The Pros and Cons of Synthetic vs. Natural Dyes
Ever since the discovery of the aniline dye Mauve by William Perkins in 1856, the first synthetic dye derived from coal tar to be manufactured on a large scale, there has been a debate about whether synthetic dyes are better then natural (plant/insect derived) dyes. We get calls all the time about whether natural dyes are more organic, or better for the environment, or safer to use, than synthetic dyes. So follows is a brief (really, believe it, we have cut this way down!) discussion of the pros and cons of dyeing with Natural Dyes vs. the most popular types (for home use) of Synthetic Dyes.
In the early days of synthetic dyes the debate was merely one of cost and performance as the industrial age had yet to see the rise of concern about pollution and worker safety issues. As it became apparent that certain types of cancers were linked to exposure to early aniline type dyes, their use and manufacture was phased out. Now we know about the health concerns of dyes and also about the environmental impact that results from their manufacture and use.
As more people have become interested in organic and other more natural options for clothing, along with heightened awareness of chemical sensitivities, there has been a growing interest in natural dyes. The feeling by some is that because they are extracted from natural sources they must be more environmentally friendly and healthier for the consumer. However, natural dyes are often neither safer nor more ecologically sound than synthetic dyes. Some folks who have a business dyeing organic clothing or yarns have actually switched from Natural Dyes to Synthetic dyes (Fiber Reactive) for reasons discussed below.
There are many downsides to naturals dyes; they are less permanent, more difficult to apply, less wash fast, and most require the use of mordants, which bond to the fiber on one end and the dye molecule on the other, some of which are highly toxic. Some natural dyes such as Logwood, which Hematein is derived from, are themselves significantly poisonous. Rhubarb leaves, which are often used as a mordant, contain Oxalic Acid, which is toxic, but the stalks can be safely made into pie. Color possibilities are more limited with natural dyes, even if one is willing to use the more toxic mordants and modifiers. Some of the metals, such as chromium and tin, are extremely toxic. Even copper and iron mordants can be dangerous if misused, because over time they can build up in the body and cause harmful effects. Alum, Aluminum Sulfate, is the most popular and least toxic of the mordants, however, it is an irritant, and may be harmful if ingested, so it should be used with care. Always wear a dust mask when working with powders. Keep all of these out of the reach of children and pets!
The last problem with natural dyes is the amount of water and heat used. First the fiber must be mordanted in one simmering bath, while the dyestuffs simmer in another to extract the actual dye molecules from the plant or insect material. Once the fiber is mordanted and the extract is ready, then the fiber must be simmered in the dye extract. At the end the fiber must be washed to remove extra dye that didn't fix to the fabric and plant material that was not properly filtered from the extract. While there are ways to mordant while the fiber is in the extract bath, the results tend to be less brilliant and only reduce the water usage by about a third. This process takes a lot of time and energy compared to Fiber Reactive type dyes, which can be used with low water immersion techniques and do not require hot water or simmering for long periods of time.
Don't get the idea though that we don't like natural dyes. On the contrary, natural dyes are aesthetically pleasing in their intrinsic variation, and of interest to groups that make historically accurate costumes and participate in re-enactment groups. They also have the benefit of being things you can often grow in a home garden and fun for hobby dyers and textile artists. They make interesting experiments for older children as well. Indigo dyeing makes for a fun chemistry lesson.
Of all the synthetic dye options available, Fiber Reactive Dyes are the most permanent type of dyes. These dyes form a covalent bond with the fiber molecule. Covalent bonds are when two atoms in a molecule share an electron, kind of like holding hands. Once the bond is formed you have one molecule; the dye molecule has become an actual part of the fiber molecule and nothing will take it off except bleach or other "discharging" chemicals. For these reasons Fiber Reactives are often the dye of choice for baby clothing and chemically sensitive folks, since what is left after a thorough rinse and wash is very very permanent. Fiber Reactive MX type dyes require only lukewarm water (~105ºF) and an alkaline environment to bond permanently to cellulose fibers. They also batch at room temperatures of 70º and above. This comparatively low temperature requirement is what makes them ideal for tie-dye and batik. Other Fiber Reactive dyes require hot water, hence more energy is used, and they don't work for tie-dye and batik. Soda Ash (pure Sodium Carbonate) creates the right alkalinity for the chemical reaction to take place. With a small amount of heat they can also be used in an acid environment on silk and wool. Because they can be manufactured with much greater consistency then natural dyes, are much more potent, wash fast and easy to apply, they are ideal for dyeing large amounts of fiber/garments/yarns. Fiber-Reactive Dyes have become the dye of choice for many organic clothing manufacturers who want a diverse palette of vibrant colors because their percentage yield (dye that fixes to the fabric) and energy use is so much better then traditional natural dyes. About the only disadvantage to the MX cooler water type of dyes is that they sometimes require a fair amount of washout to get the excess (non-bonded) dye out, depending on how concentrated you are using the dye. The hot water Fiber Reactives require more energy, but less washout, so they are more popular with commercial dye houses. The only metal found in any of the Fiber Reactives is a very small amount of Copper in Turquoise and mixes made with it. And by the time it is in the waste water, it is so dilute as to not make an impact.
Acid Dyes are another class of synthetic dyes for use on proteins. Acid Dyes are so-called because they bond to protein fibers when in an acid environment at the right temperature. Non-toxic household white vinegar or mild citric or acetic acid are all that's needed besides heat to facilitate the fixing of the dyes. Acid Dyes form Hydrogen Bonds with the fiber, which are not as strong as covalent bonds, but still very wash fast when in a neutral ph environment and cool to warm temperature ranges. Acid Dyes are considered low impact because when used in the correct proportion to the weight of goods being dyed, almost all the dye is taken up by the fabric. This is called exhausting the dye bath. Since the water is almost free of dye it can be safely disposed of or even reused with a totally different shade of dye.
Synthetic and natural dyes both have great qualities, but it is important when beginning a project to consider what is really the more efficient way to get the color you want in the quantity you need. No matter which type of dye you use it is important to follow all safety recommendations for protective gear, proper equipment and ventilation, and to keep everything out of the reach of little ones and pets.