Vat Dyes, what are they?

In 2005 I attended my first Surface Design Association http://www.surfacedesign.org conference in Kansas City, MO, USA. I was lucky to get into the master class on discharge techniques ie selective removal of colour from an already dyed ground cloth. Among other things, we worked with Vat dyes which were completely new to me.
Vat dyes are similar to Indigo. The dye powder is soluble in water, but has no affinity at all for fabrics, until it goes through a reduction process. This process involves water with a temperature of 50ºc – 60ºc, Vat dye powder, Lye, Soda Ash and Thiox (Thiourea Dioxide). The reduced dye is now in its leuco form and may differ in colour from the original dye itself. After dyeing the fabric is re-oxidized when exposed to the air again and regains the intended colour. Magic!
The beauty of Vat Dyes is that they simultaneously remove the original ground colour from a fabric and replaces it with a new colour. Do not get confused here with overdyeing which will be affected by the original ground colour ie blue dyed over a yellow will produce a green. Blue Vat dye over yellow will remain blue, it has removed the yellow and replaced it with the blue. As you can imagine Vat Dyes are great for resist techniques where parts of a fabric are bound to avoid any colour reaching within the folds.
The photos show this process on some silk seersucker.


The silk dyed chartreuse (Procion MX), and portions bound


The same piece after it has been in a navy vat dye bath


The resulting cloth being unbound, and revealed

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