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A Colour A Day: Week 30

Ruth Siddall18 October 2020

A Colour A Day: Week 30. 12th-18th October.

Jo Volley writes…

In October 2015, whilst on research leave, I travelled around Provence visiting pigment quarries, mines and factories to look at pigment manufacturing methods and processes. In this marvellous red landscape, I have never felt such a strong emotional relationship between the landscape and painting.  The trip was also something of a pilgrimage to visit the bibliotheque in Aix en Provence to view a remarkable manuscript made by the C17 Dutch artist A. Boogert who in 1692 completed an educational manual of how to mix every colour available to him. Each pigment is bound in gum Arabic and applied to paper with instruction as to their properties, proportions and potential. It is an extraordinary document of the pigments available at that time and of an artist’s dedication to learning. It was a humbling experience to hold in one’s hands this rare and beautiful manuscript, and to feel a connection with Boogert’s endeavours. The timeless and common manufacture of binding colour and making paint. The sheer pleasure of it… and its desire to communicate. It has also been the inspiration for this project.

Each red earth is bound in gum Arabic on W&N watercolour paper.

Collected: Sentier des Ocres, Roussillon.
Purchased: Sentier des Ocres, Roussillon.
Purchased: Sentier des Ocres, Roussillon.
Collected: Les Mines des Bruoux, Gargas.
Collected: Mathieu Ocre Usine, Roussillon.
Purchased: Les Mines des Bruoux, Gargas.
Purchased: Mathieu Ocre Usine, Roussillon.

 

A Colour A Day: Week 29

Ruth Siddall11 October 2020

A Colour A Day: Week 29. 5th -11th October

Jo Volley writes…

In the early 1800’s the mineralogist Abraham Werner published the Nomenclature of Colours to identify minerals by key characteristics. It was later amended by Patrick Syme, C19 Scottish flower painter, having been introduced to it by Robert Jameson who had studied for a year under Werner before becoming a professor of Natural History at Edinburgh University. Jameson had matched Werner’s descriptions with the actual minerals, Syme then used these as his starting point for the colour names, descriptions and actual colour charts introducing references to animals and vegetables. Darwin took a copy on the HMS Beagle voyage (1831-1836) and its terminology, ‘lent both precision and lyricism to Darwin’s writings, whether he was detailing the changeable ‘hyacinth red and chestnut brown’ of the cuttlefish, ‘the primrose yellow’ of species of sea hare, or the ‘light auricular purple’ and provided his naturalist contemporaries with clear point of reference, and enabled all his readers to clearly envision the creatures and settings of lands that most, in a pre-photography age, would never see.’

Extracts from Publisher’s notes Werner’s Nomenclature of Colours, Natural History Museum

 

 

A Colour A Day: Week 28

Ruth Siddall4 October 2020

A Colour A Day; 28th  September – 4th October

Jo Volley writes…

This week’s colours are seven Russian earth pigments gifted by Ruth Siddall who says of them. ‘These seven pigments are supplied by Moscow-based company Colibri Premium Pigments. Many of the earth and mineral pigments they supply are sourced in Russia. This is a selection of their ochres, which include iron ochres (red and yellow ochre) and aluminium-rich earths known as bauxite (deposits much overlooked as ochres). Siderite is iron carbonate. Also included here are two very ‘Russian’ pigments made from minerals mainly known only from deposits in Russia. Shungite is a black, carbon-rich earth pigment. It is found in very ancient, 2 billion year old rocks in Russian Karelia, in the region of Lake Onega. It is named after the town of Shunga. It formed as biogenic deposits, probably from algae preserved in anoxic conditions and then subsequently metamorphosed. Volkonskoite is a green-coloured, chrome-bearing smectite clay mineral. It is sourced from the Okhansk region of the Urals. Tuff is a volcanic ash deposit, in this case coloured purple by iron oxides.

Ruth suggests listening to Sibelius’s Karelian Suite whilst viewing the colours.

All pigments are bound in gum Arabic on W&N watercolour paper and read from left to right:

Tuff Purple
Siderite
Mumia Bauxite
Shungite
Sankirnaya Ochre
Volkonskoite

Mineral Pigment Processing: Health and Safety Tips

Ruth Siddall16 September 2020

As a geologist, I get asked lots of questions about the safety aspects of preparing mineral pigments and paints, so I thought I’d write down what I have learned during my career as a geologist and answer some of the questions I’m frequently asked. Are mineral and earth pigments safe? Well, the answer is yes and no. Most materials are safe to use if you take the correct precautions on collecting, processing, storing and using them. The more you learn about the materials, the more you understand the risks. The same goes for all artists’ materials, including glues, resins, jesmonite, white spirit and so on. For stuff you buy over the counter, material safety data sheets are readily available. Obviously, this is not the case for stuff you might collect in the great outdoors. Here are some tips, and my personal feelings, about the risks of working with rocks and minerals and how to mitigate them. This applies to people working in sculpture and pottery as well as pigment processing. If you’re grinding rocks and minerals for pigments occasionally and in small amounts, there’s very little risk. However, if this is something you’re doing everyday, you need to take some precautions to protect your health. All dusty materials can affect your respiratory system, but some materials have more damaging effects than others.

What I’m writing here applies to rocks, minerals and biominerals (shells, bone) that you might find in your environment and are regularly used to make pigments or for carving. These tips do not apply to toxic or chemical waste products (which should be avoided at all cost), plants or animal materials (including dyes and lake pigments), or processed products like metals, plastics, resins and glass. Many of you will already know this and take precautions when preparing mineral pigments. However a lot of people are find this activity for the first time. So here we go …

Learn about natural materials and their risks …
Try to learn what you can about what naturally occurring materials are made up of. You can google most things and check their chemical formula. Also look at hazard sheets for similar materials sold by paint manufacturers. Find out what is dangerous and what is not and collect accordingly. Wash your material when you get it home to remove any dirt or potentially hazardous biological material. If you are working with relatively small amounts of these materials every now and again, there is no major hazard associated with them.

It’s all about what you wear …
I’m a lab and field scientist by background and health and safety is drummed into you ALL THE TIME in these environments. There is a reason why you never see scientists in labs wearing, shorts, T-shirts and sandals. In labs, we wear lab coats, face-masks and goggles. We don’t wear this stuff just to protect our clothes or to stop stuff splashing our eyes, we wear it to protect our skin, eyes and internal organs from ingesting dangerous materials. If you’re going to prepare pigments, think like a scientist and dress appropriately. Cover your skin, eyes and mouth. If you are really worried, wear ear-defenders too. As a field geologist we had to wear goggles when we were hammering or crushing rocks. These are proper goggles with a seal round your face (like a snorkel mask but without a breakable glass front) so nothing can get in. A visor won’t stop dust getting into your eyes. This is all really obvious stuff. If you do this, your risk of ingesting or absorbing dangerous substances will be really, really low.

My friend Jim using a rock saw and wearing appropriate safety kit. This is a water saw so there’s no dust, which is why he doesn’t have a mask on. A constant spray of pressurised water washes dust down into the trough. This water has to be disposed of via a sediment trap (and not straight into the drains).

Tidy up and wash-up …
Clean up your work-space thoroughly when you have finished working. Use a vacuum cleaner to get rid of dust. Oh, and have a shower when you’re done! Wash your hair while you’re at it. Don’t eat or prepare food until you’re clean. Really straightforward stuff, but again it hugely reduces risk.

If you are doing this a lot, invest in decent processing kit …
If you are making pigments commercially, and therefore grinding stuff every day, you should probably invest in some proper kit, i.e. mechanical grinders such as disc mills or ball mills AND a proper dust extraction set up.
When I was a grad student, I earned a bit of extra pocket money by rock crushing (as well as crushing tonnes of my research project rocks). You need to crush rocks to a fine powder to study geochemistry and geochronology. I would spend days, weeks doing this. It is quite a slow process. We had a purpose built space with different machines which gradually reduced the grainsize of the rock, starting with a sledgehammer and ending with a disk mill. Also we had a huge set of extractor fans to remove all the dusts. These were like enormous vacuum cleaners which we could move and direct so they were right over the source of the dust and suck it all up. I’ve done a lot of this and I was well aware that the more you do, the more you’re at risk. I still use the lab for grinding if I need to make a lot of pigment. So if you just grind up a handful of ochre once a year, there’s little to worry about (but wear a mask), but if you’re grinding pigments and making paints regularly, you should take precautions and ideally do it in lab conditions where there is protective equipment. Dust is the enemy!

What scares me?
There are a few things that I would not normally grind by hand at home, because they produce specific hazards. Obviously radioactive minerals are a no go. This is not a huge issue with modern pigments but can be an issue when working with some 19th Century materials.

Silicosis is a horrible respiratory disease which comes from long exposure to silica dust. It cannot be cured and it is cumulative; the more silica dust you breathe in, the worse it gets. Silicosis is a killer among workers in granite quarries who have constant exposure, but potters are also at high risk. Breathing in the silica dust scars the lungs and leads to breathing difficulties. Generally, it is best to try and avoid breathing it in. The great majority of rocks on our planets are silicates (sandstones, granites, basalts, schists, slates etc), basically anything that’s not a limestone. I would not advise grinding silicates unless you have proper rock crushing and grinding facilities including extractor fans, as described above. I admit that I have crushed slates and shales by hand, but they’re at the softer end of this spectrum, and I’ve been masked up, etc. But I still regret it. Silicate rocks and minerals includes lapis lazuli and clay-rich rocks. Get it crushed in a proper lab with dust extractors. It scares me when I see people crushing basalt, sandstones, granite etc. Also most silicate minerals lose their colour when finely ground, so it’s a bit of a waste of time. The only silicate mineral worth grinding (in my opinion) is lazurite (the blue mineral in lapis lazuli), but again this should be done in the presence of an extractor fan.

Serpentinites are also silicates but in addition they contain asbestos, so just leave them well alone. Soft, attractive and colourful though these rocks are, I would never grind them, and I don’t even like sampling them at outcrop. The lung cancer caused by asbestos, mesothelioma, is not necessarily dose related. It can be caused by just one, unlucky breath.

A polished slab of serpentinite, a rock predominantly formed of the serpentine group minerals, which can be asbestiform. Serpentinites are very variable in appearance, varying in colour from black, to grey, green, bright red and yellow. They often have a distinctive ‘snake-skin’ like texture and feel soft and soapy to the touch.

What’s an ‘acceptable’ risk?
Some pigment minerals are known to be toxic because they contain heavy metals, copper, cadmium, arsenic, mercury, lead etc. Some of these are super-poisonous, but they are also very important as pigments both art historically and in the present day. As this is what I research, needs must. Heavy metals can build up in the body, so this is dose related – and can do a lot of harm. Some can kill you. You can ingest them through breathing in dust or getting it in your eyes and ears, and some you can absorb through your skin. This risk can be massively reduced by not grinding or working with these materials wearing a vest top! Cover up, mask up, wear goggles. I would never grind large amounts of these minerals. Even though I’m a pretty good mineralogist so I know what I’m looking at, one cannot be sure of precisely what is present in a mineral sample. The unknown unknowns are a problem, microminerals or inclusions of poisonous stuff, or similar looking phases that might be hidden with a mineral. The worst of these minerals are the arsenic and mercury sulphides; orpiment, realgar, pararealgar, conichalcite, cinnabar, metacinnabar etc. and synthetic pigment equivalents like vermillion. Many of the worse pigment poisoners, like Scheele’s Green, Hooker’s Green and some varieties of cobalt violets (i.e. cobalt arsenate) are now banned. These materials are very bad for you, large enough doses can kill. They are also dangerous because they begin to break down thermally and at low temperatures, i.e. not much above room temperature and start releasing straight arsenic. Also, if you have samples of these minerals, don’t have them on an open display shelf, keep them in a sealed bag or box. Keep them away from sunny windowsills! When preparing them as pigments, do so in small amounts, and make sure your skin, mouth and eyes are covered. It’s probably a good idea to wear disposable gloves too. Make sure you was hands and face afterwards, and certainly before eating.

Cadmium pigments and lead pigments are a big risk in the same way, but at least you buy these in powdered form, so there is not the grinding hazard.

Copper minerals, i.e. malachite, azurite, atacamite, chrysocolla, conichalcite (a copper arsenate, so a double whammy, and it looks just like malachite) and lead minerals (to be honest rarely collected as minerals but the main ones are crocoite, galena etc.) are also poisonous, but you would need a much bigger dose to kill you, but they can have lasting damaging effects. Again, the rule is to try to avoid making pigments from these minerals unless you’re in a lab environment, and if you are doing this in your studio, do so in small amounts, cover up and wash afterwards.

The good news is, that once incorporated into a binder, these pigments are much safer, but care needs taking, especially with the very thermally unstable arsenic sulphides.

Something to be aware of is flushing pigments and paints made from these minerals down the sink. Try and avoid doing this!

Above: Conichalcite, a calcium copper arsenate hydrate. Quite a poisonous mineral, as minerals go! This is the nearest natural equivalent to the copper arsenate pigments which were really popular in the late 19th Century.

What’s OK (assuming above precautions are taken)?
Limestones and other carbonate rocks (these don’t contain silica, so no silicosis risk)

Biominerals (shells, coral, cuttlefish bone, bone etc) are carbonates or phosphates and don’t present any major risk, although they can absorb heavy metals during the lifetime of the organism, so this is something to be aware of. Boiling these materials in water before processing, for at least 10 minutes, should kill off any biological hazards and reduce the smelliness!

Iron-ochres and earths – however many of these do also contain clays, which are silicates and they can contain other heavy metal compounds. The purer the iron ochre, the safer it is. You’ve already got a lot of iron in your body, so you need to absorb an awful lot more to make yourself poorly from it. Nevertheless, iron poisoning is a thing. Breathing in a few particles won’t do you much harm, but it’s always best to avoid it. Again, this is more of an issue if you’re processing ochres on an industrial scale but cover up to avoid ingestion of materials and you’ll be OK. However, as dust is the enemy, even if these are not going to poison you, you need to avoid the dust, they can cause respiratory problems.

NB: Not all ochres are iron-rich. Other metals (cobalt, nickel, aluminum) can also form ochreous deposits.

 

FAQs – I’m happy to add to these!

What about heavy minerals and pollution in the environment?
This is an issue. But again it’s about reducing the chances of your body absorbing this stuff. The are trace amounts of heavy minerals that are pollutants in shells, bones, cuttlefish bone etc., so treat them with care and cover up whilst you are processing them. Clays can hoover up pollution, so take care if you’re collecting clays from industrial or former industrial areas. If you wash them and they smell funny or you get that oily, iridescent film on the water, then to be honest, I would leave it at that and dump them. They can contain some really nasty stuff, like benzene (this is a problem with the London clay in some Victorian industrial areas in London). Brightly coloured mine waste products should be treated with care too. The orange ochre mineral ferrihydrite that you see in streams is usually OK but check what was being mined there as these deposits can also contain arsenic and other horrors. Anything else, personally I’d leave well alone. It is waste for a reason and also sorts of nasty chemicals were used to process it.

Are you saying that I really should be only processing pigments in a lab environment?
No, not at all. Making these things at home is good fun. You just need to follow the basic safety advice; wear a mask and ideally eye mask, wear long sleeves and clean-up yourself and your space afterwards.

I’m still anxious about the risks some of these materials pose and feel I don’t know enough about them. Should I use them?
The simple answer to this question is no. If you don’t feel comfortable using a certain material then just don’t.

I’m getting conflicting advice about the risks of working with a certain rock, mineral or shell. How do I find out what the truth is?
Again, if this is making you uncomfortable, simply don’t use that material. But remember, the hazard is caused by getting this stuff into your body. If you do everything you can to avoid that, then you will reduce the risk. Try to rely on scientific sources rather than social media and hearsay to get more information.

Should I just completely avoid using seriously toxic mineral pigments like orpiment and vermillion?
No, but you should learn about them first and understand what the risks are. There’s lots of safety info out there. These minerals/pigments need handling with care. As I keep saying, you just do everything you can to avoid this stuff getting into your body. I wear gloves for handling orpiment. However, if you have no need to use these pigments they can be easily avoided. You can buy safe modern paints which have similar colours and properties.

Is it safe to teach kids pigment processing techniques?
It’s not safe to use the toxic mineral pigments mentioned above (copper, arsenic, lead, cadmium, mercury etc. based pigments), but processing ochres and chalks will be fine and good fun; no more risky than making mud pies. Levigating clays and ochres in water is pretty safe too. Make sure they’re supervised, old enough not to know not to eat it and make sure they’re wearing a mask if they’re working in dusty environments.

Many minerals have therapeutic properties, won’t this help protect me?
No, they don’t and no, it won’t. Sorry. In fact many minerals pose risks as described above and can be very harmful. Breathing in their dust or inserting them into your body can be really dangerous. Also the mineral trade to produce therapy crystals is really, really, really unethical and exploitative a lot of the time … but that’s another story.

A Colour A Day: Week 23

Ruth Siddall29 August 2020

A Colour A Day Week 23. 24th August – 30th August.

Jo Volley writes…

This week we celebrate Goethe’s 271st birthday, 28th August, with earth pigments from Cyprus collected and processed by Ruth Siddall who says of them …

Cyprus is an island long associated with the production of pigments. These are by-products of the copper mining that has been active since the Bronze Age when Cyprus was the main supplier of copper ingots in the eastern Mediterranean region. But it was not copper-based pigments that were in abundance, it was the iron and manganese-rich ochres and umbers which were typical of Cyprus as well as the celadonite-rich green earth deposits. The Cypriot umber is a true umber in the geological sense having formed at a mid-ocean ridge plate tectonic boundary. In fact this is the environment of deposition of all of Cyprus’s ores and pigments. They originally formed in deep ocean waters, superheated by volcanic activity and then this slab of oceanic rock, ores and all, was emplaced onto the Eurasian continent during the construction of the Alpine mountain chain. The ochres formed by the weathering of the ores both before and after this emplacement onto dry land. Such a geological environment is uncommon, and Cyprus is by far the biggest example of these processes on Earth. A unique island for pigment formation.’

All pigments are bound in gum Arabic on W&N watercolour paper and read;

Left hand column from top to bottom:
Yellow Ochre, Sia Mine, Cyprus
Jarosite Yellow Ochre, Sia Mine, Cyprus
Burnt Umber, Margi, Cyprus

Middle column:
Red Ochre, Sia Mine, Cyprus

Right hand column from top to bottom:
Raw Umber, Margi, Cyprus
Terra Verte, Cyprus
Brown Ochre, Sia Mine, Cyprus

Pigment Stories: Eternal Green in Predynastic Egypt

Ruth Siddall5 May 2020

I’m delighted to welcome Matt Szafran as a guest blogger this week. Matt is an independent researcher in Egyptology, currently studying the manufacture and use of Predynastic Egyptian stone palettes, using a combination of written material study, experimental archaeology, and advanced imaging techniques such as Reflectance Transmission Imaging (RTI). He has published magazine articles and has peer reviewed articles currently in publication. Matt is also presenting an introduction to Predynastic Egyptian palettes for the Egypt Exploration Society‘s current lecture series on 16th May 2020 and he will also be presenting his research on use-wear on Predynastic palettes at this year’s British Egyptology Congress in September.

Malachite, a copper carbonate hydroxide [Cu2(CO3)(OH)2] is a naturally occurring mineral formed in the weathered zone of copper deposits. Its a bright green colour, striking in outcrop. It is a mineral that would have stood out in a landscape (with the right geology) and would have been immediately attractive as a mineral with pigment potential.

A mineral specimen of malachite, illustrating its striking colour and typically encrusting habit (photo, Ruth Siddall).

Malachite does indeed have the properties to make a good mineral pigment; it is relatively soft and can be easily ground and it retains its colour when ground (as long as it is not ground too fine). Malachite has been used as a pigment in painting from the Egyptian Dynastic era onwards, and it occurs in all cultures worldwide. However, its use as a cosmetic material is often overlooked and this glimpse of the use of malachite on Egyptian Predynastic palettes is of great interest in terms of providing a more nuanced picture of the use of malachite as a pigment in prehistory.

A photomicrograph of malachite prepared as a pigment and viewed using cross-polarised light (Photo © The Pigment Compendium, 2004).

Over to Matt …

Pigment Processing using Stone Palettes in Predynastic Egypt

To most saying ‘Ancient Egypt’ will conjure images of kings and pharaohs, glittering gold, mummies (especially that of  a certain boy king), temples, and monumental statuary. All of these are from the Dynastic era (c. 3150-30 BCE), with the earlier Predynastic era (c. 6000-3150 BCE) receiving very little attention – in spite of having its own fascinating material culture.

The Predynastic tribes mostly used stone tools, with some copper and copper alloy working, and therefore the most common material remains are pottery, woven baskets, worked stone, beads and stone tools. One of the groups of stone objects, and the third most common object found in burials, is the stone palette. Palettes have been found from different sub-periods within the Predynastic era, however they were all made from the fine-grained greywacke sandstones and siltstones found in the Wadi Hammamat in Egypt’s Eastern Desert. The shape of palettes varied stylistically across each of the different periods of the Predynastic era, with palettes having been found ranging from simple geometrical-shaped forms to animal-shaped silhouettes to later palettes which typically have large, intricately carved surfaces.

Since their rediscovery in the late 19th Century, Predynastic palettes have been associated with the processing of pigments, with the likes of pioneering arcaheologist and Egyptologist Flinders Petrie stating that they were used for processing the copper ore malachite for use in cosmetics. This assertion is in part due to palettes being rediscovered in graves with traces of green staining still remaining on their surface.

Rhomboid-shaped palette in the Bolton Library and Museums collection (accessioned as 1909.76.10).

 

Fish-shaped palette in the Petrie Museum collection (accessioned as UC4374).

Whilst many scholars repeat 19th Century statements that malachite was ‘ground’ on palettes, experimentation has shown that malachite would in fact be crushed – initially against a large anvil stone and then the resultant crystal shards should be crushed further to a fine powder on the surface of a palette. The malachite needs to be wrapped in fabric or leather; this helps to contain the very flyable shards produced during crushing – something analogous to wrapping biscuits in clingfilm before crushing them to make a cheesecake base. To create the finest possible powder the anvil needs to be as smooth and polished as possible, something for which the surface of a palette is ideally suited.

Crushing malachite against a large sandstone anvil stone, with a handheld limestone hammer stone, to produce small shards and powder.

Once the malachite powder has been obtained, it can be mixed with a base to form a cosmetic or paint. Scholars suggest that this base could have been a drying oil such as linseed or poppy, a lipid (animal fat), or even simply water.

There is no evidence for malachite being used as a paint in the Predynastic period; pottery and other objects of this time only show evidence of ochre or gypsum-based paints. It therefore seems to be logical that malachite was instead only used as a cosmetic and applied to the body. Different scholars have differing ideas on what exactly the use of this malachite application could be. Some have suggested a strictly utilitarian use, with malachite application around the eyes acting as a defence against the sun, for medicinal benefit, or even to ward off flies. Others suggest much more ritualistic uses, with the application of pigments having a tegumentary use and essentially acting as a form of mask. Palettes were not a common item and were likely only owned by the elite members of society, something which would support a more ritualistic use over a purely utilitarian one.

Whilst palettes are typically discussed for processing of malachite, there have been palettes rediscovered with traces of different pigment on their surfaces. It also appears that the difference in pigments is related to their find location, with palettes found in settlement contexts having red ochre staining whereas palettes found in funerary contexts display green malachite staining.

It is impossible to say what the everyday settlement use may have been, however archaeological evidence of the funerary uses does appear to validate Petrie’s initial assertion that palettes were used with a form of eye cosmetic. Human remains found at the site of Aidema still retain malachite residue around their eyes, additionally a painted clay head was found at the site of el-Mahasna (in tomb H.97) which has green malachite around the eyes. This does imply that a part of the funerary ritual could involve the application of malachite pigment, from a stone palette, to the eyes of the deceased. Later Dynastic practices also apply green pigment to the eyes, as a symbol of rebirth, however one should be careful comparing the Dynastic and Predynastic as they are separated by hundreds to thousands of years and it would be logical to expect beliefs and ritual changed over this time.

Whilst there have been several suggestions and interpretations of what palettes may be used for, it is clear that they played a role in the creation and use of pigments. The difference between pigments traces on palettes found in settlement and funerary contexts suggests that palettes held multiple roles in both daily life and also as part of funerary rituals, and that there is likely no single answer to their use. Sadly, we can only speculate on their uses, and we will never know the exact answer of what these objects meant to the people who owned them.

Follow Matt on Instagram and Twitter

To cite this blog:

Szafran, M., 2020, Pigment Processing using Stone Palettes in Predynastic Egypt, The Pigment Timeline Project, UCL Blogs 05/05/2020; https://blogs.ucl.ac.uk/pigment-timeline/2020/05/05/eternal-green-ma…pigment-in-egypt/

The Origin of Ochres #1: Interbasaltic Beds

Ruth Siddall1 April 2020

It is fair to say that ochres and their origins are poorly discussed in the academic geological literature. Though ubiquitous in the landscape, they are largely ignored by most geologists. They occasionally pique the interest of economic geologists but are generally dismissed and shovelled away in favour of something more shiny. Ochres can form in a wide range of geological environments on Earth and indeed, on Mars (there’s a reason it’s red), but in this post I’m going to focus on ochre forming on the weathered surfaces of solidified basalt lava flows (I may get around to writing about other ochre-forming environments in the future). At its simplest, ochre is defined as an earthy deposit predominantly composed of metal-rich oxides or oxide-hydroxides. By far and away, iron ochres are the commonest, but ochres of other metallic elements such as cobalt, nickel, copper etc. can also form. Ochres form in the surface or near-surface environment, in the presence of oxygen and water. Iron ochre formation is accelerated by warmer Mediterranean or tropical climates, and the presence of red rocks is therefore often indicative of past warm climates in the rock record.

The inspiration for this post was a photo (above) posted on Instagram for World Pigment Day by Scott Sutton of an ochre layer between basalt lava flows in the Rio Grande Gorge of New Mexico. This region’s geology is dominated by basaltic volcanism which erupted in the upper Miocene, around 10 million years ago. These eruptions were not like any basaltic volcanism we can observe from active volcanoes anywhere on Earth today. They were large-scale, effusive flows which spread out covering large areas and forming a plateau composed of a thick pile of solidified lava. Such formations are subsequently known as plateau basalts or (continental) flood basalts. The basalts of the Rio Grande Gorge are part of the Taos Plateau Volcanic Field (TPVF). Following the end of volcanism, the Rio Grande cut down through the basalt pile exposing 180 m of section. Scott’s visit into the river gorge and his photograph revealed part of this geological history. The old adage says that if you want to hunt elephants, first you must go to elephant country. The same is true for geological prospecting; horizons forming between successive basalt lave flows are typically ochre-rich, and therefore these are good places to go pigment hunting. Certain types of rocks form in certain regions, and their occurrence is generally controlled, ultimately, by the regional plate tectonic environment. The kind of basalts that are erupted into rift valleys – areas of continental extension, which is the setting of the Rio Grande – are typically iron-rich. Most basalts contain significant iron, but continental flood basalts are the richest. When they cool and become weathered, ideally in a warm, wet climate, they produce iron-rich soils; ochres. These ochres are then sealed and preserved by the next lava flow that covers them. Later in their geological history, these so-called interbasaltic beds can be further weathered and the ochre more concentrated by groundwaters which percolate through these porous layers. Layers of basalt are impervious.

This series of geological logs, from a guide to the Rio Grande basalts by Dungan et al. (1984), shows how the individual basalt flows (white) are interlayered with sediments, including ochre palaeosols (stippled). Like so many other papers on this subject, the authors record much about the basalts and little, if anything is said about the ochres.

In the British Isles the British Tertiary Volcanic Province (BTVP) is a series of flood basalts formed as the North Atlantic Ocean opened around 60 million years ago. Most famously, these are exposed in Northern Ireland on the Antrim Coast where they form the Giant’s Causeway. This basalt pile is famous for its ochreous interbasaltic horizons and is the one place where a series of papers have been published on ochre formation. Although several ochre-rich interbasaltic horizons occur between the flows of the Antrim plateau basalts, there is one 30 m thick horizon of weathered basalt and associated palaeosols which has attracted attention for many years. It is known as the Inter-Basaltic Formation (IBF) and the ochres, known locally as boles (a good painter’s term) are mostly laterites, that is aluminium and iron-rich ochres (aluminium-rich ochres are known as bauxites), The main aluminium mineral present is gibbsite. Laterites are typically orange in colour. Yellow goethite and red hematite iron ochres also occur here, along with purple-coloured ‘lithomarge’ which is rich in clay minerals and hematite.

The Antrim Basalts from an early publication by Cole et al. (1912). The huge Bole Bed is inexpertly marked out in (appropriately) red paint.

The analyses carried out on the Antrim ochres suggests they formed in warm, wet and occasionally hot, arid climates in the early Palaeogene. Similar horizons are also found in India’s Deccan Traps flood basalts.

A figure from Ghosh et al. (2006)’s paper on the boles of the Deccan Traps; interbasaltic ochre beds formed here in very similar climatic conditions to those of the Antrim Basalts. This 2 km thick pile of basalt flows was erupted towards the end of the Cretaceous, 66 million years ago. 

Basalts are composed of three main minerals, olivine, pyroxene and plagioclase feldspar. The iron minerals are produced from the breakdown of olivine and pyroxenes, whereas the aluminium-rich laterites and bauxites form due to the breakdown of the feldspars.  Hematite (iron oxide) and goethite (iron oxide hydroxide) are the main and most stable iron ochre constituent minerals. Gibbsite (aluminium hydroxide) is the predominant mineral in laterites and bauxites.

You don’t need huge plateau basalts to find ochreous interbasaltic beds. You can find them on most volcanoes that have erupted basalt. These examples below are in the lower eruptive sequences of the Greek volcanic Island of Thira (Santorini). The reddened layers are clearly seen between the layers of grey-black basalt.

A view towards Firostefani on the Santorini Archipelago. You can see the reddened ochre layers between the grey coloured basalts.

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References and further reading

Cole, G. A. J., Wilkinson, S. B., McHenry, A, Kilroe, J. R., Seymour, H. J., Moss, C. E. & Haigh, W. D., 1912, The interbasaltic rocks (iron ores and bauxites) of North East Ireland., Memoirs of the Geological Survey of Ireland., Dublin, Ireland, 143 pp.

Dungan, M. A., Muehlberger, W. R., Leininger, L.,  Peterson, C., McMilan, N. J., Gunn, G.,  Lindstrom, M. & Haskin, L., 1984, Volcanic and sedimentary stratigraphy of the Rio Grande gorge and the late Cenozoic geologic evolution of the southern San Luis Valley., in: Rio Grande Rift (Northern New Mexico), Baldridge, W. S.; Dickerson, P. W.; Riecker, R. E.; Zidek, J.; [eds.], New Mexico Geological Society 35th Annual Fall Field Conference Guidebook, 157-170

Ghosh, P., Sayeed, M. R. G., Islam, R. & Hundekari, S. M., 2006, Inter-basaltic clay (bole bed) horizons from Deccan traps of India: Implications for palaeo-weathering and palaeo-climate during Deccan volcanism., Palaeogeography, Palaeoclimatology, Palaeoecology 242, 90–109.

Hill, I. G., Worden, R. H. & Meighan, L G. 2001, Formation of inter basaltic laterite horizons in NE Ireland by early Tertiary weathering processes. Proceedings of the Geologists’ Association, 112, 339-348.

Ruffell, A., 2016, Do spectral gamma ray data really reflect humid–arid palaeoclimates? A test from Palaeogene Interbasaltic weathered horizons at the Giant’s Causeway, N. Ireland., Proceedings of the Geologists’ Association., 127, 18-28.