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A Colour A Day: Week 52

Ruth Siddall21 March 2021

A Colour A Day: Week 52. 15-21 March 2021

Jo Volley writes…

The arrangement of this week’s colours is taken from Agnes Denes’ poem Colors of the Week* and features David Dobson’s 2017 invention Deep Water Blue pigment. David explains.

‘The colours of the common minerals are dominated by the presence of iron. Iron atoms in minerals take on two charges, loosing either two or three electrons to make ferrous (Fe2+) or ferric (Fe3+) iron. Iron oxides and hydroxides commonly contain mixed valence states and exchange of electrons between the ferric and ferrous states produces the reds and yellows seen in the earth pigments. In silicate minerals, such as olivine, iron replaces divalent magnesium and Fe2+ dominates. In this case electronic transitions localised on the iron ion causes olivine to have pale green colours. Very occasionally charge transfer between iron ions in ferric and ferrous states can cause blue colouration. Vivianite is an iron phosphate where the iron is in the 2+ state. When fresh it is colourless, however exposure to air causes some oxidation to iron 3+, some of which which sits on the tetrahedral phosphate site and a blue colour develops quite quickly. It seems that this tetrahedral ferric iron might be the key to making iron-based blues. Ringwoodite is a vibrant blue silicate spinel which is stable between 520 and 660 km depth in the Earth. In this case the colour only develops when there is a charge-coupled substitution of Fe3+,H+ onto the tetrahedral Si4+ site. This substitution is quite easy in ringwoodite, and if all of the Earth’s ringwoodite were fully hydrated it would contain something like 4 times the amount of water in the oceans. The Deep Water Blue pigment uses silicate and germanate structures which can take significant amounts of ferric iron on tetrahedral sites to reproduce the colour of ringwoodite.’

David Dobson is a geologist, mountaineer and print-maker. He is interested in process, whether that is the chain of action linking winter mountaineering to a final image or developing new pigments. He is also a professor at UCL, Earth Sciences and the first Slade Scientist in Residence 2018-19


YouTube: One Minute Geology

Image: Fe-Mg ringwoodite David Dobson

Persian Orange
Grey Rose
David Dobson’s Deep Water Blue
Brown-Red Slate
*Thanks to Lesley Sharpe for directing me to this poem.

Today is both International Colour Day & World Poetry Day and the eve of World Pigment Day established in 2019 by Ruth Siddall and myself. This project started on 23rd March 2019 in an attempt to celebrate and document it’s initial year by simply dedicating a painted swatch of colour to each day and pure coincidence it was also the first day of lockdown in the UK but has somehow documented this most extraordinary year. It was inspired by A Boogert’s C17 educational manual of how to mix every colour available and influenced by Werner’s Nomenclature of Colours with Patrick Syme’s amendments. A Colour A Day is now finished (I hesitate to say complete as I haven’t painted every colour available to me in my studio) and later in the year they will be made into a series of digital prints and a publication.
Thank you to all of you who have sent me pigments and paints, writings and poems to include.

A Colour A Day: Week 2

Ruth Siddall5 April 2020

A Colour A Day is a year-long project by Jo Volley, which began on the first World Pigment Day, 22 March 2020,  to celebrate one colour each day by recording a swatch. This is the post of colours created for Week 2; 30th March – 5th April.

This week’s contributions are group of colours made from natural sources that Jo has collected and prepared as pigments.

  1. Iron Gall Ink
  2. Hampstead Heath Ochre no.6
  3. Pomegranate Ink
  4. Hampstead Heath Ochre no.3
  5. Cork Black
  6. Prespes Red Ochre
  7. Bone Black

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.

Follow Scott Sutton on Instagram, and visit his webpage here.

Download this article as a pdf document


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.

A Colour A Day: Week 1

Ruth Siddall30 March 2020

A Colour A Day is a year-long project to celebrate one colour each day by recording a swatch of it.

International Colour Day and World Pigment Day fall respectively on the 21st and 22nd of March. The project started on 23rd March which was coincidentally also the day lockdown began in the UK.

It has begun with the Liquitex Heavy Body Cadmium Free range of 7 colours, as seen here,  and tomorrow will progress onto a range of natural colours.

Jo Volley, 30 March 2020

Day 1. Yellow Light

Day 2. Yellow Medium

Day 3. Yellow Deep

Day 4. Orange

Day 5. Red Light

Day 6. Red Medium

Day 7. Red Deep

Pigment Stories: Sarah Needham’s Graphite

Ruth Siddall25 March 2020

Graphite is a naturally occurring mineral pigment, a form of  carbon, which occurs in geological environments which have undergone high temperature metamorphism or where there has been precipitation of elemental carbon from fluids. Vein carbon deposits are regarded as exceptionally pure. Graphite has been used as the main pigment for pencils. A lode of graphite was discovered in Seathwaite in the English Lake District in the 16th Century, when it was assumed to be lead (plumbago) because of it’s sub-metallic, silver grey lustre. Graphite has a very high specific heat capacity (as opposed to the metal lead), so it was initially used for moulds for casting cannon balls. The graphite pencil was exclusively manufactured in Britain because of the particular quality of the Seathwaite deposits, but they were relatively rare. The uniqueness of the Seathwaite deposit was that it could be sawn into square-section rods which could be used for drawing. Most artists and draughtsmen were using silverpoint for drawing at the time and this continued to be used until the mid 19th Century when the ruction of graphite pencils became universal.

For World Pigment Day, artist Sarah Needham wrote about a graphite pigment made from graphite rods used in the steel-making industry.

“I’m really interested in the way that pigments leave traces of our history and human interconnectedness across time and geography. The pigment in these videos is graphite, recovered from graphite rods my Uncle bought at auction when the steel works in a North Yorkshire town were closed down. This is a particular incidence of history that is close to my very own personal history, firstly because my uncle found them, and secondly because on the other side of the family there is a history of stainless steel cutlery making. The industrial graphite took some pounding to get into powder form and I did this by covering it in a cloth before pounding it.

More often I look for pigments that play a role in historical events which have resonance with current events. For example my recent collection From Alchemy to Chemistry uses pigments that were synthesised as a result of chemical analysis, to replace older natural pigments, in the industrial revolution. The connection being an era when technological change revolutionised our ways of being, living, doing and seeing…just like the technological revolutions of

What exactly are graphite rods? They are used in the steel industry for a stainless steel making technique called the electric arc furnace (EAF) process and for refining the final product (turning steel into stainless steel) in blast furnace processes. The latter were those most probably used by Sarah’s ancestors. Graphite rods are used as electrodes as they can carry huge amounts of electric current. They are made by mixing graphite and pitch and then placing this mixture into tubular moulds. These are then heated so that the pitch turns to coke, this mixture is then heated to extremely high temperatures, in modern process, these are typically 3,000 °C, so that the entire mix of hydrocarbons is reduced to pure graphite.

Follow Sarah Needham on Instagram.

Pigment Stories: Margot Guerrera’s Colour Wheel

Ruth Siddall24 March 2020

This beautiful colour wheel was created for Work Pigment Day by Margot Guerrera, and artist from Santa Fe in the South-West USA and the pigments she has used here have been inspired by the desert landscape in which she lives. Starting at the top, the pigments used are Heart Wuda (heartwood), Campeche Wood, New Mexican Ochre, Copper Vitriol, Azurite and Mixtec Indigo. They represent a mixture of plant-based dyes, geological deposits and synthetic materials, reflecting the world of pigments.

Follow Margot in Instagram and see more of her work here.

Pigment Stories: Polly Bennett’s Pigment Rainbow

Ruth Siddall23 March 2020

Yesterday (22 March 2020) we launched the inaugural World Pigment Day. There was an huge amount of engagement on social media and particularly on Instagram. Over the next few days I will be sharing images and pigment stories from people who posted to celebrate World Pigment Day. First up is artist Polly Bennett, a resident of St Ives in Cornwall, who contributed a series of posts on the colours of the Rainbow. Over to Polly …

Red: Cinnabar Cinnabar is a toxic mercury sulfide mineral that has been used as a pigment for thousands of years due to its bright red colour. It is a pigment in its own right, however, it was also used to make the red pigments known as “vermilion” and “Chinese red”. Cinnabar is a hydrothermal mineral that is usually found in rocks surrounding recent volcanic activity but can also form near hot springs and fumaroles (an opening in or near a volcano). Because of cinnabar’s toxicity, it is a lot less commonly used nowadays.

Orange: Ochre Ochre is a family of earth pigments that includes yellow ochre, red ochre, purple ochre, sienna, and umber. It consists of varying amounts of iron oxide, clay and sand, and ranges in colour from yellow to deep orange or brown with an array of shades inbetween. I have found huge amounts of ochre earth in St Ives, where I am currently staying, and have been slowly but surely grinding and separating the ochre into different shades. The mineral goethite, an iron oxide hydroxide and the main constituent of most yellow ochres, is named after Johann Wolfgang von Goethe, the colour scientist whose death marks the date of World Pigment Day

Yellow: Sulphur Although Sulphur is not a pigment, I found some in a curio shop and was curious to see if it ground into a powder; would it work in the same way?  So I intend to turn it into watercolour and test it out. Historically it has been used to bleach cloth, so it might do something similar when applied over the top of other watercolours. Sulphur occurs naturally as the element, often in volcanic areas, and as the extraction of pigments is very alchemical, I thought it was interesting to note that for centuries, along with mercury and salt, it was believed to be a component of all metals and formed the basis of alchemy, whereby one metal could be transmuted into another.

Green: Green Earth from St Ives Yesterday I was super excited to find a little green sparkly rock on the eroded foreshore. I set about grinding it down and managed to get two shades of green from it, the darker one I immediately made into watercolour.



Blue: Azurite Azurite is a soft copper mineral, named for its beautiful “azure blue” colour. It has been ground and used as a pigment in blue paint as early as ancient Egypt, and through time, its become much more common. During the Middle Ages and Renaissance, it was the most important blue pigment used in Europe, and through the early 19th century, it was also known as chessylite, after the type locality at Chessy-les-Mines near Lyon, France, where much of the pigment was mined. Here I have mullered the Azurite into glaze.

Indigo: Mussel Shell Blue from St Ives Since landing in St Ives I have been going down to the foreshore every morning to collect mussel shells as I wanted to create a blue pigment to represent the sea, however after being ground the mussels create a light indigo colour that I love! Historically painters used shells as paint pans, so I thought it very appropriate to make watercolour paint with the mussel pigment and use one of the mussel shells as the pan for the paint.

Violet: Cochineal Cochineal is a bright scarlet insect lake pigment that has been used for centuries to dye textiles, drugs, food and cosmetics. A lake pigment is a pigment made by precipitating a dye with a mordant. Unlike mineral pigments, lake pigments are organic.Cochineal is the result of harvesting the female cochineal parasitic insect that live on the cacti native to Mexico, Central and South America. Using soda ash and alum, I extracted the pigment from the insects and added honey and gum arabic to make watercolour.

Follow Polly on Instagram.