Image courtesy of SixRevisions; image source: Flickr Indigo: recreating Image courtesy of Jeff Dahl; image source: Wikimedia Commons Wikimedia Dahl; image source: Image courtesy of Jeff 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 40 I Science in School I Issue 24 : Autumn 2012 www.scienceinschool.org Image courtesy of dudchik / iStockphoto Science education projects Chemistry Image courtesy of Jeff Dahl; image source: Wikimedia Commons Wikimedia Dahl; image source: Image courtesy of Jeff 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 Wikimedia Image courtesy of H Zell; image source: 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 Wikimedia source: Commons 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 Wikimedia source: Commons 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 42 I Science in School I Issue 24 : Autumn 2012 www.scienceinschool.org Image courtesy of PKM; image source: Wikimedia Commons Science education projects 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 Flickr image source: Image courtesy of naturaldyer; 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.