While we were researching dyes for our dyeing lab, I came upon the history of woad, a plant that produces a blue dye used by Europeans for millennia before indigo was imported. It originated somewhere in the steppes north of Asia Minor and was grown, traded, and transplanted all across Europe until it reached Germany, France, and England. During the time of the Romans, warriors from one tribe of Britons would dye or tattoo themselves with woad in elaborate patterns to frighten their enemies. The Romans called them Picts because of the pictures they drew on themselves.
During the Renaissance, woad trading and dyeing made whole towns wealthy. In England, many acres were planted to woad. The leaves were harvested and mashed, then rolled into balls and allowed to ferment in a shed. The fermentation allowed the indigotin dye molecule to precipitate out of the plant leaves. The process smelled rather awful, and laws were passed banning any woad dyeworks within two miles of a town.
The other major source of blue dye before synthetics were invented was the indigo plant, which is native to warm and humid climates. Different cultures worldwide have invented their own methods of extracting the indigotin dye from the plants. Japanese dyers would allow the indigo leaves to ferment in a vat to remove oxygen. In India, the leaves were also soaked in vats then treaded by humans to mash the indigo and release the pigment, which was dried and pressed into cakes. Once indigo became known in Europe, it replaced woad as the choice for blue dye because the indigo plant has more indigotin and is therefore cheaper to produce. One wealthy indigo trader, Heinrich Schliemann, used his wealth to explore the ancient site of Troy. Another, Percival Lowell, used his family’s indigo wealth to build the Lowell Observatory in Flagstaff, Arizona to look for life on Mars. Levi Strauss used indigo to dye his original blue denim pants. As you can see, it’s had an impact on history.
As part of my research, I discovered something completely unexpected: dyers had imported woad to Utah in the early 1900s and tried to grow it here. Since it originated in a high desert environment, it did well in Utah’s climate. In fact, it did too well. It got away from the dyer’s fields and went wild, growing all over the western United States. It is now considered to be a Class 3 Invasive Weed, which means it is almost out of control. The only way to prevent it from spreading further is to pull up the plants before they go to seed.
I memorized the characteristics of woad plants, knowing I would want to try to find some and take my students on a “Woad Twip.” Woad has dark green leaves with a white vein. These leaves are clustered at the base of the plant. It sends up tall flower spikes with yellow flower clusters in the late spring. By fall, the flowers become brownish-red pendular seed pods with many small black seeds.
By mid October the rabbitbrush blossoms were beginning to fade. I needed to collect as much as I could for continued experiments, so I found a spot in the middle of Mountain View Corridor in southwestern Salt Lake Valley where the rabbitbrush blossoms were still bright, and I stopped there after school on a Friday. While I was out collecting the rabbitbrush blossoms, I noticed a plant I hadn’t seen before. It was woad! So I collected several bags full of leaves and some seed pods, with the idea of trying to grow some in my back yard.
My chemistry students cut the leaves into pieces and also separated out the seeds. We looked up the ancient process of woad extraction and found some websites that describe how it is still being done at small farms in England. The process involves quite a bit of chemistry. I am attaching a PDF file that describes the steps. Here it is:
In summary, the indigotin dye in woad and indigo is a rather delicate molecule. Too much heat will destroy it, but some heat is needed to extract it from the leaves. The chopped leaves are steeped in water at 90° C for 10-15 minutes. The leaf fragments are strained out and the liquid has soda ash added to make the solution basic. To precipatate the indigotin, the solution must be whipped with an electric mixer for 15-20 minutes to add oxygen to the solution. The solution is poured into dishes and allowed to settle. The supernate is carefully poured or filtered off and the final pigment allowed to dry.
We took it this far in chemistry class. Our next step will be to put the purified indigotin in a 50-60° bath and add some sodium hydrosulfite to the solution. This is a reducing agent that converts the blue indigotin into leuco (white) indigotin, which will dissolve in water and turn yellow-green. It takes about an hour of careful heating without stirring to do the conversion. While it is simmering (but not boiling), the fabric or yarn must be heated up in a bath with some soda ash. Once the solution turns yellow-green, the hot fabric or yarn can be carefully added without dripping or splashing. After about ten minutes, the dye has absorbed into the fabric but not yet bonded. When it is removed and exposed to the air, the fabric will turn from green to blue as the indigo converts from leuco back to blue indigo. As it precipitates out, it bonds with the fabric. Hopefully.
I purchased some pure indigo from Dharma Trading Company and was in the middle of heating the dye bath with sodium hydrosulfite after school on Tuesday when our fire alarm went off – an exterior pipe in our fire suppression system had frozen and burst, so we had to evacuate the building. I quickly unplugged everything and left. It was the last day before Winter Break. I hope to return by tomorrow and continue the experiment. If it works with pure indigo, I will demonstrate the process in chemistry with our own woad pigment when we return from break. I’ll update this blog post then.
All cultures around the world have found ways to solve the problem of how to dye fabrics. They’ve found dyestuffs in plants, minerals, and animals; through continual experimentation they’ve realized that certain salts (mordants) will help make the dyes colorfast. The process of oxidizing, then reducing indigo must have taken a long time to discover. It amazes me that such a complicated process could be developed in many countries and cultures, each with their own way of accomplishing the same thing, and all to get a permanent blue dye.