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.
In ancient Egypt, only one boat had a purple-dyed sail. It belonged to the Pharaoh and was a vibrant and powerful sign to other Nile users that they should move aside to let the royal boat pass.
Even today, deep blue, purple and crimson are traditionally associated with royalty, luxury and wealth. This is because they are difficult and expensive colours to achieve using natural dyes. Until the advent of synthetic dyes 100 years ago, natural dyes (from plant, animal or mineral sources) were the only way to colour fabrics.
The vivid purple of the Pharaoh’s sail was achieved using indigo – a natural dye that can be extracted from certain plants and animals. Thanks to Roman naturalist Pliny the Elder (see box on Pliny the Elder), we even know the dyeing methods used by ancient civilisations. This activity allows your students to follow in the footsteps of these early chemists, extracting indigo from the leaves of the woad plant.
Using basic lab equipment, secondary students of all ages could carry out this extraction in one or two practical sessions.
Younger students (ages 11-15) could simply do the extraction without going into much detail about the chemical reaction [although note that the reviewer suggested the activity was suitable for students aged 14+]. More advanced organic chemistry students (ages 16+) could investigate the compounds and reactions more thoroughly. The experiment is also relevant to the use of science in industry and could be used as part of a project on this topic.
Indigo is an organic compound that is found in three differently coloured forms – indigo itself, which is blue; purple mono-bromoindigo; and red-purple di-bromoindigo (figure 1). Naturally derived dyes can contain just one of these pigments or a mixture of two or three in variable proportions, leading to a range of colours from red to blue (Cooksey, 2001). The intensity of the colour is also influenced by the way it is processed, such as whether the dyed cloth is dried in the light or shade.
In ancient Egyptian times, the best quality indigo was extracted from Murex sea snails, which were once common in the coastal waters of the eastern Mediterranean. Archaeological evidence from Crete suggests that indigo extraction from Murex snails had begun by 2000 BC. By 1000 BC the Phoenicians, a civilisation centred in what is now Lebanon, were exploiting this valuable dye.
Their entire economy was based on trading Tyrian purple, a violet-purple indigo dye derived from the snail Murex brandaris (now known as Bolinus brandaris). They were so famed for it that the name Phoenician is derived from the Greek word ‘phoin?sso’, meaning ‘to make red’.
Murex-derived indigo was so expensive because 12 000 snails yield just 1.4 g Tyrian purple – enough to dye a handkerchief! The compounds used to make the dyestuff are secreted by the snail’s hypobranchial gland, found between the bowels and the branchial organ. The snails appear to produce these indigo precursors as a defence response and for their antimicrobial properties. The fresh secretions are colourless, but darken when exposed to air. According to Pliny the Elder, the dye was extracted by crushing the snails, leaving them to putrefy for three days in alkaline salt water, and then boiling them for up to ten days – imagine the smell!
The Phoenicians perfected this process over hundreds of years and discovered that by varying the species of snail, extraction methods and processing, they could produce a range of colours from red to purple to blue (table 1).
|Species||Appearance||Dye precursors found in the organism||Dye name||Colour||Chemical composition of dye|
|Bolinus brandaris (formerly Murex brandaris)||
|6-bromoindoxyl||Tyrian purple / imperial purple / argaman||Red-purple||Mainly di-bromoindigo|
(formerly Murex trunculus)
|Indoxyl and 6-bromoindoxyl||Royal blue / tekhelet||Blue-purple||Mixture of indigo, mono- and di-bromoindigo|
|Indigofera tinctoria (true indigo)||
|Isatis tinctoria (woad)||
Lower-quality indigo can also be extracted from certain plants, and this technique actually pre-dates the Phoenicians’ snail-derived dye. In India, methods of extracting indigo from the ‘true indigo’ shrub Indigofera tinctoria were known before 2000 BC. In Europe, the dye was extracted from woad (Isatis tinctoria). Although these two plants contain different precursors, indican in true indigo and isatan B in woad (table 1 and figure 2), they both ultimately yield the same dye – indigo.
The production of indigo from woad takes place in three main stages:
For each group of students:
Wet the wool and place in the extract for 10 minutes. Remove and leave to air dry.
To use the indigo powder as a dye, it needs to be dissolved in water and mixed with a reducing agent, such as sodium hydrosulphite (Na2S2O4). When the dyed material (e.g. wool) is exposed to air, it will turn blue.
Remember that indigo and many of its precursors are dyes, so take care not to spill them on clothes or skin. When handling concentrated ammonia solution, as well as indigo and its precursors, wear gloves and chemical splash glasses, and use a fume hood. See also the general Science in School safety note.
You could also ask your students to further investigate the chemistry of indigo and other dyes with some of the following activities.
Weigh the leaves before the extraction and the dried indigo powder. Calculate the percentage yield of indigo as follows: (mass of indigo / mass of leaves) x 100. You could compare the yield from fresh and dried leaves, or compare it with that of other indigo sources such as Indigofera tinctoria.
Our experiment involves using indigo to dye wool. Can indigo be used to dye other materials? You could compare natural materials such as cotton, linen and silk with synthetics or synthetic mixes (e.g. polyester cotton, cotton lycra).
My students produced dyes from onion skins, from madder (Farusi, 2006), and from myrtle berries. In addition, they tested how colour-fast their dyes were by washing the dyed materials using different methods.
This activity is part of a larger interdisciplinary project, developed together with 14- to 15-year-old students, to explore ancient scientific techniques. Pliny the Elder (23-79 AD) was a Roman author and naturalist. His encyclopaedia, Naturalis Historia, brings together much of the scientific knowledge of the time. We began each topic by discussing a passage from Naturalis Historia and then worked out how to recreate 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 knowledge on each of the topics. The process motivated even the most unenthusiastic students.
Other activities in the project include recreating ancient perfumes (Farusi, 2011), preparing glass tesserae with boric acid, simulating the luminescence of the shellfish Pholas dactylus, and preparing iron-gall ink (Farusi, 2007).
The author would like to thank Dr David Hill from the University of Bristol, UK, for his tips on extraction of indigo from woad.