When Mel Gibson, playing the 13th-century Scottish warrior William Wallace, smeared his face in woad and hollered “Freedom!” for the Scots against the English in the film Braveheart, he was following a tradition that went back more than 1,000 years to the days of Boadicea.

Blue faces may have gone out of fashion since the ancient Britons, but indigo dye, extracted from woad and other plants, remains in use to this day, as an exhibition about the dyestuff and its history proves.

Jenny Balfour-Paul, its curator, greets me at her four-bedroom Tudor cob and thatched cottage, near Tiverton in Devon. She is wearing her own indigo-dyed trousers and a silk shirt, although she has resisted the Scottish freedom-fighter make-up. Throughout the house, which she shares with her husband, Glencairn, are cushions in various shades of blue. Even the paintwork on the outdoor furniture is the colour of faded French shutters.

In Balfour-Paul’s herb garden, growing alongside soapwort (to make soap) and southernwood (which keeps moths away from fabrics), is a rosette of dark green leaves. This is Isatis tinctoria, or woad, a biennial belonging to the Brassicaceae family, along with cabbage, cauliflowers and broccoli. Although there are several different plants that produce indigo dye, woad is the one that grows best in temperate climates such as ours, as it will survive cold winters. Balfour-Paul, 55, also cultivates Japanese indigo, Polygonum tinctorum, a relation of knotweed, with pink tassel-like flowers. It is not frost-hardy, so she collects the seeds and starts off the young plants beside the Aga before planting them out in spring, when they thrive.

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Half the large vegetable patch has been given over to the polygonum, because Balfour-Paul doesn’t grow it for curiosity value or its pretty flowers: she harvests the leaves in summer to use as a dyestuff. She also grows madder, which produces a red dye, as well as weld and Impatiens tinctoria, both of which are yellow.

Balfour-Paul’s favourite dye is indigo, however, not least because of the many shades of blue it produces. An honorary research fellow at the Institute of Arabic and Islamic Studies at Exeter University, she has delved thoroughly into the history of the dye plants, which have been used to colour materials since ancient Egyptian times.

“Every single thing that was dyed blue in the world until 1900 [when a synthetic alternative was created] was dyed with indigo,” says Balfour-Paul. And without indigo, there would be no denim and no jeans. First designed by Levi Strauss as American workwear in the 1850s, more than a billion pairs are made annually – although nowadays, the vast majority are dyed with synthetic indigo.

The plants that produce natural indigo are undergoing something of a revival, as an alternative to the widely used synthetic dyes that rely on petrochemicals. Spindigo, a £2.2m European project that ran from 2001 to 2004, looked into ways to produce natural indigo commercially but sustainably, and planted trial crops in Spain, Italy, Germany, Britain and Finland. Since then, farmers, such as Woad-inc, based in Norfolk, have taken up the baton and are producing dyes and pigments from their products.

“The indigo molecule is a miracle,” says Balfour-Paul. “It is stable and strong and has many properties.” One of the most important of these is that it will dye any natural material – be it wool, rayon (which is cellulose-based), silk or cotton. Many natural dyes, such as the madder she grows, will only dye certain fabrics.

In colonial times, before synthetic dyes were invented, indigo made vast fortunes for its growers and traders. “When the British took over Jamaica from the Spanish, all the Spanish had been growing was a species of indigoferae, which is tropical,” Balfour- Paul says. “The British then turned to sugar, as the indigo was highly taxed.”

Woad is generally used in a dried and fermented leaf compost, whereas indigo from the East and West Indies was often produced as extracted and concentrated dyestuff. One of Balfour- Paul’s treasured possessions is a block of it that was given to her by one of the men who discovered the wreck of the Nuestra Señora de la Concepcion, which sank on its way from Mexico to Spain in 1638. “They were looking for pieces of eight,” she says, “and found that masses of them were blue.” In those days, the indigo was a highly valued trading good; to modern bounty hunters, it is almost valueless in comparison with gold and silver.

Balfour-Paul dips a piece of cloth into a solution made from some of the block and it comes up first green, then, as it oxygenates, it turns to the familiar strong blue that it would have done had she been using it 400 years ago.

SUPPLIERS

Flowers: Chiltern Seeds; 01229 581137, www.chilternseeds.co.uk. Nicky’s Nursery; 01843 600972, www.nickys-nursery.co.uk. Suffolk Herbs; 01376 572456, www.suffolkherbs.com. Cotswold Garden Flowers; 01386 833849, www.cgf.net. Cally Gardens; 01557 815029, www.callygardens.co.uk.

Colours: Woad-inc (01362 860218, www.woad-inc.co.uk) has a variety of indigo clothes and textiles, as well as dyes and pigments. In France, Bleu de Lectoure (00 33 5 62 68 78 30, www.bleu-de-lectoure.com) produces paints for that quintessential faded-shutter look.

An exhibition curated by Jenny Balfour-Paul – Indigo: A Blue to Dye For – is at the Brighton Museum & Art Gallery and Hove Museum & Art Gallery until January 6. For details, visit www.virtualmuseum.info

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