Vat Recipe

Iron Vat Recipe

This recipe is a proven in-house recipe, enjoy!

The iron vat is the ideal recipe to get beautiful saturated blues on cotton, linen, and other cellulosic fibers.  It is not recommended for protein-based fibers, as the iron may cause discoloration or brittleness on these materials.  Once set up, the vat does not require heating and may last for weeks or months when covered and occasionally replenished. To make a vat, you must make a concentrated stock solution containing dissolved (reduced) indigo and then add it to the large vat container in reduced conditions.


Ingredients: All ingredients are based on using a 5-gallon bucket

  • 50g SCC natural indigo of a known purity  
  • 35g Iron (II) sulfate heptahydrate, (FeSO4) also known as copperas or ferrous sulfate
  • 44g Calcium hydroxide (Ca(OH)2), also known as slaked lime or hydrated lime
  • 5 gallon tall and narrow bucket with a lid to serve as a vat container
  • A lidded pot or large jar for the stock solution one fifth to one quarter the size of your vat. A 2 quarts pot to one gallon will serve nicely as a stock pot.

Stock Recipe

Begin by determining the total final volume in gallons of your vat, then measure out the following quantities of each material. The stock solution will be about ¼ to 1/5 the volume of the final vat.


Use 17g/gallon of Stony Creek Colors Natural Indigo for a very strong vat on cotton. For a slower-building vat, use 9g SCC indigo/gal. The recipe below uses about 13g natural indigo/ gallon. In 100% active indigo this ranges from 3-6 g/gal equivalent. So if you are making a strong 5-gallon vat with 35% purity indigo, use 85g of Stony Creek Colors Natural Indigo powder.


To make a 4-gallon vat from 50g of SCC natural indigo (about 17.5g active dye)


Per 50g natural indigo powder, use 35g FeSO4 and 44g Ca(OH)2.

Paste indigo thoroughly in hot water. Dissolve Ca(OH)2 (also called Hydrated lime or quicklime) in 0.5 L warm water. Dissolve FeSO4 (Iron Sulfate) in .5 - 1 L warm water. Add more water if necessary.


Fill a large container, such as a pot or 2-quart mason jar with a lid with warm water (50 C or 120 F), leaving room for the above ingredients. Add indigo, then FeSO4, then Ca(OH)2. Let sit with the lid on for 2-3 hours. The stock will change color from blue to yellow or brownish.


If you purchase a different purity from us or you want a different vat size, calculate inserting your purity or vat size. For example, in the following formula, .35 stands in for 35% purity. You would use 5 gals * 6 g/gal /0.35= 85 grams of indigo powder.  Per g indigo, use 2 g FeSO4 and 2.5 g Ca(OH)2.

Preparing the Vat

Use a container, preferably plastic, that is as tall and narrow as possible. A good container to use is a 5-gallon bucket with a lid.  Fill the vat, leaving enough room for the stock solution, with 100-120 F water.

For the 4-gallon vat recipe above 92g Ca(OH)2 and 30 g FeSO4.  

Per gallon of final vat volume, add 23 grams of Ca(OH)2 and 7-8 grams FeSO4. Paste the lime with water and dissolve the iron in warm tap water, then add the lime followed by the iron to the vat. Stir well then wait thirty minutes to one hour before adding the stock solution.  Avoid stirring roughly and pour the stock carefully to minimize the amount of air that is introduced to the vat. The final color of the vat should be brownish-yellow. Refer to the troubleshooting section if it is not this color.


Allow the sediment to settle completely before using the vat, and take care to avoid letting the fabric touch the sediment.  We like to put a raised stainless metal grating at the bottom of the vat to keep our fabrics off the bottom. If the sediment does discolor the fabric, a good soak in vinegar will sometimes remove the stains.

Maintaining and Troubleshooting the Vat

To keep the vat working as long as possible, stir it thoroughly once per day. If the vat turns greenish, add a maximum of 15g/gal iron sulfate, stir thoroughly, and allow to settle before checking the color again. If the vat turns blue-ish, add a small amount of lime (1-2g/gallon). Again, stir thoroughly and allow to resettle before checking the color.