The problem of coloring cloth is not as simple as you might at first expect. Pliny has told you about colored minerals in Chapter 11, but while these compounds make useful pigments, they make lousy dyes. You see, a pigment by definition does not stick to the surface to be colored. A binder mechanically sticks the pigment to the surface like glue. A crayon, for example, contains pigments in a wax binder. Plaster is the binder for fresco, linseed oil for oil paints, and gum Arabic for water colors. If you wish to make paint, you add a solvent, usually oil or water, to the pigment and binder. While paint is useful for coloring wood and stone and stucco, it is useless for clothing unless, of course, canvas is your idea of slipping into something more comfortable. No, colored clothing requires a coloring agent which will penetrate each fiber so that the color is as flexible as the yarn. Such a color is called a dye. Now there are two hard bits about dyeing: getting the dye into solution so that it may be absorbed by the cloth and then preventing this dye from washing out again.

One approach to this problem is to use the juice from colored vegetables. Just think about all of the foods you would hate to spill on your frock accidentally: coffee, tea, beets, red onions, to name but a few. Now if you think about it, food stains generally wash out because the colored compounds had to be water-soluble to get into the cloth, and being water soluble they wash right out again. Even the most persistent food stain leaves only a faint brown, tan, or yellow stain; dull, faint colors are prized by neither queens nor hookers. A mordant is a metal compound which chemically binds to the fiber on one hand and to the dye on the other. Whereas paint is a heterogeneous mixture of pigment and binder, a mordant forms a homogeneous compound with the fiber and dye molecules. Alum, potassium aluminum sulfate, is the most popular mordant of all times and may be used to render coffee (tan), tea (rose), beets (gold), and red onions (orange) colorfast. Alum also mordants other vegetable dyes, including fern (yellow-green), elder-berries (lilac), madder (red), and saffron (yellow). A few animals provide dyes, notably red cochineal and kermes extracted from insects and Tyrian purple extracted from a sea-snail. If you are interested in these dyes, you should consult The Weaving, Spinning, and Dyeing Book [1] or The Art and Craft of Natural Dyeing.[2]

Indigo requires no mordant for reasons that are both intrinsically interesting and, as it happens, important to the historical development of chemical industry. The blue color comes from the compound indigotin, C16H10N2O2, which is insoluble in water. It is easily reduced to leucoindigotin, C16H10N2(OH)2, which is colorless and soluble in alkaline solution. You are possibly considering that if wool could be soaked in a solution of leucoindigo, oxygen from the air might oxidize it back to blue indigo which, being insoluble in water, would be extremely colorfast. If so, you have hit the snail on the head, so to speak. The problem, then, is to reduce indigotin to leucoindigotin.

So far we know only two reducing agents, charcoal and sugar. We used charcoal at high temperature to reduce metal oxides to elemental metals. We forced yeasts to oxidize glucose anaerobically to produce alcohol. Since heating the bejeezus out of indigo just makes smoke, let us try the glucose route. Equation 12-1(a) shows the reduction of indigotin, (b) the oxidation of glucose, and (c) the balanced redox reaction. Now, leucoindigotin is soluble only in alkaline solution and yeasts are not particularly tolerant of alkali. There is a bacterium, however, which eats urea, farts carbon dioxide, and pisses ammonia. You will be familiar with this bacterium if you have ever used an outdoor privy.

Equation 12-1. From Indogotin to Leucoindigotin

When mammals metabolize protein, the nitrogen is excreted as urea. What is waste to us is food for the bacterium. Equation 12-2 shows the reaction by which the bacterium converts urea to ammonia, NH3. Ammonia gas is soluble in water and ionizes to give hydroxide ion and so it is an alkali. The bacterium has just what we need for dissolving indigotin; it produces alkali and oxidizes glucose. You are probably thinking that we should add honey and indigo to a 2-liter bottle of urine, burping it from time to time as we did with mead. The indigo vat would be much more pleasant were that possible. You will recall that whereas yeasts thrive anaerobically, bacteria require oxygen to live. We must carefully balance the air intake of our vat, providing enough oxygen for the bacteria to live, but not so much that our hard-won leucoindigotin is oxidized back to insoluble indigo.

Equation 12-2. From Urea to Ammonia

Once the vat is in order, alkaline and saturated with colorless (or pale green), soluble leucoindigotin, we are ready to dye our yarn. Simply soak the yarn in the vat until it is thoroughly saturated with liquid. When the yarn is pulled from the liquid, oxygen from the air oxidizes leucoindigotin to indigotin as shown in Equation 12-3. The yarn will change from yellow to blue and this blue will be colorfast. It looks like a magic trick.

Equation 12-3. From Leucoindigotin to Indigotin

Actually, we might have saved ourselves a good deal of trouble if we had been able to extract leucoindigotin from woad or indigo directly. Unfortunately, the indigotin is oxidized by the air very quickly. But we can use the same idea to extract dye from black walnuts if we are able to collect them when they have just fallen from the tree. The walnut tree has been kind enough not only to produce the dye juglone, C10H6O3, in its colorless, water-soluble state, but to package it in an air-tight container. I am speaking, of course, about the hull of the walnut itself. When the nuts fall in the fall they are initially soft and green, but after they have been lying around for a few weeks, the air oxidizes the juglone and the nut becomes hard and brown. If you are able to harvest black walnuts as soon as they fall from the tree, you will be able to make a beautiful brown, colorfast dye with no mordant and no urine required. You will be pissed at yourself if you miss the harvest.

WarningMaterial Safety

Biological hazards tend to be far more insidious than chemical ones. You see, one molecule of even the most toxic chemical is absolutely harmless. Such a molecule might react with one of your molecules, but you have so many that it makes no difference. With a chemical compound the dose makes the poison; a larger dose is more hazardous and a smaller dose is less hazardous. Biological hazards are not like this at all. You know from making mead that one yeast becomes two, two become four, and so on until the mead is chock full of them. If you choose to ferment urine, the bacteria behave the same way. Of course, you have been exposed to these particular bugs all your life, so you have some immunity. Nevertheless, it is a good idea to wash your hands regularly, particularly when you have been brewing your own juices.

The squeamish may prefer to use something other than fermenting urine as a reducing agent and a popular alternative is sodium hydrosulfite (CAS 7775-14-6) in household ammonia (CAS 1336-21-6). Summarize the hazardous properties of these materials in your notebook, including the identity of the company which produced each MSDS and the potential health effects for eye contact, skin contact, inhalation, and ingestion. Also include the LD50 (oral, rat) for each of these materials.

Your most likely exposure is eye or skin contact. If you get some in your eyes, you should flush them with cold water and go to the emergency room. Exposed skin should be washed with soap and water. Be aware that sodium hydrosulfite will bleach clothing. Be aware that walnut hulls will stain skin.

You should wear safety glasses and rubber gloves while working on this project. Leftover dye solution may be washed down the drain.

NoteResearch and Development

You are probably wondering what will be on the quiz.

  • You should know the meanings of all of the words important enough to be included in the index or glossary.

  • You should have studied the Research and Development items from Chapter 6 and Chapter 8.

  • Know the equation for the reduction of idigotin.

  • Know the equation for the oxidation of leucoindigotin.

  • Know the equation for the conversion of urea to ammonia and carbon dioxide.

  • Know the hazardous properties of indigo, walnut hulls, sodium hydrosulfite, and urine.

  • Know why humans wear clothing and why dyes are so important.

The Stockholm Papyrus which began this chapter contains 154 recipes, of which I have chosen only a wee sample. Indigo will remain the number one dye even down to modern blue jeans. Progress in dye technology will come in dribs and drabs until the eighteenth century, when a new way to dissolve indigo gives a leg up to the infant alkali industry. Cheap alkali, in turn, will relieve a soap industry starved for soda. Industrial waste flushed out of the alkali trade will become the mainstay of the bleach industry. All of this in the service of clothing so that we can tell the whiz-kids from the pee-ons. Now, if you are dyeing to get started and you cannot hold it any longer, urine in for a treat.



Reference [91].


Reference [92].