Previously, we discussed some basic ways of categorising pigments, looking at the organic/inorganic and natural/synthetic definitions and then progressing a little into qualities in use.

I am going to continue this, in a series of four further posts, diving a little deeper in the categorisations of pigments, starting with natural inorganic pigments.

Natural inorganic pigments are some of the oldest pigments ever used, having been found in burial sites from up to 60,000 years ago, and artworks from around 20,000 years ago. Our cave dwelling ancestors were pounding rocks and mixing the powder with fat long long ago.

Obviously as aesthetics and technology advanced these have been refined through various processing methods, but my definer of a natural inorganic pigment is that it has been gathered from a natural source, and prepared by no more than grinding, washing, purifying and heating.

I would include in this where the colour has been naturally altered by the effects of human activity, for example iron staining from industrial processes or soot from atmospheric pollution, and in this I would have to include (in the rare examples where this is the case) foraged verdigris and other copper compounds.

Essentially this is where the compound itself is not synthesised in a laboratory for the purposes of pigment making.

So, I would also include artificially calcined (heated) earths, so long as other specific compounds are not reacted. Essentially here the chemical substance is not significantly altered, other than perhaps different levels of oxidation or hydration (i.e. calcining which could remove water from the crystal structure or convert oxides of a metal to a different oxide of the same metal). One could use the qualifier “processed inorganic” but then I would argue virtually any substance used nowadays as a pigment as been processed in some way.

In relation to my practice of foraging I would include (although opinion here will differ) naturally found industrial byproducts such as naturally occurring iron oxides on discarded metal, or kiln slag. As Lucy Mayes notes “all pigments come from the earth”¹, to an extent on the other end of this definition – human beings are a part of nature and their actions therefore natural.

Catagorising these pigments succinctly could be tricky.

MacEvoy states that: “With few exceptions, inorganic pigments are combinations of a mineral element with oxygen and other elements (most often sulfur, silicon or carbon) that fall in the chemical classes known as oxides, oxide hydroxides, sulfides, sulfates, silicates and carbonates.”² . In my mind, however, catagorising pigments solely by their cations in this way in unclear for the artist as in most cases it does not explain much about their properties or behaviour.

A classic historical break down would likely be:

• Red earths
• Yellow earths
• Green earths
• Mineral pigments

But already this creates issues. While artists would often break the earths down to colours (and I would seperate browns out of this too) the “earths” are mostly clays, and as such could be one catagory of “minerals”. Other minerals have been used over history as colours of one sort or another, with the added complexity now that many exist in natural and man made forms. To an extent, there is the difference that most “earths” are mixtures of compounds and most minerals are relatively pure.

We could perhaps expand and sub-catagorise as follows:

• Earths
  • Red³
  • Brown
  • Yellow
  • Green
  • Violet
• Minerals
  • Oxides
  • Oxide Hydroxides
  • Sulphides
  • Sulphates
  • Sillicates
  • Carbonates

There’s some sense to this, perhaps in part due to traditions of use. Most Earths are Iron Oxide or Iron Hydroxide based minerals (deeper browns including manganese and green earths tending to be Iron/Potassium compounds) and as such could simply be seen as one catagory of minerals, but as there are so many historical examples there’s some sense in breaking them down and as we mentioned they tend to be mixed. Here the minerals are catagorised by their cations – but as we’ve noted this may not be helpful.

The other option is to catagorise minerals by their anions, which goes along with how synthetic compounds (which of course may mimic natural compounds) tend to be grouped:

• Minerals
  • Iron Minerals (the basic constituents of most Earths)
  • Copper Minerals
  • Arsenic Minerals
  • Manganese Minerals
  • etc, ad (almost) infinitum.

I am minded that in many cases gives a better idea of their colour and properties, as it is often, but by no means always the anion which has the greatest impact on colour and is responsible for the structure/mechanism which could be defined as the chromophore in the compound.⁴ As an extension of this we could catagorise by actual mechanism, e,g trapped radical anions, ligand field transitions and so on, if one wants to get very technical.⁵

Other catagories could be Elemental Compounds – for example sulphur which can be used in pure form as a pigment, and indeed some metals in elemental form.

I think that some of these are perhaps based on tradition and usage. As I have previously noted I would, at least in mineralogical terms, most likely catagorise Lapis Lazuli as “Sillicate Minerals” and include Azurite and Malachite as a “Copper Mineral” – though if I was defining as I have with Lapis, these would be more accurately grouped as “Carbonate Minerals”.

To some extent, it’s nessecary to be guided by usage and artistic merit here and accept that catagories may be partial, overlapping and indistinct.

Going forward perhaps catagories, for completeness are

Type, Anion/Metal, Cation, giving for example:

• (Colour) Earth, Iron Mineral, Oxide
• Mineral, Copper Mineral, Carbonate

with perhaps additional catagories for elemental pigments, e.g.,

• Metal, Copper
• Non-Metal, Sulphur

I will aim to make some sense of this in the following outlines.

Earths

Earth pigments are, as we’ve noted, some of the oldest pigments used. While it is possible (and not at all uncommon) to synthesize pigments which are based upon the main compounds in earth pigments, natural earths are often a mixture of compounds and “impurities”⁶ which is what gives them their more variable colours and properties.

Red Earths

As the name would suggest these are red(ish) coloured earths (clay and sand). They are composed mostly of haematite which is a hydrated form of iron(III) oxide (or ferric oxide, Fe₂O₃·nH₂O), along with impurities such as other iron oxides and hydroxides, for example, as orange or yellow lepidocrocite (Iron(III) oxide-hydroxide or ferric oxyhydroxide, γ-FeO(OH)), black magnetite (Iron(II,III) oxide or ferrous ferric oxide,  Fe₃O₄), and manganese oxides.

Hydrous (water containing) oxides are lighter, and as the water content is reduced anhydrous oxides become darker (this being how the calcining process reddens ochres). Red Earths would typically contain around 50% iron oxide and less than 1% manganese dioxide.

The naming of these vairies. Some may fall into what is known as known as Siennas, (from the italian region where they were often mined) or Umbers⁷, Indian red, English Red and or various geographical names such as Terra Ercolana, Venitian Red and and so on, however Ochre – the overriding common name for this type of sedimentary rock can also be used.

They would normally be designated with the pigment code PR102, some darker varieties could be classed under PBr7. I would still class these as Natural if they have been processed with nothing more than artificial calcining (and some may be calcined yellow earths). The designation here comes broadly from the colour, though see below under brown earths, where a clearer defintion has been proposed.

Yellow Earths

These are the yellow(ish) coloured clays and sands, their colour being characterised by increased proportion of sillicates (sand, micas), and their iron compounds being Limonite and Geothite (iron(III) oxyhydroxide, α-FeO(OH)). Limonite is commonly defined as a single ore, but more recenly has been accepted refer to a mixture incuding Goethite, Lepidocroite (another form of iron(III) oxyhydroxide, γ-FeO(OH)), Akaganeite (Chloride-containing iron(III) oxide-hydroxide,
FeO(OH,Cl)) and Jarosite (hydrous sulfate of potassium and iron(III), KFe₃(SO₄)₂(OH)₆)

These are commonly known as (Yellow) Ochres or Siennas (and Sienna tends to be associated with the Oxide-Hydroxide ores), with the pigment code PY43.

Brown Earths

Brown earths tend to be a mixutre, of the above minerals and clays, other oxides such as maghemite (another form of iron(III) oxide, γ-Fe₂O₃) and often, especially in Umbers (which most brown earths would be named), manganese oxides and hydroxides.

Brown earths would, unsuprisingly, be designated PBr7. These begin to exemplify the fact that natural mineral pigments (particularly earths) may not be a discrete compound, and as such the colours and properties thereof are greatly influenced by factors other than the composition, such as particle size and shape.

Green Earths

Green earths are somewhat different. Rather than simpler Iron Oxides, these are composed predominantly of two main minerals – Glauconite – an Iron-rich potassium phyllosilicate, approximately represented by compositions in the vicinity of⁸ (K,Na)(Fe³⁺,Al,Mg)₂(Si,Al)₄O₁₀(OH)₂ and Celadonite – chemically related but generally more structurally ordered and commonly represented approximately as K(Mg,Fe²⁺)(Fe³⁺,Al)Si₄O₁₀.

These are designated PG23.

Violet Earths

This is an historical and artistic catagory, also with a somewhat amorphous nature. At the deeper end of the colour scale this term could include bluer red earths such as morellone which is essentially a red earth with a smaller and more regular particle size (affecting it’s colour) but also violet and lilac shales with lower amounts of iron oxides in a clay/sillica base. In general the redder ones would be classified as PR102, lilac shades may lack a CI pigment number, though slate (a metamorphic phase of shale) is catagorised as PBk19.

Briefly, under violet/green earths one could also include vivanite – (hydrated iron phosphate Fe₃(PO₄)₂·8H₂O), sometimes known as Blue Earth, though it can tend to green or brown – this would generally not be classified with a CI pigment code.

Minerals

Virtually any coloured mineral substance can, to some extent, be used as a pigment and over time many have. In general a mineral pigment (and for this they were prized in antiquity) would be purer in colour and composition than an earth. Minerals such as Cinnabar, Malachite, Azurite, Lapis Lazuli, Purpurite, Jade, Garnet and so on (as I have noted almost ad. infinitum) can and have all been used as pigments througout history.

Notwithstanding the above discussion, for the purposes of this introduction I am going to look at the principle cation – the reason for this is I wish to seperate compounds such as orpiment, cinnabar and lapis lazuli, for which the colour is produced by sulphur containing systems because historically they sit very differently. However, Lapis Lazuli I will discuss as a sulphur pigment (rather than a sillicate) as a special case – in this the sulphur is anionic, but not a simple anion.

Iron Minerals

I’ll get these out of the way first, as we have discussed these at some length in our consideration of Earths. However, two important iron minerals appear as pigments in their own right, these being haematite (red, or tending to black) and magnetite, this latter often artificially synthesised as a black pigment – mars black. Vivanite and celandonites also fall into this list to an extent, and do appear as mineral pigments rather than earths. The Iron pigments oxides, vivanite is a phosphate and celandonite a sillicate with a far more complex structure.

In this catagory, as I am choosing to also include naturally formed substances which derive from human activity, we must include rust – this is predominantly Iron(III) Oxide (analogous to Haematite), with other oxides present.

Arsenic Minerals

Two very important mineral pigments, Orpiment (arsenic trisulfide, As₂S₃), known as Kings Yellow is a yellow pigment and Realgar (tetraarsenic tetrasulfide; α-As₄S₄), both with the CI Pigment Index number PY39. In the most basic catagorisation of the cation, these are sulphur compounds.

Copper Minerals

I have been waxing lyrical about homebrewed synthetic forms of copper minerals, in my work on bice. The main naturally forming copper minerals used as pigments are the green malachite: (copper oxide hydroxide, Cu₂CO₃(OH)₂) and the blue azurite: (also copper oxide hydroxide, Cu₃(CO₃)₂(OH)₂). These are obviously carbonates in the most basic catagoriation of the cation. cuprite (copper(I) oxide; Cu₂O), is also used as a pigment, though mostly in ceramics as has been copper hydroxide (syntheticly produced and often a mixture with copper carbonates).

As above I would also include naturally formed verdigris, often a mixture of the above carbonates along with copper hydroxides and copper acetate, copper sulphate etc. from atmospheric pollution.

Manganese Minerals

Two key examples – purpurite (mostly manganese(III) phospahte MnPO₄, with some iron impurities) which is as the name suggests, purple, and pyrolusite (Manganese Dioxide, MnO₂) and manganite (manganese oxide hydroxide, MnO(OH)) which are brown to black – the former obviously a phosphate and the latter two an oxide and an oxide hydroxide – various other mineral forms exist which could co-exist within raw mineral pigments.

Lead Minerals

Though more commonly produced synthetically since antiquity, minium (Lead(II,IV) Oxide or Lead Tetroxide, Pb₃O₄) designated PR105 is an important red pigment, and to a lesser extent wulfenite (Lead Molybnate, PbMoO₄) is a yellow/orange mineral historically used. These are Oxides.

Mercury Minerals

One very important compound – cinnabar (Mercury(II) sulfide, HgS), which is a bright red, the synthetic form being Vermillion, both may be listed as PR106.

Lapis Lazuli

This, historically has been set apart, and chemically is makes sense also to do so. Lapis Lazuli is a blue metamorphic rock predomminantly consisting of the vibrant blue mineral lazurite ((Na,Ca)₈[(S,Cl,SO₄,OH)₂|(Al₆Si₆O₂₄)]) along with pyrite, diopside, and calcite⁹. The raw rock itself is used, but processing with mastic and washing can isolate the lazurite, which was the historical pigment of such repute. The synthetic form of this is ultramarine (PB29).

Calcium Minerals

I’m just going to include this here as an important historical pigment is of course calcium carbonate – CaCO₃ – better known as chalk (designated PW18), or marble or limestone – various mineral forms exist. Gypsum (PW25), Calcium Sulphate, CaSO₄·2H₂O is also considered a white pigment.

Others…

As I noted this list could go on for some time. Other white pigments for example obviously have natural forms – the most common historically being Kaolin clays (aluminium sillicate).

Notably, Daniel Smith’s Primatek range includes a huge number of different mineral compounds ground into watercolour¹⁰, and Wallace Seymour uses a number of natural foraged and procured earths, and some rarer minerals, such as eskolaite (which is essentially natural Oxide of Chromium), augites, sandstones, slates and so on. Other ranges such as Roman Szmal and A. Gallo also specialise in geographically specific and local earths and other minerals. I will just briefly, for fun if nothing else, mention various striking green and yellow uranium minerals, but one would really not want them in a pallete…

Native Elements

This is a small set, and one that is naturally disparate. Native sulphur can be used as a pigment in oils and acrylic mediums. I have used pure copper to good effect in watercolour, and some metals do have their own pigment designations. It’s obviously arguable whether these should be classed as natural or synthetic as few exist naturally in elemental form.

Perhaps the most common, however is carbon, specifically as graphite, though (especially if including foraged human-influenced materials) charcoal could be included here, though it sits on the boundary between organic and inorganic; while mostly carbon it is from an organic source.

This is, as ever, an introduction and the classification of pigments is very much a multifacited and subjective exercise – as this first stage has shown. At some point I will tie this up further, after we have looked at synthetic inorganics, as what I intend to do – expand on the debate on the structure/mechanism of the primary chromophore – applies to both.

Next up, natural organic compounds.

1. 2005, The Natural Pigment Handbook. Exeter, David and Charles. ↩︎
2. https://www.handprint.com/HP/WCL/pigmt1a.html ↩︎
3. Sometimes a classification is included for orange earths, I have omitted this as the Pigment Index does not include a code for orange earths, and broadly one can fit these into red, yellow or brown. ↩︎
4. This is a gross simplification which I will unpack in a future post ↩︎
5. Again, I will discuss this in a future post. ↩︎
6. By which in this context I mean something of minor but still noticable effect in the structure/mechanism producing the colours. ↩︎
7. It is disputed whether this refers to the region of Umbria or umbra meaning shadow ↩︎
8. Its composition is highly variable because glauconite is essentially a compositional series rather than a perfectly fixed stoichiometric substance. ↩︎
9. Slide over to Wikipedia if you want to know more I am not discussing these in depth here as they are irrelevant. ↩︎
10. The linked posts discuss debate on the integrity of these formulations ↩︎