Dyeing with Woad

Natural dyeing was something that came out of combining my interest in spinning and weaving and home education. There is nothing like a little magical dyeing to teach chemistry!

I have been growing woad for several years now. It is classed as an invasive weed in some countries, and I was hoping that it would give that other invasive weed in my garden, stinging nettles, a run for its money, but so far it hasn't worked; we still have far more stinging nettles than woad, and the dye from stinging nettles is not nearly so spectacular! Woad also has medicinal properties, and our chickens have been self medicating by stripped off the leaves on the woad plants on the patio. Luckily they can't get into the veggie patch where I most of my woad growing.

Woad is a member of the brassica family which includes cabbages and mustard. I haven't dyeing with cabbage, but mustard does give a wonderful yellow. Woad though, when it comes to dyeing, is in a whole different class.

Ok, here is the science bit! Woad has molecules that absorb light because it has lots of double bonds. This means that some of the the electrons can effectively spread across the whole length of the molecule in a strange quantum mechanical process. The significance of this is that if the incoming light (the photon) has a wavelength similar to the length of this double bonded chain then the photon can be absorbed and in the case of woad this is very easy - so pretty well every photon of red light hitting the molecule is absorbed giving you the intense blue colour.  

Unfortunately the woad molecule is a lousy dye in itself because it is not very soluble. However, if you raise the pH and either give it a reducing agent, or alternatively raise the pH and let some bacteria at it, then the molecule can be reduced - electrons are added to it - and it loses its colour and becomes very soluble at the same time. The reason it loses its colour is essentially that the double bond in the middle is broken and this means the molecule has two much smaller regions of double bonds - this means that only ultraviolet photons now match the length and the absorption shifts away into that part of the spectrum. This reduced form is a white form of woad. It is the perfect dye precursor because you can easily saturate your cloth with this material as it's so soluble, and when it's exposed to air and dries out, the oxygen in the air plucks off the electrons, oxidizing it back to the blue form. Since this is not soluble in water, the indigo is now entombed in the fabric, and unlike many natural dyes, it is “permanent”.

Now, to the practical bit. We used the reducing agent way this time. Last year we tried the bacteria method by using stale urine, but we didn't fancy doing that again. However, I will post up that story another day!

We picked the leaves, tore the ones the chickens didn't eat into shreds, and poured on boiling water (woad is temperature sensitive and you don't get good results if the temperature gets too high) to scald the leaves. We let that sit for a while, then strained the liquid and squeezed out all the juice. We added washing soda (alters the pH) and with an rotary hand wisk set about beating the water until it no longer had blue bubbles.







The temperature is supposed to be about 50 degrees Celsius and because it was a hot day we barely had to reheat it before adding some sodium hydrosulphite (the reducing agent) and leaving it to sit for a while. This resulted in a yellowish green clear liquid.






We then added the wetted wool (I haven't got round to spinning it yet so it was in the form of raw fleece) trying not to disturb the dye too much, let it sit for a while longer, and then the magic began!














After washing the wool, we set it out to dry, and I embarked on a second batch (I don't have pans big enough to do it all at once), and then decided to try to revitalise the vat, but in a way that is supposed to give a pink. I thought it was by raising the temperature, but while I had the pan on the heat I reread the book and it was not temperature but alum, a common mordant used in dyeing, that had to be added! So with a quick dig in the larder, alum was added and the result is the batch on the right:




It looks grey when on its own, but beside the blue of the woad it does have a pink tinge. But then this is the excitment of natural dyeing, you never know how things are going to turn out!




This page on woad has more details on how to dye with woad as well as how to grow it.


I found some spun fleece on Easter Sunday, and some more woad in the garden that the chickens didn't get, so I will post up some more photos of that later.