A healthy woad vat should have a bloom of bronze blue bubbles above a yellow-green liquid. Two hours at 50 C should be enough to complete the reduction process.
If you have too much blue scum at the top, add a little dithionite and wait for another hour. (Sometimes you will need to add soda ash too, but it is best to add the dithionite first, keep the vat warm for an hour and then if necessary add some soda ash).
If you get a white deposit at the bottom (which is difficult to see inside a saucepan), add a little soda ash.
If your vat has become yellow with little or no bloom on top, add dithionite or more woad.
2) How do I get a good blue?
Failure to clean fibres properly is one of the main reasons for not getting good colours from your woad.
Wool, cotton and silk need to be well scoured before being dipped in the woad vat even if they have just been bought and look clean. Scouring is much more than washing; when you scour you remove grease and oils from the fibre as well as dirt. See www.wildcolours.co.uk for more information on how to scour fibre. It is also important to soak the fibres well for at least a couple of hours before dipping them in the vat.
3) How do I get a dark blue?
It is difficult to build up a dark blue using a chemical vat as the dithionite strips the colour from the fibres. It is much easier to get a dark blue using a fermentation vat. It is also easier to get a darker blue on wool than on cotton or silk. I find Dupion silk particularly difficult to get a dark blue.
4) The colour becomes lighter not darker. What am I doing wrong?
If your fibre was medium blue after the first dipping but turned a paler blue after the second or subsequent dippings, you probably left the fibres too long in the vat in the second and subsequent dippings. Do not leave the fibres in the vat for more than a minute after the first dip as the dithionite is removing blue from the fibre almost as fast as new pigment is being deposited in the fibres.
5) My vat is not working. What have I done wrong?
a) Overheating the vat will destroy the pigment. Keep the vat at 50 C.
b) Your dithionite may be too old. dithionite only has a shelf life of one year.
c) Did you actually remember to add the dithionite? On one occasion I weighed the dithionite, then there was a fire drill and I forgot to add it to the woad vat.
6) How do I mix the woad pigment with water?
Add a little bit of water to the woad pigment and mix it into a fine paste before adding any more water. I find it helps to use the back of a spoon for this. This is a similar process to mixing corn flour and water.
7) I don’t like the idea of using a urine vat. Is there any other way?
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Last updated on 10 January 2022
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