Bombay Cutch Extract

A variety of plants are considered to be forms of “cutch” by natural dyers because they contain a related set of pigment compounds, namely catechins and catechutannic acids. Catechutannic acids are an interesting class of natural dyes that behave somewhat differently from standard mordant dyes. They operate primarily by oxidation, rather than through the formation of color lakes (the compounds that form when dyes combine with metal mordants like iron or aluminum). Because the various cutches behave so similarly in the dye bath, they are usually addressed together in dyeing manuals and similar strategies are employed for them. The most common form of cutch is acacia cutch, which is harvested from the inner wood of acacia catechu by boiling out the sap and then drying and grinding it to a powder. Another common form of cutch is gambir, which is harvested in a similar manner from the twigs and branches of an Indonesian tree, uncaria gambir. The third main source of cutch is the seed of the betel palm areca catechu. In the past dyers referred to this areca cutch as “Bombay Cutch,” presumably because it was exported from Mumbai on India’s west coast rather than from Bengal in the east.

All three of the cutches behave in a similar manner and produce various shades of brown, although acacia cutch tends a bit more yellow and Bombay Cutch tends toward rosy or reddish hues of brown, especially in an alkaline dye bath. However, another factor that separates the various species of cutch is the relative state of oxidation in which the catechutannic acids exist in each plant. In their most basic state, catechutannins are nearly colorless and extremely soluble (although this is hard to credit when looking at a cutch dye bath, since the other compounds in the plant make the dye bath appear dark brown). Textile fibers will soak the catechutannins right up, but since they are soluble they also tend to wash right out. The only way to apply them permanently is to oxidize the catechutannins directly onto the textile fibers. A mordant is technically not even necessary to precipitate cutch dyes onto fibers, provided a strong enough chemical oxidant is present in the dye bath (although in practice a mordant is almost always used in order to fix the precipitated dye more permanently). The catechutannins found in acacia cutch are already relatively oxidized, so precipitating them onto textile fibers is not difficult; even mild oxidants can finish the job. Bombay Cutch, however contains natural catechutannins that are almost completely unoxidized and are thus much harder to precipitate onto textile fibers. Boiling fibers in a strong solution of Bombay Cutch will yield pale rose-brown shades that almost entirely wash out during rinsing, unless a strong chemical oxidant is used to fix them to the fiber. As a result, Bombay Cutch has excellent potential to produce a variety of dark shades of brown, but it can also be difficult to apply successfully.

Victorian dyers used Bombay Cutch almost exclusively with potassium dichromate, which is both a mordant and an extremely potent chemical oxidant. They would boil cotton in a strong solution of Bombay Cutch until the fibers were saturated with soluble catechutannins, then plunge the saturated cotton into a hot dichromate bath to instantly oxidize the catechutannins and precipitate them onto the fiber (simultaneously mordanting it with chrome). This apparently produced exceptionally dark, fast, and pleasing shades of brown. We know now that potassium dichromate is a dangerous carcinogen, however, so it is no longer a part of the natural dyer’s toolkit. To dye successfully with Bombay Cutch we will need to adopt alternate strategies. For example, we can add a small amount of copper mordant to the dye bath; copper sulfate is a modest oxidant in its own right, and it will help to precipitate the soluble catechutannins in the dyebath onto the fibers. Next, we can replace the traditional dichromate bath with a secondary bath of iron or copper, which will finish precipitating the dye and mordant it at the same time. Of course other oxidizing strategies could be tried instead (simple air exposure, while slow, would work in theory) and a great variety of shades might be obtained by varying the oxidation and mordanting strategies. These challenges make Bombay Cutch Extract an interesting dye that will reward natural dyers who enjoy experimentation.