Q: My Indigo vat no longer has a flower and is no longer yellow-greenish. When I dye, the cotton reacts blueish and not greenish. How do I keep the vat healthy? Do I need to warm it up?
A: Take a look at the photos below. One shows the surface of a spent vat, the other is a healthy vat that was just mixed up complete with a flower and yellow-green sediment that needs to be allowed to settle out before dyeing. Eventually all vats will become exhausted and will need disposing.
When your vat begins to look opaque green/blue or blue/grey, you need to stop dyeing or rehabilitate your vat via a process called 'sharpening'. If your vat is dark and opaque, no more pigment will transfer onto your piece, no matter how long, or how many times you dip. That said, you probably still have some residual pigment left in the vat, it is just not in proper shape to transfer onto the fabric.
You do not NEED to heat your vat, or warm it up, to sharpen it, but it will help! If you have made your vat in a large metal container, you can place it on the stove and bring the contents up to a simmer, this will induce your vat back into a state of reduction and you’ll be able to transfer more color from the vat onto your fabric.
For information about sharpening your vat and extracting more dye, feel free to take a look at my website that goes more in depth on the subject : www.grahamkeegan.com/indigo-vat-basics