1. Make a paste with 30 grams indigo and 120 ml hot water. You will find detailed instructions on how to make the indigo paste here.
2. Fill your pyrex jug with 350 ml very hot water and add 75 grams iron (ferrous sulphate) to the jug. Stir well. The iron liquor will be rust coloured.
3. Fill your large container with 7 litres of hot tap water.
4. Add, stirring and in this order, indigo paste, iron liquor and 100 grams lime.
5. Stir well for 2 to 3 minutes and cover.
6. Stir again 2 or 3 times over the next hour. Let stand for an hour. When the vat is ready it should have bronze sheen at the surface with some bubbles, below the surface it will have turned amber or yellow-green.
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This vat is caustic, therefore make sure you wear rubber gloves and be careful with any splashes. Make sure the sediment has had time to settle to the bottom of the vat. The bottom 1/3 of the vat is sediment and your fibres must not touch the sediment. You will find tips of how to do that here.
Slowly immerse your fabric or yarn into the vat and avoid introducing bubbles. The fibre should stay in the vat for 15 to 20 minutes but try to move your fibre from time to time to make sure the indigo is penetrating everywhere. You can use shorter dips if using a paste resist. It is best to hold the fibre a few centimetres under the surface.
After about 15 to 20 minutes squeeze the fibre under the surface of the vat, and holding it compressed, remove it from the vat. Keep a bucket next to the vat and squeeze the fibre again (return the drips to the indigo vat at the end of the day). Take your fibre outside to air, and watch how the indigo changes colour from yellow to green and then blue. Let it air for at least 30 minutes, then rinse them well and repeat the dyeing process. The second and subsequent dips can be 10 minutes long. You may need several dips for a dark blue, and for really dark blues you may need as many as 20 dips.
When you have finished dyeing it is very important to give it a proper rinse (you will find a clear explanation on how to care for your dyed fabric here). With the iron vat you may get better results by neutralising it with citric acid instead of vinegar. Use about a tablespoon of citric acid per 4 litres of water and let your fibre soak in that for half an hour. Rinse well after that.
The chemical reaction in the vat will continue to work in the next day or two, keeping the vat strong. After you have finished dyeing for the first day, stir the vat well then put the lid on. You should stir the vat every day, even if you are not using it.
If the colour of the vat has turned green or bluish, it has oxidised and it will need sharpening. Dissolve 5 to 10 grams of ferrous sulphate in a little hot water, add it to the vat and stir well. You may also need to add 8 to 15 grams of slaked lime directly to the vat. Avoid adding too much iron or lime, as this will make it difficult to build up good blues. It is best to add a small amount of iron and lime, stir, and then wait a couple of hours. Check the vat and if necessary add more iron and lime.
If you have done a lot of dyeing and the vat looks the correct colour (yellowish green or amber) but it is not producing good blues, it may be exhausted and may need more indigo. In this case it is best to start again, otherwise there will be too much sediment.
You can keep this vat going for a week or two.
Whisk the liquid well to neutralise the lime which will turn it into chalk. Let the sediment settle and then you can dispose of the liquid in the drains (or pour it on waste ground if you have a septic tank). The sediment, however, is better dried and disposed in the bin.