It’s not easy being green if you’re denim
The word ‘‘denim’’ originally meant ‘‘from Nimes’’, a city in the south of France. Centuries ago, the word became attached to a sort of cotton cloth produced in that city, cunningly woven so that it appeared blue on the outside, but white inside.
The word ‘‘jeans’’ originally meant ‘‘from Genoa’’, for the Genoese first turned denim into jeans.
In the 1900s, Americans got in on the act, the firm of Levi Strauss adding the distinctive metal rivets and double back pockets of jeans. The rest is history.
A certain indigo dye has long been used to give denim its distinctive colour. Egyptians used the indigo cloth to wrap mummies 4000 years ago.
In the 13th century, Marco Polo reported that the dye was being used in India. Until the 18th century, French people produced their own indigo and traders faced the death penalty if they imported the dye from India.
The purplish dye of jeans has always been made from a kind of bean plant, Indigofera tinctoria, which grows abundantly in many warm countries, but is also cultivated in New Plymouth.
Indigofera leaves are soaked in water and fermented, and the precipitate mixed with lime. The cloth is soaked in this boiling toxic dye then repeatedly dried in the sun. The more dips, the stronger the colour.
A potent bleaching agent is added to create denim’s distinctive fading characteristics.
Most denim garments are made in China, which has 186 denim mills. The rest of Asia has 104 mills, then come Latin American and European denim mills.
The denim capital of the world is Xintang, a city in southwest China, where more than 200,000 workers make half a million pairs of denim pants a year. It is said that workers in Xintang snap buttons on jeans so fast you can barely see their hands move. Xingtang jeans are made for more than 20 brand names.
Once the boiling dye has served its purpose, a cocktail of steaming indigo bleaches and detergent is disgorged into Chinese waterways, discolouring rivers purple and black for miles downstream.
The concoction threatens the local drinking water quality. Xintang spews so much dye into the Pearl River that Greenpeace claims it to be one of the worst examples of industrial contamination in the world. Scientists are on the job. Writing in the latest Nature Chemical Biology, Californian genetic engineer Tammie Hsu and his colleagues describe a more environmentally friendly way of making and using indigo.
They have taught bacteria to make an indigo dye and use an enzyme extracted from fungi to complete the process.
The synthetic dye can be decoloured before it is thrown away, does away with previous wasteful steps and skips the damaging bleaching process.
More than 200 million pairs of pants are made each year, plus dungarees, skirts, belts, bags, sneakers, lampshades, hats, jackets, shorts and interior car trim.
Worldwide, the denim market is worth more than US$60 billion a year, and is on the up.
The Berkeley team hopes their new technique will make blue genes a lot greener.