On Pigments (Part 3) – Natural Organic Pigments

So, after an introduction and an exploration of natural inorganic pigments, we now turn our attention to natural organic pigments.

Here, of course things could become more complex as we noted previously. The natural inclination and one I have seen in some of the extant literature¹ is to create the top level classification as origin – typically plant and animal. Alongside this, we may need to include geological organic (asphaltum, van dyk brown, lignite/coal) as the origin of these is more diverse. Of course chalk is another outlier, while inorganic in chemical terms it is produced from the remains of biological organisms. and of course we have Indian yellow – ultimately of plant origin (historically suggested to be mango leaves) yet ultimately produced via an animal². Added to this is that in the case of pigments many will exist laked to a substrate (chalk, aluminium hydroxide) – while I will maintain these are effectively natural despite processing, the actual structure/mechanism of the what can be classed as the chromophore may be affected by this substrate.

A basic list of some common pigments catagorised by origin could be

Animal Origin:

• Bone black/Ivory Black/Hartshorn³
• Cochineal/Kermes
• Tyrian Purple
• Sepia
• Indian Yellow

Plant Origin:

• Vine Black⁴
• Logwood
• Indigo
• Woad
• Safflower
• Reseda/Weld
• Curcumin
• Saffron
• Stil de grain
• Gamboge
• Madder/Alizarin
• Dragons Blood
• Chinese Green/Buckthorn
• Blackthorn
• Brazilwood
• Avocado⁵
• Beetroot
• Buddleia
• Spirulina

Geological

• Asphaltum
• Van Dyke Brown
• Lignite/Coal/Jet

However, this tells us very little about the actual pigment itself. Tyrian purple for example (extracted from a mollusc) is very similar to indigo (from a plant) and very different to sepia (from another mollusc). Madder (from a plant) and cochineal (an insect) are very similar.

While many natural organic pigments, perhaps even more so than inorganic pigments, are mixtures of compounds in many there is a primary colour producing compound, and grouping by these can give us a better classification schema which I will tentatively suggest below.

Pyrolytic Carbonaceous

Here we are talking about Carbon, basically, formed from burning organic material, and right at the start we hit a crossover. vine black, or wood black and various other blacks occasionally found from burning fruit stones are obviously clearly of organic origin, but processed by burning. So this bridges the classification to synthetic already. They will naturally also, depending on the process and level of carbonisation contain other carbon structures, remaining organic material and mineral ash. We must then consider Lamp Black, which is created by burning petrochemicals – in chemical terms organic, but geological in origin.

Bone black, ivory black, and hartshorn are rather more complex. Bone and horn consists mostly of a mixture of collagen and calcium phosphate. So the resulting pigment is also far more complex than pure carbon.

Indigoids

Both indigo and weld are essentially the same substance – indigotin which is a pair of indole⁶ derived ring structures, linked via a central C=C bond. The raw material of this can be used as a blue in its own right, and also laked pigments exist as Mayan Blue and Morocan Nila where this is bound to an organic clay.

Tyrian purple is predominantly 6,6′-dibromoindigo, which is the same compound with two hydrogen atoms substituted with bromine. It is a dye in it’s primary form, though Roman artists also used this laked to chalk.

Indole rings occur in some synthetic dyes, though do not tend to be the primary chromophores.

Quinonoids

Quinones are oxidised aromatic compounds consisting of one to a number of rings.

Madder (Rubia tinctorum) is a well known and still used historic plant lake red pigment (NR9). This is formed of a number of anthraquinones (three ringed quinones), most prevelently alizarin (NR8) (which is sometimes extracted, and may also be synthesized artifically) and purpurin. A historic laked preparation is known as lac de garance.

Cochineal (NR4) and Kermes (NR3) are both derived from insects, and consist mostly of the anthraquinones carminic acid and kermesic acid respectively, again these can be laked or sometimes simply ground to give red pigments.

Logwood (NBk1 or NBk2) and Brazilwood (NR24) contain the phenolic quinonoids haematein and brazilein respectively. These are formed by oxidization of neoflavinoids and give brown or purple to red dyes.

Synthetic examples are dibromoanthanthrone (PR168) and anthraquinone (or indanthrone) blue (PB60).

Dragons Blood (NR31) is a complex resin from various plants, notable dracaena, ground and used as red pigment. The main compound is Dracorubin, a highly conjugated penolic aromatic system, but the composition of the resin is complex.

Flavinoids

Flavonoids are molecules with the general structure of two phenyl rings and a heterocyclic ring. 

Weld (Reseda luteola) is an important historic yellow plant dye giving the laked pigment known as Reseda (NY2) predominantly from Luteolin and Apigenin and glycosides thereof. It’s important to note that processing is needed to hydrolyse these glycosides to the more intense free flavinoids.

Stil de grain is another historic lake pigment (NY14) made from buckthorn, tending to a greenish yellow containing Quercetin as the most important constituent, though again it is a mix containing other flavinoids and their glycosides. Chinese green is very similar, with rhamnetin/rhamnazin and related compounds as the main compounds.

Buddleia is not commonly listed in the more formal lierature, but is a favourite of some artisan natural dyers, also giving a yellow colour from various flavinoids.

Synthetic compounds based on similar structures exist, but are uncommon.

Xanthanoids

Xanthonoids are plant polyphenols built around the xanthone framework: two benzene rings joined by a central oxygen-containing ring with a carbonyl group.

Indian Yellow (NY20) is a pigment of much debate. While this is sometimes disputed it is widely held to be the dried urine of bovines which have been fed a diet high in mango leaves, The Magnesium salt of Euxanthic acid is the main constituent.

Gamboge (NY24) is a resin, which can be used directly as a pigment, consisting of a number of complex and diverse highly oxygenated xanthonoid compounds.

Quinochalcones

Quinochalcones are highly conjugated plant pigments structurally related to chalcones – open-chain flavonoid relatives in which two aromatic rings are connected by an unsaturated carbonyl-containing chain.

Safflower has been and still is widely used as a dye. The principal red pigment is carthamin (NR26, if isolated), a water-insoluble quinochalcone glycoside. There are also yellow colourants such as precarthamin and related chalcone derivatives. The red dye is the most highly regarded and can be extracted by washing out the yellow compounds. Various other plants such as dahlia and cosmos contain similar dyes, but do not tend to be used as pigments.

Diarylheptanoids

Diarylheptanoids are plant polyphenols containing two aromatic rings linked by a seven-carbon chain.

Turmeric has been used as a dye historically, and finely ground can be used as a pigment in its own right. The principle constiutent is curcumin (NY3).

Apocarotenoids

Apocarotenoid are carotenoid-derived compounds formed by oxidative cleavage of carotenoid chains (these being large C₄₀ polyene chains).

Saffron (NY6) has historically been used as a dye (and given its value, perhaps could be considered the Asian continent’s equivalent of Tyrian Purple). Crocin is the primary colour constituent – a complex C₂₀ apocarotenoid.

Tetrapyrroles

Tetrapyrroles are a a large class of colorful, naturally occurring molecules built from four pyrrole rings that are essential for life processes like oxygen transport, photosynthesis, and catalysis. I’ve included these here as I have been experimenting with Spirulina as a source of colour – notably when Nestlé removed artificial colours from Smarties this famously resulted in the demise of the Blue Smartie – until this was reintroduced using spirulina. There’s actually two related pigments that could be derived from this algae, a green and a blue.

The green pigment uses the whole algae and as such is a chlorophyll a-rich algal pigment material, with carotenoids as secondary colourants; the Magnesium ion-containing tetrapyrrole chromophore produces the characteristic green colour.

The blue pigment is phycocyanin, a phycobiliprotein containing covalently bound phycocyanobilin chromophores; the extended linear tetrapyrrole system produces the intense blue colour.

Complex biological materials.

Sepia is the key example here. The primary colour producing compound is Eumelanin a complex, cross-linked heteropolymer composed of indole-based units. However the compsition of the ink itself is far more diverse, containing a mixture of proteinaceous material, polysaccharides / glycoproteins, lipids and other organic constituents and inorganic salts/mineral components.

Complex geological organic materials

Asphaltum, Van Dyke Brown and lignite/coal/jet are chemically heterogeneous carbon-rich organic materials rather than single molecular pigments. Their colour arises from complex mixtures of high-molecular-weight organic compounds, carbonaceous material and, depending on source, mineral constituents.

Afterword

One thing that this exploration shows us, is that the quest for colour provokes the use of some rather interesting substances. We’ll move next to synthetic inorganic pigments, many of which, particularly the more older examples are recreations of natural minerals. After which we will move on to the wonderously complex array of synthetic organics, and see how they compare to the myriad colourants that nature thought of first.

1. For example, Lucy Mayes, 2025, The Natural Pigment Handbook. Exeter, David and Charles. ↩︎
2. For those who don’t know, it’s cattle piss. ↩︎
3. While of organic origin, the composition of this is more complex and not truly organic chemically ↩︎
4. Ditto ↩︎
5. These latter four do not appear commonly in many lists, but are beloved of natural dyers, and subject of some of my experiments ↩︎
6. I’m not going to explain these building blocks in huge detail here – google them if you don’t know ↩︎