The Chemistry of Colour

Fashion, like so many other things, exists at a crossroads of art and science. The endless artistic possibilities of colour are made possible by the chemistry of dyeing (as well as the physics of light and biology of how our eyes and brains process the colours we’re surrounded by). The long history of textile dyeing and the study of dye chemistry have had consequences not just for fashion but for numerous other fields—the invention of colour photography and of medications including aspirin were indirect results of early dye research—and of course for our planet’s environmental health.

All methods of dyeing require an effective combination of fibre: the material that you are trying to colour, and dye: a compound containing a colourant, assisted by some auxiliary chemical components and, in most cases, heat to trigger the chemical reactions that bond the fibre and colourant together. What distinguishes dyeing from staining is the degree of relative “fastness” or resistance to fading or washing out. Some dye-fibre marriages are more fast than others, and fastness is measured in resistance to light and common laundering agents as well as plain old water. Here we’re going to describe four common dye types (acid, natural, reactive, and vat dyes). Different systems exist to categorize types of dyes; the one we’re primarily using here is based on the binding mechanism.

Acid dyes (a.k.a. anionic dyes)

Acid Dye Fleece

Acid dyeing is effective on protein fibres from animal sources, including sheep, goats, camelids (alpacas and llamas), and other mammals such as rabbits and muskoxen, and silk, although silk behaves differently than other animal fibres. It also works on some synthetic fibres such as polyamide.

Despite the scary name, acid dyeing is a relatively safe and simple technique to use on a small scale, in your own home or urban workshop. Acid is used in the dye bath or soaked into the fibre as a pretreatment, in order to lower the water’s pH level, making it easier for the anionic (negatively charged) dye molecules to bond with cationic (positively charged) sites on the protein fibre (there are basic or cationic dyes that bond to anionic sites with help from alkali chemicals raising pH but they’re used more often on non-protein fibres). Animal fibre can be dyed at any stage of processing from clean fleece to finished fabric. Independent dyers typically use citric or acetic acid (vinegar), both common in food preservation, to get a pH level around 4, so the solution is only moderately acidic and minimally caustic. The dyes usually come as a powder and colours are produced on a CMYK model from a mix of “primary” or “pure” blues, reds, yellows, and black, blended in the packaged powder by dye manufacturers or in solution as dyestock by the dyers themselves.

Natural dyes (a.k.a. mordant dyes)

Natural Dye

Nature is full of colours, and people have spent thousands of years experimenting with different parts of plants, insects, mollusks, and so on to colour both protein and cellulosic plant fibres. Very few of these natural dye sources bond directly with the fibre or are wet-fast on their own, and most will tend to wash out. Convincing these colourants to stay where they’re put requires the use of compounds called mordants, which can be applied to the fibre before or with the dye although pre-treating is usually preferred.

Different combinations of dyes and mordants can produce very different colours, as can the addition of modifiers, the freshness of the dyestuff, the mineral content of the water used, and many other factors. This versatility might be wonderful if you’re playing around but frustrating if you’re looking for consistency across multiple batches. That, along with other factors including material scarcity or monopoly by another business or political power, prompted early organic chemists to examine how dyeing works and to synthesize new dyes, expanding the range of achievable colours and launching the wave of innovation responsible for many of the medicines, food additives, plastics, and other substances we take for granted today.

The tradition of natural dyeing has many appealing attributes—some dyers love gathering or growing their own dye plants, love feeling self-sufficient and connected to nature or history, or prefer not to support synthetic dye manufacturers. Natural dyeing also has drawbacks: in addition to being temperamental about repeating results, the materials can be difficult to obtain and work with. Some plants are toxic if ingested, as are many of the metallic salts used as mordants (chrome in particular is a nasty poison and pollutant and leftovers should be considered hazardous waste) and over-mordanting fibre can ruin it. It’s not a very efficient method, requiring big piles of plants or beetles—up to 8 times the mass of the fibre quantity they’re able to dye. Aranya, a fair trade organization in Bangladesh that’s been working for years to revive the local natural dyeing tradition, takes a conscious approach to mitigate the method’s negative aspects. They’ve developed a range of 30 colour-fast dyes using mostly locally gathered dyestuffs, recycle waste materials for other projects like gardening and paper-making, and support workers with fair wages, training, and other benefits.

Vat dyes

Vat Dye Indigo

Some traditional dyes don’t require mordants but are applied to textiles using a different process; these are known as vat dyes. The most popular example is indigo, which historically was extracted from plants via fermentation and treatment with urine but can now be derived by other means or synthesized chemically.

Vat dyes don’t dissolve in water, so in order to dye fibre you must first chemically reduce or convert them to a soluble form. Removing fibre from the vat or dyebath and exposing it to light and air reverses the reaction, converting the dye back to its insoluble form and making the finished product extremely water-fast. Indigo isn’t blue in the vat. Instead, prepared indigo is combined with lye and sodium hydrosulfite to create a white or yellowish compound called indoxyl which, once it has been absorbed by the fibre, magically turns blue again as it dries thanks to oxidization.

Reactive dyes

Reactive Dyes

The youngest of the types described here, invented in the 1950s, reactive dyes fuse directly with the molecular structure of cellulosic fibres without mordants to bind them together. Their method of application mirrors acid dyes, and in fact some can be used with acid to dye protein although they’re recommended for plants. Alkali substances such as soda ash in the dye bath raise the pH, releasing negative hydrogen ions which “activate” the dye, allowing it to form a permanent electron-sharing or covalent bond with the fibre. They work at lower temperatures than other kinds of dyes (between 35*C and 80*C, depending on formulation, colour, and strength of alkali helper) and permit bright, water-fast hues.

Keep watching our blog for the second in this series of articles, where we will look at the ecological damage that dyeing does as a part of the textile industry plus ways of mitigating that damage.

Written by Claire Dalmyn.

Photo credits:
Acid dyes photo by DharmaTrading.com
Reactive dyes photo by Micracolour.com
Natural and vat dye photos by Wikimedia Commons.