The Blue Reversal
The deep blues of Sumba's ikat textiles begin with the leaves of the indigo plant, Indigofera tinctoria. The leaves themselves are not blue; they contain a colorless glucoside called indican. The first chemical step is fermentation. Weavers soak the leaves, allowing enzymes to hydrolyze the indican, splitting it into glucose and a molecule called indoxyl.
To become a dye, this indoxyl must be oxidized, which weavers achieve by aerating the mixture, often by pouring it between vessels. This transforms the indoxyl into indigotin, the familiar blue pigment. A major chemical challenge arises here: indigotin is insoluble in water. It cannot penetrate the cotton fibers.
The solution is a second, anaerobic fermentation. In a reduced (oxygen-free) vat, bacteria go to work, converting the insoluble blue indigotin into a soluble, yellow-green form called leuco-indigo. The cotton yarn is submerged in this vat, allowing the water to carry the soluble leuco-indigo deep into the fibers. When the yarn is removed and exposed to the atmosphere, a final oxidation occurs. The leuco-indigo reverts to insoluble blue indigotin, now physically trapped within the yarn. To achieve the darkest blues, this dipping and oxidizing process is repeated dozens of times over many months.
The Red Complex
The deep red-brown color comes from the root bark of the Morinda citrifolia tree, known locally as kombu. The primary coloring agent is a molecule called morindone. Unlike indigo, morindone is an adjective dye, meaning it has no natural affinity for cotton's cellulose fibers and requires a chemical intermediary, a mordant, to form a bond.
The mordanting process is complex and begins with pre-treating the yarn in an oil bath, often using candlenut oil (Aleurites moluccanus), which helps the subsequent chemical agents penetrate the fibers. The way to make the red color permanent is the introduction of aluminum ions. The local source for this metal is the leaves of Symplocos trees. These plants are hyperaccumulators of aluminum, absorbing it from the acidic soil.
The yarn is soaked in a solution made from these leaves. Aluminum ions from the leaves bind to the cellulose fibers of the cotton. Only then is the yarn submerged in the red Morinda dye bath. The morindone molecules form a stable coordination complex with the aluminum ions already fixed to the fibers. This creates a large, insoluble pigment that is locked onto the yarn. This entire sequence of oiling, mordanting, and dyeing is repeated multiple times over the course of a year or more to build up the saturated red color.