LONDON -- Ever wonder why so many traditional uniforms -- such as those of English policemen and the British Navy -- are (or were) blue? The answer is the same reason why blue is always the dominant color when you look at a medieval tapestry: because it doesn't fade or run.
In the days before chemical dyes, indigo blue was one of the few reliable pigments available. It came from the woad plant, which was used in a notoriously smelly woad vat in order to produce indigo. The popularity of indigo -- to dye blue jeans, for example -- has remained, although the pigment is today produced by modern chemical methods, complete with environmentally harmful waste products.
Investigations by a collaboration of agricultural botanists and microbiologists, together with a keen historian, have now established exactly how indigo was produced from woad in medieval times.
The group has identified the species of bacterium responsible, and suggests that it could be used to replace some of the chemicals currently used, thus reducing the potentially harmful environmental impact of the production process. The collaborators' observations are published in a letter to Nature in the Nov. 19 edition, titled "Lostridium used in mediaeval dyeing."
Nikki Padden, postdoctoral research fellow at the department of agricultural botany at the University of Reading, in the U.K., and first author of the letter, told BioWorld International: "We have characterized the bacterium concerned, which is a new species, and have called it Clostridium isatidis. We are looking for partnerships with biotechnology companies to develop this work further. We have some wonderful ideas, but can't say what any of them are at this stage."
Reduction Process Can Harm Environment
Indigo is a very insoluble pigment, which is one reason why it doesn't run in the rain. In order to dye cloth with indigo, the indigo first has to be chemically reduced into a soluble, colorless form. The cloth is dipped into this solution, and as soon as it is taken out and exposed to the air, it goes blue as the air oxidizes the soluble form back to indigo.
It is the reduction process which makes the production of indigo-dyed cloth potentially environmentally harmful: sodium dithionate is currently used to reduce the indigo, and this produces wastes, including sulphites and sulphates.
In medieval times, however, indigo was produced directly from the woad plant, Isatis tinctoria. The leaves were chopped up, rather like mint for mint sauce, rolled into balls and dried for about a month. The woad balls could be stored in this way, or used immediately. When people needed the balls for dyeing, they broke the balls open, wetted them and "couched" them, a process which involves aerobic microbial activity. The couched woad was then put into a woad vat.
John Edmonds, a former engineer and a volunteer for more than 20 years at Chiltern Open Air Museum at Chalfont St. Giles, in the U.K., explained how his interest in the subject unfolded. The museum exists for the restoration and preservation of historic buildings which would otherwise be pulled down, and includes a medieval field system with strip farming. Edmonds told BioWorld International: "We started growing appropriate crops in the medieval field project, and one of these was the woad plant. Then, of course, we wanted to know what to do with it, and we ended up recreating the process of making it into woad balls." With the help of various historical works -- including books written by English dyers who emigrated to America around 1800 and who were hoping to stimulate a home-grown American textile industry -- Edmonds was able to establish the basic operation of the woad vat.
'Working Backward' Provided Clue
"I managed to work backwards from what they were doing in the 18th century, which was fairly well documented, and ended up with a pure woad vat," he said. "Just in the past couple of years, a French historian found an authentic recipe for woad from 1418 from the Dyer's Guild in Florence, Italy. This was a secret account written for the instruction of the apprentices in the guild, but it was very valuable because it confirmed what we had been doing."
The method involved adding water to the broken-up woad balls, to allow them to ferment into "couched woad." The couched woad was then placed in a vat with boiling water, and wood ash and bran was added. The temperature was allowed to drop to about 50 to 60 degrees centigrade, and the contents of the vat fermented for about three days.
"What is interesting," Edmonds said, "is that nothing should happen. You have sterilized the material with boiling water, added wood ash, which brings the pH up too high for most bacteria, held it at 50 or 60 degrees, which is too hot for most bacteria, and -- lo and behold -- it ferments."
Edmonds was able to dye cloth. But, he said, "I wanted someone to explain what was happening. It wasn't simple fermentation and we didn't understand it." He approached the Reading researchers and they undertook to find out, with funding from the Leverhulme Trust.
Padden takes up the story at this point. "We took woad balls, couched them and made a woad vat in the laboratory -- in a fume cupboard, of course!" she said. "We took samples from the vat and put these onto plates supplemented with indigo to try to find indigo reducers. A lot of the bacteria present in the vat didn't reduce indigo, but we selected a strain that did and characterized it. We analyzed it to see if it could do the same thing in pure culture, and it could."
They concluded that it was the reduction of indigo by Clostridium that enabled the indigo to dissolve, enabling its use as a dye. Padden and her colleagues wrote: "The medieval vat operators were aware of several requirements for the system to work: They had to keep the vat alkaline, adding wood ash to counter acid produced by fermentation; they provided the Clostridium with polysaccharide substrate, adding bran to maintain the fermentation for several months; and they had to maintain the temperature at around 50 degrees centigrade, as the Clostridium is thermophilic."
The discovery of Clostridium isatidis could be of interest to companies wanting to use natural dyes, Padden added. "Before this discovery, you could obtain indigo naturally, but you still had to treat it chemically to use it, whereas with this bacterium you can use natural indigo and reduce it naturally, just as they did in medieval times," she said. *