Indigo: Recreating Pharaoh's
Image courtesy of SixRevisions; image source: Flickr
Indigo: recreating Image courtesy of Jeff Dahl; image source: Wikimedia Commons
Indigo being extracted Pharaoh’s dye from plant leaves What links your jeans, sea snails, woad plants and the Egyptian royal family? It’s the dye, indigo. Learn about its fascinating history and how you can extract it at school.
Image courtesy of gitane; image source: Wikimedia Commons
By Gianluca Farusi (see box on page 46), we even know All about indigo the dyeing methods used by ancient Indigo is an organic compound that n ancient Egypt, only one boat had civilisations. This activity allows your is found in three differently coloured a purple-dyed sail. It belonged students to follow in the footsteps of forms – indigo itself, which is blue; to the Pharaoh and was a vibrant these early chemists, extracting indigo purple mono-bromoindigo; and red- Iand powerful sign to other Nile users from the leaves of the woad plant. purple di-bromoindigo (figure 1). Nat- that they should move aside to let the Using basic lab equipment, second- urally derived dyes can contain just royal boat pass. ary students of all ages could carry one of these pigments or a mixture of Even today, deep blue, purple and out this extraction in one or two prac- two or three in variable proportions, crimson are traditionally associated tical sessions. Younger students (ages leading to a range of colours from red with royalty, luxury and wealth. This 11-15) could simply do the extraction to blue (Cooksey, 2001). The intensity is because they are difficult and ex- without going into much detail about of the colour is also influenced by the pensive colours to achieve using natu- the chemical reaction [although note way it is processed, such as whether ral dyes. Until the advent of synthetic that the reviewer suggested the activ- the dyed cloth is dried in the light or dyes 100 years ago, natural dyes (from ity was suitable for students aged shade. plant, animal or mineral sources) were 14+]. More advanced organic chemis- In ancient Egyptian times, the best the only way to colour fabrics. try students (ages 16+) could inves- quality indigo was extracted from The vivid purple of the Pharaoh’s tigate the compounds and reactions Murex sea snails, which were once sail was achieved using indigo – a more thoroughly. The experiment is common in the coastal waters of the natural dye that can be extracted from also relevant to the use of science in eastern Mediterranean. Archaeologi- certain plants and animals. Thanks industry and could be used as part of cal evidence from Crete suggests that to Roman naturalist Pliny the Elder a project on this topic. indigo extraction from Murex snails
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Modern painting of an Egyp- tian royal boat, with a furled Chemistry imperial purple sail. Note that Biology ‘purple’ is not always what we Organic chemistry would expect today Botany Ages 14-19* In this and several other Sci- ence in School articles, Gi- anluca Farusi successfully blends ancient history and chemistry in an unusual mixture – in this case, an
Images courtesy of Gianluca Farusi innovative and simple prac- had begun by 2000 BC. By 1000 BC the tical activity to isolate the Phoenicians, a civilisation centred in dye indigo from the leaves what is now Lebanon, were exploit- of the woad plant (Isatis ing this valuable dye. Their entire tinctoria). In my experi- economy was based on trading Tyrian ence, practical activities in- purple, a violet-purple indigo dye volving intense colours are derived from the snail Murex brandaris an effective way to attract (now known as Bolinus brandaris). the students’ attention. They were so famed for it that the The additional activities name Phoenician is derived from the and supplemental ques- Greek word ‘phoinísso’, meaning ‘to tions suggested in the arti- make red’. cle should help the students Murex-derived indigo was so expen- gain yet more practical sive because 12 000 snails yield just skills and also stimulate in- 1.4 g Tyrian purple – enough to dye a triguing questions, as well handkerchief! The compounds used as offering sound answers. to make the dyestuff are secreted by Vladimir Petruševski, the snail’s hypobranchial gland, found Republic of Macedonia between the bowels and the branchial *Note that the author recom- organ. The snails appear to produce Figure 1: Structures of indigo (top), mended the activity for 11- to
these indigo precursors as a defence mono-bromoindigo (middle) and REVIEW 19-year-olds response and for their antimicrobial di-bromoindigo (bottom)
www.scienceinschool.org Science in School I Issue 24 : Autumn 2012 I 41 A O B OH
Indoxyl (A) and Image courtesy of H Zell; image source: Wikimedia Commons indolin-3-one (B) are tautomers – isomers that exist together in N N equilibrium H H
properties. The fresh secretions are The Phoenicians perfected this colourless, but darken when exposed process over hundreds of years and to air. According to Pliny the Elder, discovered that by varying the species the dye was extracted by crushing of snail, extraction methods and pro- Shell of the snail Murex brandaris, the snails, leaving them to putrefy for cessing, they could produce a range the secretions of which were first three days in alkaline salt water, and of colours from red to purple to blue used to make indigo in 2000 BC then boiling them for up to ten days – (table 1). imagine the smell!
Species Appearance Dye precursors Dye name Colour Chemical found in the or- composition ganism of dye source: Wikimedia Commons source: Wikimedia Image courtesy of H Zell; image Bolinus 6-bromoindoxyl Tyrian purple / Red-purple Mainly brandaris imperial purple di-bromoindigo (formerly / argaman Murex brandaris)
source: Wikimedia Commons source: Wikimedia Image courtesy of Dezidor; image Hexaplex Indoxyl and Royal blue / Blue-purple Mixture of indigo, trunculus 6-bromoindoxyl tekhelet mono- and (formerly di-bromoindigo Murex trunculus)
Image courtesy of Kurt Stüber; image source: Indigofera Wikipedia Commons Indican Indigo Blue Indigo tinctoria (true indigo)
Image courtesy of Pethan; image source: Isatis tinctoria Wikipedia Commons Isatan B Indigo Blue Indigo (woad)
Table 1: Natural sources of indigo
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Indigo-dyed linen from 1873 Chemistry
A B Images courtesy of Gianluca Farusi
Lower-quality indigo can also be Figure 2: Indigo precursors found in plants: indican (indoxyl-β-D-glucoside, found in extracted from certain plants, and true indigo; A) and isatan B (indoxyl 3-ketogluconate, from woad; B) this technique actually pre-dates the Phoenicians’ snail-derived dye. In Images courtesy of Gianluca Farusi India, methods of extracting indigo HO OH from the ‘true indigo’ shrub Indigo- fera tinctoria were known before 2000 HO OH O A: Boiling the leaves to BC. In Europe, the dye was extracted extract the water-soluble from woad (Isatis tinctoria). Although Boiling O isatan B these two plants contain different O precursors, indican in true indigo and isatan B in woad (table 1 and N H figure 2), they both ultimately yield the same dye – indigo. The production of indigo from HO OH woad takes place in three main B: Adding alkali (ammo- O stages: HO OH nia) to hydrolyse the O molecule, removing the Ammonia 1. Boiling the leaves to extract the O carbohydrate group to water-soluble isatan B (figure 3A; O leave indolin-3-one (or N steps 1-6 in the activity below). H its tautomer, indoxyl) 2. Adding alkali (ammonia) to N H hydrolyse the molecule, removing the carbohydrate group to leave O O O indolin-3-one or its tautomer, indoxyl (figure 3B; step 7 in the H C: Exposing the extract to activity below). N N N H H H the air, leading to the 3. Exposing the extract to the air, O oxidation and coupling H leading to the oxidation and N of two indolin-3-one coupling of two indolin-3-one OO molecules to form the molecules to form the blue- N blue-coloured indigo H coloured indigo (figure 3C; O N N dyeing the wool and obtaining H H indigo powder in the activity Figure 3: Chemical conversion of isatan B to indigo below).
www.scienceinschool.org Science in School I Issue 24 : Autumn 2012 I 43 Student activity: extracting indigo from woad leaves
Materials Image courtesy of naturaldyer; image source: Flickr For each group of students: • Fresh woad (Isatis tinctoria) leaves (enough to fill a 1 l beaker) Woad seeds are easily obtainable from garden centres, and in some countries woad is a very common plant. • 20 wt% ammonia solution • Three 1 l beakers • Distilled water • Octanoic acid (optional) • Bunsen burner and tripod • Stirring rod Powdered indigo against a background of woad from which it was extracted • Sieve or strainer • Measuring cylinders and/or 10 ml pipettes water, stirring both the extract in To use the indigo powder as a dye, the beaker and the water in the • Heatproof gloves it needs to be dissolved in water and tray. You now have a solution mixed with a reducing agent, such • Funnel and filter paper containing, among lots of other as sodium hydrosulphite (Na S O ). • Knife for cutting the leaves 2 2 4 things, isatan B, which is an When the dyed material (e.g. wool) is (optional) unstable glycoside of indoxyl. exposed to air, it will turn blue. • Wool for dying (optional) 7. When cool, measure the volume Safety notes: I used a ball of white knitting wool, of your extract and add ammonia Remember that indigo and many and some rough grey-white sheep’s equivalent to 1 % of that volume, of its precursors are dyes, so take care fleece. e.g. for 1 l extract, add 10 ml not to spill them on clothes or skin. ammonia; for 100 ml extract, add 1 Procedure When handling concentrated ammo- ml ammonia. The solution should 1. Fill a beaker with woad leaves. nia solution, as well as indigo and its immediately change from brown Remove the leaves from the beaker precursors, wear gloves and chemical to brown-yellow then green. You and cut or tear into small pieces. splash glasses, and use a fume hood. now have a solution containing, 2. Fill the beaker two-thirds full with among lots of other things, Questions for discussion distilled water and bring to the indoxyl. boil over a Bunsen burner. • Why do we boil the leaves? What is To dye wool 3. Add a few torn leaves to the happening? boiling water, stir and continue 1. Wet the wool and place in the • What happens when the ammonia heating. When the water has extract for 10 minutes. Remove is added? returned to the boil, add a few and leave to air dry. • When you dye the wool, why does more leaves. Continue adding To obtain indigo powder the blue colour appear as the wool leaves and heating until all the 1. Aerate the extract by pouring dries? woad has been added. the solution from one beaker to • When you produce the powder, 4. When all the leaves have been another for 10 minutes. what is happening when the added, continue boiling for 1 2. Optional: if too much foam solution is poured from beaker minute. develops during aeration, add to beaker? Why does the indigo 5. Strain all the liquid from the boiled some octanoic acid, drop by drop. precipitate? leaves into a clean beaker. This acts as a surfactant. • Can you find out how indigo is 6. Cool the extract to room 3. Filter the extract through ordinary produced industrially today? What temperature by floating the beaker filter paper to obtain the indigo are its major uses? in a shallow tray full of cold powder.
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Image courtesy of Marcel Douwe Dekker; image source: Flickr Chemistry
Made in the 11th century, the Bayeux Tapestry celebrates the tion to produce the ten colours found in the 70 m embroidery: Norman conquest of England; a turning point in English history. woad (blues and greens), madder (Rubia tinctorum; reds) and Shown here is the Battle of Hastings at which William, Duke weld (Reseda luteola; yellows and dark green). To achieve the of Normandy, defeated Harold, Earl of Wessex, on 14 October desired colour intensity, the wool was successively soaked and 1066. Three plant-based dyes were used alone or in combina- air-dried.
Extension activities
You could also ask your students to further investigate b) Grow plants with and without added nitrogen the chemistry of indigo and other dyes with some of fertiliser. Aim for 2-4 g nitrogen per m2 of soil the following activities. (you will need to calculate the amount of fertiliser Calculating the yield of indigo that gives this amount of nitrogen). Does adding nitrogen fertiliser increase the amount of indigo Weigh the leaves before the extraction and the dried indigo powder. Calculate the percentage yield of produced? indigo as follows: (mass of indigo / mass of leaves) c) Try putting the plants in the dark for a few days x 100. You could compare the yield from fresh and (cover them with black plastic). Does this have an dried leaves, or compare it with that of other indigo effect on the amount of indigo produced? What sources such as Indigofera tinctoria. can you conclude about the metabolism of isatan Using indigo as a dye B in the plant? Our experiment involves using indigo to dye wool. d) Investigate the effect of other treatments to the Can indigo be used to dye other materials? You could plants or the cut leaves. compare natural materials such as cotton, linen and e) Test the effect of varying the method of the silk with synthetics or synthetic mixes (e.g. polyester extraction, for example using different kinds of cotton, cotton lycra). alkali. The effectiveness of indigo production Investigating other dyes a) Which produces more indigo, larger, older leaves My students produced dyes from onion skins, from or younger, smaller leaves? Express the amount of indigo obtained by weight and per leaf (count madder (Farusi, 2006), and from myrtle berries. In ad- leaves before extracting them). Which produces dition, they tested how colour-fast their dyes were by
BACKGROUND the most indigo? washing the dyed materials using different methods.
www.scienceinschool.org Science in School I Issue 24 : Autumn 2012 I 45 Image courtesy of Marcel Douwe Dekker; image source: Flickr
The Battle of Hastings begins: under the leadership of William, Duke of Normandy, the Norman forces Image courtesy of PHGCOM; image source: Wikimedia Commons charge the English forces, led by Harold, Earl of Wessex. Studying
chemistry If you found this article useful, you with Pliny may like to browse the other inter- Science in A 12th century manuscript disciplinary articles in the Elder of Naturalis Historia School. See: www.scienceinschool. org/interdisciplinary This activity is part of a larger interdisciplinary project, developed to- gether with 14- to 15-year-old students, to explore ancient scientific Acknowledgement techniques. Pliny the Elder (23-79 AD) was a Roman author and natu- The author would like to thank Dr ralist. His encyclopaedia, Naturalis Historia, brings together much of David Hill from the University of the scientific knowledge of the time. We began each topic by discuss- Bristol, UK, for his tips on extraction ing a passage from Naturalis Historia and then worked out how to rec- of indigo from woad. reate either the experiment described in the text or something similar. In this way, the students began in the same pre-scientific state as Pliny and, through laboratory work and discussion, gained modern scientific Gianluca Farusi teaches chemis- knowledge on each of the topics. The process motivated even the most try at the technical school (Istituto unenthusiastic students. Tecnico Industriale) Galileo Galilei in Other activities in the project include recreating ancient perfumes Avenza-Carrara, Italy, and since 2004, Image courtesy of def110; image source: Flickr (Farusi, 2011), preparing glass tesserae with boric acid, simulating the he has lectured in stoichiometry at the luminescence of the shellfish Pholas dactylus, and preparing iron-gall University of Pisa, Italy, for the degree programme in medicinal chemistry BACKGROUND ink (Farusi, 2007). and technology. He is also the regional tutor for the Italian ministerial project ‘Insegnare Scienze Sperimentali’ References Science in School 6: 36-40. (‘teaching experimental sciences’). For Cooksey CJ (2001) Tyrian purple: www.scienceinschool.org/2007/ secondary schools, Gianluca is also 6,6’-dibromoindigo and related issue6/galls the regional REACH (registration, evaluation, authorisation and restric- compounds. Molecules 6: 736-769. Farusi G (2011) Smell like Julius tion of chemicals) tutor. He has been www.mdpi.com/1420-3049/ Caesar: recreating ancient perfumes teaching for 16 years and nothing 6/9/736/pdf or use the shorter link in the laboratory. Science in School gratifies him more than the delight on http://tinyurl.com/tyriancooksey 21: 40-46. www.scienceinschool. his students’ faces when they grasp a Farusi G (2006) Teaching science and org/2011/issue21/caesar difficult chemical concept. humanities: an interdisciplinary approach. Science in School 1: 30-33. Resources www.scienceinschool.org/2006/ Seefelder M (1994) Indigo in culture, issue1/francesca science and technology. Landsberg, To learn how to Farusi G (2007) Monastic ink: linking Germany: Ecomed Verlagsgesells- use this code, see page 65. chemistry and history. chaft. ISBN 978-3609651606
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