed early Miocene forest sites near Seben, northwest Turkey., Review of Palaeobotany and Palynology 235., 31–50.

Ash, S.R. and Creber, G.T., 1992. Palaeoclimatic interpretation of the wood structures of the trees in the Chinle Formation (Upper Triassic), Petrified Forest National Park, Arizona, USA. Palaeogeogr., Palaeoclimatol., Palaeoecol., 96:299 317.

Ding, Q., Tain, N., Wang, Y., Jiang, Z., Chen, S., Wang, D., Zhang, W., Zheng, S., Xie, A., Zhang, G. & Liu, Z., 2016, Fossil coniferous wood from the Early Cretaceous Jehol Biota in western Liaoning, NE China: New material and palaeoclimate implications., Cretaceous Research, 61, 57-70.

Mandang, Y. I. & Kagemori, N., 2004, A Fossil Wood of Dipterocarpaceae from Pliocene Deposit in the West Region of Java Island, Indonesia., Biodiversitas, 5(1), 28-35.

Maria da Conceiçao, D., Saturnino de Andrade, L., Cisneros, J. C., Iannuzzi, R., Pereira, A. A. & Machado, F. C., 2016, New petrified forest in Maranhao, Permian (Cisuralian) of the Parnaíba Basin, Brazil., Journal of South American Earth Sciences 70, 308-323.

Petrified Wood Park, Lemmon, South Dakota.

Petrified Woods from the Indonesian Islands of Java and Sumatra.

Saltarelli, M, G., 2009, ‘Irreplaceable Works of Art’: Petrified wood treasures and dinosaur tracks create a paradise of geology.

Sigleo, A.C., 1979. Geochemistry of silicified wood and associated sediments, Petrified Forest National Park, Arizona. Chem. Geol., 26: 151–163.

Snider, T., 2012, Texas Road Trips: From Dinosaurs to Drive-Ins.

Snider, T., 2013, A to Z Texas: P is for Petrified Wood Buildings.

Teachout, G. E., 1995, Petrified wood of South Dakota.

Trendell, A. L., Nordt, L. C., Atchley, S. C., Lebland, S. L. & Dworkin, S. I., 2013, Determining floodplain plant distributions and populations using paleopedology and fossil root traces: Upper Triassic Sonsela Member of the Chinle Formation at Petrified Forest National Park, Arizona., Palaios, 28, 471-490.

University of Arizona, Laboratory of Tree-Ring Research; Fossil Trees or Petrified Wood.

Vasileiadou, K. & Zouros, N., 2012, Early Miocene micromammals from the Lesvos Petrified Forest (Greece): preliminary results., Palaeobio. Palaeoenv., 92, 249–264.

Williams, D. B., 2009, Chapter 7: Pop rocks, pilfered fossils and Phillips Petroleum – Colorado Petrified Wood., Stories in stone: travels through urban geology., Walker Publishing Inc., New York., 133-151. & Blog.

Yoon, C. J. & Kim, K. W., 2008, Anatomical descriptions of silicified woods from Madagascar and Indonesia by scanning electron microscopy., Micron 39, 825–831.

 

Posted in Building, Building Stone, Dinosaurs, Fossils, Geology, Stone, Urban Geology, USA | 4 Comments

Urban Geology in Birmingham

I was invited to come back to Birmingham by Julie Schroder of the Black Country Geological Society to update and expand previous building stone walks of the city created by Julie, Eric Robinson and Paul Shilston. I was very pleased to do this, having graduated in Geology from the University of Birmingham in 1989. The city has changed a lot since then, with a brand new development around the Bull Ring and New Street Station. Julie, myself and fellow Birmingham Geology graduate Laura Hamilton hit the streets in Easter 2016. We have produced three guides to the city centre which can be downloaded as pdf documents here:

1. The Town Hall to the Cathedral

2. Centenary Square to Brindleyplace

3. Around the shops

The pictures below provide a snapshot of the geodiversity of Birmingham’s built environment …

Posted in Birmingham, Building, Building Stone, Fossils, Geology, Stone, Urban Geology | Leave a comment

Prehistoric Animals: A series of illustrations by David Roland

I bought this set of postcards when I was a kid in the 1970s. I can’t remember exactly where I bought them, but it was probably Manchester Museum. They were produced by scientific illustrator David Roland for Birmingham Museum and Art Galleries and represent what was then state-of-the-art interpretations of the appearance of dinosaurs and other prehistoric animals, including Dimtetrodon and Pteranodons. All are very green and scaly. What impressed me at the time, though, is that they all fitted together to make a single, continuous panorama. I loved them!

Featured are: Dimetrodon, Stegosaurus, Diplodocus, Brontosaurus, Iguanodon, Pteranodon, a portly Tyrannosaurus Rex and Triceratops.

Posted in Art, Dinosaurs, Fossils, Geology, SciArt, Science, Zoology | Tagged , | 3 Comments

The mis-appliance of science in cultural heritage?

Science applied to archaeology and cultural heritage is a thing. It has been happening for decades. Scientific analysis of materials can provide much needed information about materials, trade, manufacture, provenance, foodstuffs, populations, individuals. With today’s kit we can make analyses on tiny samples, or even acquire semi-quantitative analyses without the need for the destruction required to remove a sample. We can identify the components in building materials and pigments, the type of honey a vessel once held, the isotopic signature of metals and therefore their provenance, the isotopic signatures of bone material can tell us were a person lived, where they migrated, where they came from. Science can revolutionise established archaeological chronologies. Amazing information. So why is so much science applied to archaeology and cultural heritage so bad?

I have been to several conferences this year where there has been a cross- and multi-disciplinary approach to materials in cultural heritage. This if course should be a GOOD THING. However, myself and colleagues have been commenting that with the proliferation of analytical techniques and access to them, the science is actually getting worse, and this is not good. We feel that the study of science in cultural heritage is not moving forward. I also see this in papers I review. Scientists are collaborating with archeologists and art historians, but it seems that they are not communicating well and there is little effort on either side to learn about each other’s discipline and what the questions the ‘science’ aims to answer. Inappropriate analytical techniques are used and poor data are produced and these data are, again inappropriately, under- or over-interpreted. The discussions at conferences and recent press activity on the use of a synchrotron to identify the presence of calcium on a Greek vase have spurred me to write this. I admit I have only read the press releases on this latter research, and I presume that the actual publication will provide much more depth to these analysis.

I’m a geologist. My PhD used geothermochronological techniques to look at landscape evolution on a continental scale. I used a radiometric dating technique called fission track analysis which used on the mineral apatite. It was known that the chemistry of apatite, with substitutions between chlorine, fluorine and the hydroxyl group, affected the lengths of the fission tracks and therefore the thermochronometric data obtained. I looked at the subtle changes of chemistry in apatites using Fourier Transform Infra-Red spectroscopy (FTIR). Although I went on to lecture in ‘orthogeology’, much of my research has been associated with the application of geological techniques to cultural heritage. I am so glad – and lucky – to have learned how to use petrology/petrography, geochronology and spectroscopy from first principles, rather than to have stumbled across these techniques as  ‘black box’ methods to reveal more about the material I am working on. I understand what these techniques actually tell us about the materials they are applied to. They are measuring bonds and their configurations on a molecular scale, the number of undecayed v. decayed isotopes (or their proxies), elemental weights or excitation of electron energy levels. These data can then be interpreted to give us information such as a radiometric age, they may be diagnostic of the identification of a mineral (or analogous compound) or the nature and origin of an organic compound, they can elucidate the presence or absence of a certain element or the precise chemical compound present in a sample.

The important word here is interpreted. Yep, that’s right, some guy sits down and writes a computer programme to interpret the results that a machine turns out. This is what the software that comes with your new black box analytical machine is. It is NOT the machine telling you the answers. So if you click on the sample image in your back-scattered electron image of your sample (yes, that’s right it is not a photo) and your spectra tells you that a peak is assigned to silicon, this is a programmers interpretation of what fits that peak. Now of course the vast majority of these analyses will be correct and completely reliable, but they should still be used with caution. I have always found pertinent this quote from the 1990 film The Hunt for Red October, in which a submarine sonar operator Jones (played by Courtney B. Vance) discovers that his interpretation software is not telling him the truth. He has detected the faint but rhythmic sounds of another submarine which is using a new propulsion mechanism and is trying to explain this to his commanding office Bart Mancuso (Scott Glen) …

Jones: When I asked the computer to identify it, what I got was ‘magma displacement’. You see, sir, SAPS software was originally written to look for seismic events. And when it gets confused, it kind of ‘runs home to mama’.

Mancuso: I’m not following you, Jonesy.

Jones: Sorry, sir. Listen to it at times speed. [he plays a tape in which a rhythmic noise is heard] Now that’s gotta be man made, Captain.

Mancuso: Have I got this straight, Jonesy? A forty million dollar computer tells you you’re chasing an earthquake, but you don’t believe it? And you come up with this on your own?

Jones: Yes, sir.

Mancuso: Including all the navigational math?

Jones: Sir, I-I’ve got-

Mancuso: Relax, Jonesy, you sold me!

This is so true. That peak fitting software on your FTIR, WDS, EDS, EMP, XRF, whatever was probably not written with reference to archaeological materials. It was written to identify pure compounds used in pharmacy or precise mineral compositions.

So the lesson to learn here is if you get something unexpected, like a massive peak assigned to tellurium, you should probably question your results, not just accept it as something really unusual and therefore cool. Any scientists watching your presentation or reading your paper (or indeed reviewing it) will immediately see through this. Here is an example from the field of petrology. What you need to know before reading this is that basic mineral identification in rocks is carried out by (experienced) analysts using optical polarising light microscopy (PLM). So for example, a basalt may include a mineral from the pyroxene group where chemistry ranges between iron, magnesium and calcium and variable silica amounts. This group can be conveniently subdivided into two main groups orthopyroxenes and clinopyroxenes which have clear association with different geochemical environments. The ‘ortho’ and ‘clino’ bits refer to differences in crystal structure and these two groups are most easily distinguished using PLM. It would take about three seconds to distinguish the two pyroxenes and we generally just refer to them as orthopyroxene (opx) and clinopyroxene (cpx). If you really want to, you can further subdivide the pyroxenes into named types, say for cpx, the main ones are are augite, hedenbergite and diopside. However most of the time we geologists don’t use this classification unless there is a reason for doing so, i.e. we want to answer specific questions about zoning within a single crystal and how that relates to say, fluctuating melt chemistries (and we need a reason for deciding that fluctuating melt chemistries are important). We just call it clinopyroxene. Therefore when I see a presentation that shows someone has identified diopside in a pot sherd I know they haven’t looked properly at the material and a computer programme written for igneous petrologists has told them it is diopside and they didn’t question it. It is also important to note that the name ‘diopside’ does not denote a specific phase chemistry that will direct you to a particular source/provenance. It doesn’t and it won’t. And it isn’t ‘more scientific’.

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Clinopyroxene. I really DON’T CARE whether it is diopside, augite or hedenbergite.

On the subject of mineral names, the misuse that infuriates me most is the use of the mineral name cuprorivaite to describe the synthetic pigment Egyptian Blue. If you Google cuprorivaite, you will get far more references to Egyptian painting than to minerals and this demonstrates the scale of this problem. Cuprorivaite is a rare, naturally occurring mineral. When it occurs it is as a micromineral (very small crystals, < 1 mm) and often disseminated. It occurs nowhere in masses worth of economic extraction, even as a cottage industry. Therefore it has never been used as a pigment and no one has ever detected cuprorivaite on archaeological paintings. However, you would not believe this from the literature. What researchers have found is an analogous synthetic compound called Egyptian blue. It’s a calcium copper silicate if you want to sound scientific. Why is this terminology important? In the world of pigment analysis, there are many phases which can and have been used as pigments derived from natural minerals and analogous synthetic compounds and it is important in answering archaeological and art historical questions to be able to distinguish these forms. Examples are the red pigment mercury sulphide which when natural is cinnabar and when synthetic is vermillion, or (natural) azurite and (synthetic) verditer for the blue copper carbonate hydrates. It can be important to know the differences between these minerals to detect fakes and assess knowledge of technologies or mineral provenances. As a consequence it is essential to have a terminology that clearly differentiates between natural and synthetic pigments.

A photomicrograph of the mineral cuprorivaite.

I think a major problem here is that a lot of the time, scientists and cultural heritage people don’t really get each other. They don’t know enough about each others subjects. Sure, they watch the TV programmes and remembered being into the Ancient Egyptians at school, or they maybe had a toy chemistry set. But now we are all grown-up academics, we are all set in our ways and stuck in the silos that the university academic departments give us (and this structure is a problem). A chemist is probably not likely to meet an art historian at their institution unless they sit together on a college committee – and many academics will avoid these committees like the plague. So we don’t talk and learn about other disciplines, and don’t get me wrong, scientists are just as bad about this as anyone. Many can be arrogant, thinking that they can answer the ‘simple’ questions archaeologists ask and they think its cool to have something they have only seen before in a museum or on the TV in their lab. Something to tell their colleagues and hey, it might just tick some of those ‘impact’ boxes. So an art historian comes along with a Greek vase and the scientist thinks ‘OK I can analyse this for you using my synchrotron. This will cost hours of beam time and thousands of dollars, but we may get a paper from it that will enable me to patronise humanities people with my superior knowledge and show that I can also do novelty science. I have seen Greek vases in museums, I am surprised that no one has made these analyses before because it is so easy to do’. The art historian thinks ‘Brilliant, this will tell us something that all those other techniques won’t tell us. I don’t really understand those techniques, but surely this synchrotron thing is so big and so expensive that it must provide better, richer, more accurate and more precise results than anything else. This will make me look amazing when I present this work! So few people in my position have access to this sort of kit. Just using this technique makes this study novel and innovative. And the scientist guy says it will be easy. He seems to know what he’s doing.’. Maybe, just maybe, one of the parties will search the literature, but because scientific literature database on the whole doesn’t search books and memoirs and vice versa, they don’t find out that this is something that has already been done, using simple analytical techniques which have given better and more informative results.

And this happens within science disciplines too … this (modified) cartoon is on our geochron lab wall. ‘We’ are the geochronologists …

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Despite what you may interpret from the above, a geologist or geochemist would never use complex, expensive kit to do routine mineral analyses, so why would you do this to analyse the pigments on a wall painting or on a Greek vase? Many simple techniques such as microscopy and wet chemistry give reliable, accurate and precise results and can’t be improved on. The press releases surrounding the analysis of pigments on an Attic lekythos carried out on a synchrotron only told us that the white pigment present ‘contained calcium’. It stopped short of making the logical (i.e. totally f**cking obvious) observation that this was a form of calcite, a polymorph of calcium carbonate. If I could have taken a tiny sample from the vase (so small that it could not be noticed by the naked eye) I could have told the museum/art history guys that not only was it calcite, but would have been able to differentiate chalk, eggshell, seashell (OK the aragonite polymorph in this case), coral or any other form of calcite derived from a limestone. If sampling was not possible, I could have identified the presence of calcite/aragonite (very unlikely that it would be aragonite though) using a UV light torch and a pXRF in less than a minute. I would have probably charged £50 max for experimental costs. To present a paper on pigment analysis and conclude that you have a Ca-rich pigment or an Fe-rich pigment is simply not good enough and is not moving the subject on.

Mind you, on the subject of portable X-Ray fluorescence machines, these will only give semi-quantitative results and users should be aware of this, but they are good for identifying major elements when sampling is not possible. However pXRF analyses should not be used in isolation. These machines were designed for quick, rough, field checks to look at say, arsenic pollution in ground water. The manufacturers are horrified at how their data has been used to provide ‘confirmed scientific analyses’ in cultural heritage.

My examples here are mainly to do with geological and analogous materials but I am sure that many scientists can quote examples from metallurgy and biosciences where glaring misuse of equipment and terminology are used. How can this be improved? We all have to learn more from each other. Most importantly cultural heritage people need to know what questions they want to ask of their objects and materials. Is it as simple as ‘what is it?’ (a very valid question) or do they want to know more; where does it come from? How was it made? If there are more complex questions to be answered then find a scientist that can work with you to help find the best analytical method to get the answers you need.

My top tip is that scientists and cultural heritage partners work together in a truly collaborative manner, so that the scientists are not just used as technicians, they are an integral part of the research project. I have always worked in close partnership with colleagues and have made time to learn about the materials I am analysing and the contexts of the artefacts or building from which they are derived. I know about comperanda, I read the literature, I know what the expected range of materials are. I double check my results when I find something outside that. If its true I design further experiments or do field work to answer these new questions. I don’t just hand over the data and walk away. And I always, always use the most appropriate technique to perform scientific analysis. As a petrologist I am experienced with polarised light microscopy so that is my ‘go to’ technique. It is great for ceramics, mortars, pigments and of course, stone. I do accept that this takes learning and experience, but go on a course! Learn it! I do a lot of work on pigments so I use FTIR and Raman spectroscopy too. If I have questions about organic binders then I would turn to gas chromatography mass spectrometry, but as this is costly and wouldn’t go there unless it was really important. I use old-skool spot tests and wet chemistry for quick analyses of phases to detect things like lead or phosphate (there are loads of books on how to do this, and you can do it in you kitchen). I use pXRF and SEM/EDS, again for major elements only, but am cautious on how interpret results, knowing these are not quantitative techniques. I occasionally use X-Ray diffraction (XRD) to look at crystallinity in some minerals (mainly hematite and other iron oxides and iron oxide hydroxides in ochres). These techniques answer all the questions I can think of. I used a synchrotron once to try and design some experiments to further the understanding of the blackening of cinnabar/vermillion paints. It didn’t work.

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I used a synchrotron to analyse a pigment and all I got was this crappy picture.

Posted in Art, Artists' Pigments, Geology, Materiality, Minerals, Pigments, Science | Leave a comment

Portland Bone

There is a lot of Portland Stone in London, so much of it in fact that I almost blank it out. I am trying to change the way I think about it, partly thanks to Gill Hackman’s inspiring book “Stone to build London” and also, whilst working on the London Pavement Geology project, to give this most iconic of London’s buildings stones its rightful coverage.

London is a good place to see all the varieties of Portland Stone quarried today and in the past, and a variety of facies and fossils can be seen in many buildings (see Siddall & Hackman, 2015; Siddall 2015, Hackman 2014). Notable examples are Green Park Underground Station and BBC Broadcasting House. However the bulk of Portland Stone Buildings in London are of fairly standard Whitbed, with little variation in facies and fossils. Typically these are white to pale-grey weathering, oolitic limestones, sometimes showing cross-bedding and with variable fossil content, mainly shells, shell fragments and occassionally pieces of Solenopora algae. The stone used at St Margaret’s Westminster is, on the whole, fairly characteristic of this description.

Easily overlooked, dwarfed as it is by its next door neighbour, Westminster Abbey, St Margaret’s Westminster is a neat little church clad in Portland Stone Whitbed. Dedicated to Saint Margaret of Antioch, it is the parish church of the House of Commons. There has been a church on this site since the 12th Century. The current building including its Portland Stone cladding dates from the 1730s refurbishment by the architect John James (1673-1746). St Margaret of Antioch was swallowed by a dragon, but was coughed up alive after she had tickled the dragon’s rib cage from the inside with her cross.

In the passage between St Margaret’s and the Abbey, towards the SE corner of the church, a block sits just above eye-level, containing a pen-shaped, brown clast, truncated by the lower edge of the block, but strikingly different from the standard allochems of the Portland Limestone. The preserved piece is around 15 cm long and 3 cm wide and has been eroded into a flattened pebble (below).

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My knowledge of vertebrate palaeontology is scant and more influenced by Ray Harryhausen than anything I learned when an undergraduate. But I guessed this looked more ‘bone’ than ‘stone’. However, I have never before seen bone in Portland Stone. In the Purbeck Beds it is relatively common, but usually preserved as jet black phosphate. Thanks to the fantastic research tool that is social media, I contacted geologist Mark Godden of Portland quarry firm Albion Stone. Mark agreed that this worn pebble of brown stuff was ‘probably’ bone, as these occasionally turn up when quarrying and Mark sent some pictures for comperanda. Certainly confirmed bone from Portland Stone, recognisable as vertebra etc are a similar colour and texture. Other options where that it is an infilled burrow; however, if so, what is it that has infilled it? There is extensive bioturbation in Portland Stone, but it is all infilled with white oolitic limestone or shell fragments. So I am fairly convinced that it is a fragment of a disarticulated, much eroded and fairly large vertebrate skeleton.

According to Delair & Wimbledon (1993), the bones from several vertebrates have been found in the Portland Limestone member and equivalent strata of Tithonian (Upper Jurassic) age; crocodile and turtle bones, as well as those of marine reptiles Ichthyosaurs and Pleiosaurs are not unexpected, but dinosaur bones also occur. These may have been derived from paddling saurischians who subsequently keeled over, or more likely, were washed in from adjacent dry land, where dinosaurs such as Megalosaurus and Iguanodon were knocking around.

Reconstructions of these types animals can be seen at Crystal Palace. These Victorian effigies somewhat dated (to say the least) and are not exactly Jurassic Park. The real things are now interpreted to be the sleeker, more streamlined beasts with which the average film goer is more familiar. Nevertheless, you get the idea!

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Above, left: Crocodiles and Right: Iguanodons at Crystal Palace. Below, a Megalosaurus surveys the scene.

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Other notable bones to be found in St Margaret’s include those of William Caxton, England’s first printer of books, Sir Walter Raleigh and poet John Milton.

St Margaret’s Westminster is listed here on London Pavement Geology.

Many thanks to Mark Godden of Albion Stone.

References & Further Reading

Crystal Palace Dinosaurs: http://cpdinosaurs.org/visitthedinosaurs

Delair, J. B. & Wimbledon, W. A., 1993, Reptilia from the Portland Stone (Upper Jurassic) of England: A preliminary survey of the material and literature., Modern Geology, 18, 331-348.

Dino Directory: Megalosaurus: http://www.nhm.ac.uk/nature-online/life/dinosaurs-other-extinct-creatures/dino-directory/megalosaurus.html

Dino Directory: Iguanodon: http://www.nhm.ac.uk/nature-online/life/dinosaurs-other-extinct-creatures/dino-directory/iguanodon.html

Godden, M., 2012, Portland’s quarries and its stones. http://www.dorsetgeologistsassociation.com/Portland-Stone/Portland_Stone_Document_-_7_June_12.pdf

Hackman, G., 2014, Stone to build London: Portland’s legacy., Folly Books Ltd., Monkton Farleigh., 311 pp.

London Pavement Geology: http://londonpavementgeology.co.uk

Siddall, R., & Hackman, G., 2015, The White Cliffs of St James’s: Portland Stone in London’s Architecture., Urban Geology in London No. 31, http://www.ucl.ac.uk/~ucfbrxs/Homepage/walks/PortlandStJames.pdf

Siddall, R, 2015, An Urban Geologist’s Guide to the Fossils of the Portland Stone., Urban Geology in London No. 30, http://www.ucl.ac.uk/~ucfbrxs/Homepage/walks/PortlandFossils.pdf

St Margaret’s Westminster: https://en.wikipedia.org/wiki/St_Margaret’s,_Westminster

Posted in Building Stone, Fossils, London, Portland Stone, Urban Geology, Westminster | Tagged , , , , | 1 Comment