Activity 5: Student Activity Packet

Student Activity Packet

Activity #5: Dyeing with the Synthetic Dye Indigo

NOTE: This website contains activities that are intended for schoolchildren, grades 5 and up, under the supervision of adults. The Smithsonian Institution cannot be responsible for any accidents or injuries that may result from conducting the activity without proper supervision, from not following specific directions, or from ignoring the cautions contained in the text.

Description

As part of a hands-on science experiment, you will make observations before performing the experiment, react to what happens as you perform the experiment, and further your understanding through follow-up investigations once the experiment has been completed.

Natural and Synthetic Indigo

See a slide show from the Smithsonian's Hands On Science Center about how to mix an indigo vat. Notice that this demonstration does not heat the indigo vat (unlike the activity described below). Many dyeing recipes exist and expert dyers experimented until they found a method that worked for them.

Since about 1900, chemists have been able to make synthetic indigo, and because it was cheaper, synthetic dye replaced natural indigo except for some home use or its by crafts specialists (for instance, in kimono dyeing in Japan). Indigo does not require a mordant, but it will not dissolve in water. Consequently, dyers first have to perform a chemical reaction called reduction (deoxidization), changing the indigo molecule to make it soluble. That process turns the solution yellow and in the soluble form indigo is readily taken up by the fabric being dyed. (In the following experiment you will use sodium hydroxide and hydrosulfite to deoxidize or reduce the indigo.) However, when the material is pulled out of the dyebath and exposed to oxygen in the air, another reaction--oxidation--takes place. The yellow molecule reacts with the oxygen, the indigo returns to its insoluble form, and the material turns the familiar shade of blue known to all wearers of denim jeans.

The experiment outlined in "Dyeing with Synthetic Indigo and Chemicals -- Experiment Instructions" demonstrates these stages. In it, you will:

1. Make the indigo soluble, a reaction you can see by the yellow color it produces.
2. Oxidize the indigo by exposing it to the air.
3. Identify the oxidation, which makes indigo insoluble again, by noting the change from yellow to blue.

Crafts dyers achieved reduction through the bacterial action of stale urine, a technique still employed by craft traditionalists and by people in remote parts of the world. This simple fermentation process also removes oxygen and makes indigo soluble, and for centuries dyers preferred this method over all others. In the following activity you can read about reduction using a urine bath.

Dyeing with Synthetic Indigo and Chemicals -- Experiment Instructions

Two Warnings:

1. You are making a dye and it may stain your clothes. Wear an apron. Also, protect your eyes by wearing goggles.

2. CAUTION: THE CHEMICALS USED IN THIS EXPERIMENT ARE DANGEROUS. DO NOT LET THEM COME IN CONTACT WITH YOUR SKIN.

Experiment

Materials:

- 0.2 grams of indigo (equal to the volume of 2 peas; to get a darker color, use more indigo)
- 0.5 grams of sodium hydroxide (about twice as much as indigo)
- 1.0 grams of hydrosulfite (about five times as much as indigo; Rit White-Wash is a grocery store source of hydrosulfite)
- 200 ml. of water (one cup of water)
- 400 ml. beaker (two-quart glass container)
- glass stirring rod
- four 4" x 4" white cotton squares (could use an old cotton sheet, but wool won't work)
- a heat source

Procedure

See a slide show about how to mix an indigo vat. Notice that this demonstration does not heat the indigo vat (unlike the activity described below). Many dyeing recipes exist and expert dyers experimented until they found a method that worked for them.

Add the indigo to the water. Before going on to step 2 of the procedure, answer question # 2 of "During Experiment" below.
Add the sodium hydroxide and sodium hydrosulfite to the indigo and the water.
Sir and heat until the solution turns a clear yellow and the indigo has dissolved. (A little blue froth remains at the top but this doesn't affect the dyeing.)
Place four small pieces of white cotton in the dyebath and stir in the hot solution for five minutes; remove from the dye and watch as the indigo reforms and changes color when exposed to the air.
Rinse thoroughly under running water and hang up to dry.

Dyeing with Synthetic Indigo and Chemicals -- Questions

Pre-Experiment :

Florence H. Pettit, in her book America's Indigo Blues, notes that "none of the growing forms of indigo, woad, pastel, or anil showed any outward traces of blue color in the flowers or leaves, nor did they give any hint that a substance useful as a blue dye could be extracted from them." Hypothesize on how humans thousands of years ago first discovered that part of the indigo plant could be used as a blue dye.
During Experiment :
Describe what happens when you add the indigo to the water.
Describe what happens when you stir and heat the indigo bath (indigo, sodium hydroxide, sodium hyrdrosulfite and water).
Describe what happens when you put the cotton squares into the indigo bath. Describe what happens when you take the cotton squares out of the indigo bath. Describe what happens when you rinse the cotton squares.
Post-Experiment Questions:
Examine and describe the dyed piece of fabric labeled "Control." Is the entire piece of fabric dyed the same shade of blue? If not, hypothesize why this occurred. Are there any white spots or streaks on the piece of fabric? If so, hypothesize why this occurred. Indigo is the dye used to color jeans. How does the uniformity of your dyed fabric compare to the commercial dyeing of jeans?
Why don't your blue jeans turn yellow when you wash them?
Test your dyed cloth for color fastness.
1. Put the dyed cotton square labeled "Dark Location" in a dark location. Place the dyed cotton square labeled "Exposed to Sunlight" in bright sunlight. After several days compare the results.
2. Wash the square labeled "Washed" in soap and water and compare to an unwashed piece (the square labeled Control).