Tina de Hidrosulfito
february, 2021

“We want to share our history through our own experience as dyers specialized in indigo vats. We also intend to pay tribute to the efforts of many colleagues who throughout the world: Asia, Europe, Africa, North America, South America, Oceania, work in the management of traditional indigo techniques as well as in their study to preserve an entire culture”.

Indigo has been used as a dye for centuries around the world. And throughout the world, there are many different ways to make indigo vats. We are not going to talk about all the indigo vats in the world, but yes, we are going to focus on those that we have tried and to which we have access.

Indigo is the trickiest dye to use because the indigo molecule is not soluble in water. If it is rubbed on the fabric, it may leave a stain, but it will not permanently dye the fiber in this state. In order for indigo to permanently dye fabric, it must be transformed into a soluble material. And this is done by creating a vat.

For us, this point is what makes it truly special, many cultures throughout the planet came to the same conclusion to be able to dye with plants that contain indigo. And each culture knew how to adapt to its own means a common formula for dyeing blue.

All cultures came to the conclusion that the purpose of creating a vat is to alter the indigo molecule. For this to happen, two requirements must be met within the indigo vat:

  1. It must be alkaline or have a high pH
  1. It needs to be a reduced solution. In this case, a reduced solution means that it is devoid of oxygen.

High pH is what creates the ideal environment to remove oxygen from the water. And then, why is there no oxygen?

Removing oxygen from solution creates excess electrons. Indigo molecules take away those extra electrons. And that is what transforms indigo into leuco-indigo, which is soluble in water.

 Leuco-indigo means white indigo and is so named because it no longer appears blue. It doesn’t look white or clear, as you might think.

Instead, the solution is a translucent greenish-yellow. The surface of the vat is usually a coppery blue color and may have bubbles. Bubbles often contain trapped indigo molecules and appear blue in color. This is called  flower.

When the fiber is immersed in the vat, it first appears the same greenish-yellow color as the solution. When exposed to oxygen, the leuco-indigo molecules lose those extra electrons and become indigo again. You can witness this change by watching the color of your fiber transform from green to blue before your eyes!

In our principles on indigo dyeing we experimented with the so-called CHEMICAL VAT.

In this tub, Sodium Carbonate is used to increase the pH and Thiourea Dioxide (thiox) or Sodium Hydrosulfite (hydros) as reducing material.

Thiourea Dioxide is sometimes found under the name Spectralite. Hydrosulfite is the active ingredient in RIT Color Remover dyes.

Among the ADVANTAGES of using this type of chemical vat, it can be said that it is a fast-acting vat, that is, it does not take long to be active and be able to dye in it.

When heat is applied it speeds up the reaction time and does not require continuous heating.

Another advantage is that it does not sediment on the bottom, it is easy to rebalance and can be maintained for a long time.

HOWEVER, they are vats that give off a strong sulfate odor, especially the Hydrosulfite vat. Dyed fibers need to be thoroughly washed to remove any traces of thiourea or hydrosulfite. And in addition, dyeing with Hydrosulfite makes it difficult to over dye with another color to achieve mixtures.

It must be taken into account that they are chemical products that must be handled with care and with protective equipment.

In this sense we would like to clarify that Thiourea is a substance considered dangerous for human health, inhalation or oral route being the harmful ways in which it enters the body. Thiourea has made it onto the priority list of substances within the ATSDR, the United States Agency on Poisons and Diseases, or the Hazardous Substances List.

Regarding its effects on the environment, the greatest risks, in addition to its mutagenic effects on animals (In biology, a mutagen is a physical, chemical or biological agent that alters or changes the genetic information of an organism and this increases the frequency of mutations above the natural level. When numerous mutations cause cancer, they acquire the designation of carcinogens.), is in the possible decomposition of thiourea into substances that are harmful to the environment, such as thermal decomposition into oxides of sulfur and nitrogen. which can cause severe damage. The EPA (Environmental Protection Agency of the s United States) establishes thiourea as a substance to be monitored, although not as one of the highest risk.

Sodium Hydrosulfite dust irritates the lungs. Ingested in large quantities it is harmful. Especially in sensitive people, the vapors cause headaches and even nausea and can irritate the intestinal system.

In our opinion, today there are many indigo dyes who use natural pigment mixed with these chemicals. If we would like to add that it seems to us a real aberration to mix a natural pigment with how expensive it is to extract it with chemical substances to sell consumers a final product that defines it as organic and natural.

Coming soon: Part II: Our experience with rapid reduction vats

Muestrario de teñido en algodón con Hidrosulfito de Sodio
Cotton dyeing sample with Sodium Hydrosulfite

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