woad extraction

Woad--Sarah's Natural Colour

Woad extraction
This is one of many methods of extracting indigo from woad. This is the one that I have had the most success with but there are many variables involved in the process that are yet to be quantified.

Download instructions (pdf):

Steps in indigo extraction process

What is happening on a molecular level

1. Harvest woad leaves, weigh the leaves, wash the leaves.

This removes impurities which may reduce yield and quality.

2. Boil a large pot of water. Add vinegar to reduce the pH.

Isatan A and B are more stable in acid conditions. Acidifying the extraction vat will improve yield.

3. Put leaves one handful at a time into boiling water. Allow water to return to boiling after each addition. Boil for 1 minute after the last addition of leaves.

Although common advice suggests that indigo will be lost if the woad extraction vat is allowed to boil, I have found that brief boiling increases yield by breaking the glucose and glutimate bonds that hold the indigo precursors within the plant and dissolving the waxy surface on the leaves.

4. Immediately plunge pot into cold water and reduce extraction vat temperature as quickly as possible to room temperature.

The cold water may be replaced several times to bring the vat temperature down to room temperature as quickly as possible.

This preserves the indigo precursors (Isatan A, Isatan B, and indican primarily) leaving them available to form indigotin in the next step of the extraction.

5. The leaves should remain in the extraction vat for 20 minutes.

Remove leaves from the extraction vat. Squeeze out excess moisture before setting aside. These leaves may be used for a boiling water dye bath and will give varying shades of pink and beige, depending on the genetics of the plant.

Prevents indigo from forming prematurely on leaves and being lost to the extraction process.

6. Add soda ash to raise vat pH to 10

There will be a change in colour from tea brown to yellow green.

7. Oxidize vat. There will be a colour change in the foam at the top of the vat. It will begin as yellow.

8. Foam on top of vat will change to green and then to blue.

Isatan A, Isatan B and indican combine to form indoxyl, in the presence of oxygen

2 indoxyl molecules join to form indigotin. Free Isatan molecules join with radical indoxyl molecules to form indirubin (red), isoindigo (blue) and isorubin (brown)

9. Continue oxidizing till foam changes from blue, back to green and then to yellow again.

Indigotin, indirubin isoindigo and isorubin molecules form indigo precipitate.

10. Allow indigo that is suspended in the vat to precipitate out -- 4 to 8 hours -- up to 72 hours. Filter or decant liquid. I use a turkey baster to remove the top liquid , without disturbing the indigo sediment. Indigo will be visible as a blue sludge in the bottom of the vat. Allow to dry or use immediately to dye in a reduction vat.

Indigo precipitate settles in the bottom of beaker, ready to be decanted, washed and dried

To be ready for use. This indigo sediment is not pure indigotin, but contains "impurities" that give woad indigo its permanence, and fullness of shade.