Not to be semantically confused with :
Woad-leaved Ragwort (Senecio glastifolius) [a plant with similar name]
Might easily be mistaken for : the tall 1.5m high Oil-Seed Rape (Brassica napus ssp. oleifera) which also has leaves with Auricles, but these go around the stem rather than curling backwards and avoiding the stem as does Woad. Or for Black Mustard which grow to 2m high but has lobed leaves.
Could be mis-identified as :
Gold-of-Pleasure (Camelina sativa) which also has leaves with auricles, but these actually do go around the stem rather than curling backwards and avoiding the stem as does Woad. But Gold-of-Pleasure is shorter at 70cm.
Some similarities to : Great Yellow-Cress (Rorippa amphibia), Creeping Yellow-Cress (Rorippa sylvestris),
Tower Mustard (Turritis glabra)
Uniquely identifiable characteristics
Distinguishing Feature : The long-lanceolate leaves which are without teeth or lobes and have a prominently visible pale mid-rib. And its height of up to 1.5m.
Woad is infrequently found in scattered locations through the British Isles, but is found on cliffs in Surrey and East Gloucestershire since at least 1800AD. The fruit is purple-brown when ripe and between 1cm to 2.5cm long.
Woad is a cultivated Archaeophyte and yellow-flowered Brassica which was introduced into the UK in the dim and very distant past for the production of a dye called indigo, a deep blue dye. Blue dyes were once scarce, so Indigo was a valuable commodity in times past. As more modern dyes go, it is not a very amenable dye; there are some problems with the dying process, the first obstacle is that indigo is not soluble in water! As a result Woad is not often used for dyes these days; there are more dye-fast and easier to obtain modern dyes available with brighter colours. But there are still some dyers in the UK who still use Woad from which to obtain indigo. The botanical name for Woad, (Isatis tinctoria), includes reference to its use as a dye in the word 'tinctoria'. For the production of the blue dye
Indigo Woad was superseded in the 20th Century by
Himalayan Indigo (Indigofera tinctoria) which also produces the dye
Indigo but which somewhat inexplicably yields a richer blue colour than Woad, possibly because Indigo is present in greater quantities(?).
Woad was first heard about by the western world after 1498 when the Portuguese navigator Vasco da Gama discovered a sea-route to India. Woad got a bad reputation when concoctions of its roots were eaten or imbibed for pleasure. It also has weak toxins within which can cause some harm when ingested. The Indigo within it is toxic (and quite possibly other compounds as well). Its use as a blue dye started before 1495. But long before then the illustrator of The Lindisfarne Gospels had used a Woad derived blue pigment in about AD720. So it's use as a blue dye or pigment must go back even further into obscure antiquity. However, there is another plant which also produces the dye Indigo, and that is (Indigofera tinctoria), and moreover that contains a greater concentration of Indigo and thus is better that Woad. Your Author wonders if the dye derived from Woad can be distinguished from that derived from
Himalayan Indigo, but in antiquity neither would have been pure indigo; they would be contaminated with other (and different) secondary metabolites produced by each plant - so it should be possible to differentiate between the two derivations analytically from ancient dyed artifacts... Err, maybe...
Indigo is now much less used as a dye than it once was. Only small amounts are now produced from Woad for the UK and French craft dyers. However Indigo can, since the first years of the 20th Century, now be synthesized by the chemical industry. Indigo has found a new lease of life in inkjet printer ink (presumably blue printer inkjet inks only - although mixed with other dyes it might be possible to use it for some other colours as well?).
Herewith is a list of all(?) UK-grown plants which included the 'tinct' monika and which were also historically used for the dye that they can produce:
[There are many other plants lacking the 'tinct' monika which were also used for dyeing]
- Common Name (Binomial name); Dyes; Dye Colour(s).
Yellow Chamomile (Anthemis tinctoria);
Tinctosid (flavonoid glycoside); golden-orange, yellow, buff
Safflower (Carthamus tinctorius);
Carthamin(e) - red; Safflor Yellow(A/B) - yellow (not 'fast')
- Dyer's Greenweed (Genista tinctoria); isoFlavone:
Genistine, Luteolin & Apigenin; golden-yellow (or Lincoln Green when mixed with Indigo)
- Giant-Rhubarb (Gunnera tinctoria);
Tannins (from root); black
Madder (Rubia tinctorum); Alizarin; red
- Saw-Wort (Serratula tinctoria); the flavonoids Luteolin, Apigenin &
Golden Tickseed (Coreopsis tinctoria), flavonoids
Marein + 15 more flavonoids; maroon, yellow, orange, brown.
- Woad (Isatis tinctoria);
- Plus 4 absent from Clive Stace's 'Flora - 4th Ed' - so don't now occur in UK:
True Indigo (Indigofera tinctoria);
Indigo; blue (two other Indigofera species also produce indigo)
Dyer's Knotweed (Polygonum tinctorum / Persicaria tinctoria);
Bramble (Rubus rubrotinctus); (?) ; probably red (?)
Turnsole (Chrozophora tinctoria); 5 flavonoid glycosides including
Indoxyl is an Indole-type oily substance which will dimerise readily with oxygen from the air to become Indigo, the blue dye.
Isatin is another Indole derivative, this one with two carbonyl moieties. It is a bright-red coloured powder and is found not only in Woad, but also in a non-native Orchid Calanthe discolor and the non-native
Cannonball Tree (Couroupita guianensis).
Indican occurs in plants which produce Indigo. It is the glycoside of Indigo. Indican is naturally and readily oxidised by the oxygen in the air to produce Indigo and β-
D-Glucose, which is how Indigo occurs. Because humans also normally produce Tryptophan, another similar Indole compound, those human babies born with a syndrome called 'Blue Nappy Syndrome', erroneously convert Tryptophan first to Indole then by some abnormal gut bacteria into Indican, which is then excreted in their urine, which subsequently turns blue on exposure to air (which contains 21% oxygen) as the Indican is converted to Indigo. Indigo, as such, does not occur naturally in those plants which produce Indigo, it is Indican which is produced, and that turns to Indigo when exposed to oxygen in air. The intermediate product as Indican is oxidised (not depicted) is not in itself stable and two of these unstable identical molecules readily dimerise to produce Indigo.
Indigo is a natural dark-blue crystalline compound which yields a somewhat dirty blue dye (used to dye denim cloth used in 'blue jeans'). Indigo is obtained from several plants: Woad (Isatis tinctoria),
True Indigo (Indigofera tinctoria) (plus two other species from the same Indigifera genus) and
Dyer's Knotweed (Polygonum tinctorum. It is the
dimer of an Indole with a carbonyl atom attached to each. The two identical halves are bound together by a double bond.
Tyrian Purple does not occur in any terrestrial plant. Tyrian Purple is a muddy purple dye almost chemically identical to Indigo but with two additional
bromine atoms where on Indigo there are hydrogen atoms. Bromine is a toxic halogen that is hardly ever (if at all) found in land-based plants because Bromine is very scarce on land. They might be found in some plants which grow near the sea or where the land was once inundated by the sea. One such bromine-containing plant is Dittander. The sea contains a lot of bromine (as well as
chlorine, another halogen) and that is where Tyrian Purple is made: within the bodies of several differing species of Sea Snails. The two Bromine atoms, being much heavier than the two hydrogen atoms they replace, make the Tyrian Purple molecule vibrate at a differing frequency, absorbing light at differing frequencies than Indigo, thus altering the colour from a dullish blue to muddy purple. Tyrian Purple, a very expensive dye, is thought to have been used in antiquity by the Greeks in the 4th Century and by the Phoenicians as early as 1570.
It has recently been found that Tyrian Purple makes a good organic semiconductor but is probably still at the research stage. Like most high-chroma pigments, the RGB triangle used in monitors, TVs and computer screens does not encompass Tyrian Purple, for that colour is outside of the RGB colour triangle.