Natural blue dye is primarily made using two plant sources: Indigo and Woad, both of which contain the blue-colored chemical compound called Indigotin.
Here's a more detailed breakdown:
1. Sources of Natural Blue Dye
- Indigo: Indigo dye comes from plants of the Indigofera species, with Indigofera tinctoria being the most widely used.
- Woad: Woad (Isatis tinctoria) is another source of Indigotin, although it produces smaller quantities and may contain other coloring compounds in trace amounts.
2. The Process: General Overview
Both Indigo and Woad require similar processing steps to extract the Indigotin and convert it into a usable dye. The basic process involves:
- Harvesting: The plant leaves are harvested.
- Fermentation: The leaves are submerged in water to ferment. This process releases indican, a precursor to Indigotin.
- Aeration: The fermented liquid is aerated (often by beating or stirring) to convert indican into Indigotin.
- Precipitation: The Indigotin precipitates out of the liquid as a solid.
- Drying: The Indigotin sediment is collected and dried, resulting in a dye cake or powder.
- Dyeing: The dye is then solubilized in a dye vat using a reducing agent (historically, fermentation was used for this as well) before the fabric is immersed. The fabric appears yellow or green when it's first removed from the vat, but the Indigotin oxidizes in the air, turning it blue.
3. Key Steps Explained
| Step | Description |
|---|---|
| Fermentation | Plant leaves (Indigo or Woad) are steeped in water. Enzymes within the plant break down indican into indoxyl, releasing it into the water. |
| Aeration | Introducing oxygen converts indoxyl into Indigotin, the insoluble blue pigment. The aeration helps encourage the reaction to occur fully. |
| Precipitation | Because Indigotin is insoluble in water, as it forms it separates out of the solution as a sediment. |
| Drying | The Indigotin sediment is dried to create a stable dye product that can be stored and used later. |
| Vatting | The dried Indigotin needs to be reduced to a soluble form (leuco-indigo) to dye the fabric. This is done in a 'vat', originally a large vessel. A reducing agent is required. |
| Dyeing | The fabric is immersed in the vat, allowing the leuco-indigo to penetrate the fibers. |
| Oxidation | When the fabric is removed from the vat and exposed to air, the leuco-indigo oxidizes back into Indigotin, which is now trapped in the fabric fibers, resulting in the blue color. |
4. Modern vs. Traditional Methods
While the fundamental principles remain the same, modern techniques can involve more controlled fermentation and chemical reducing agents for vatting. Traditional methods often rely on natural fermentation processes for both the release of indican and the reduction of Indigotin for dyeing.
