Getting Pigments to Hang On
It's generally easier to dye animal fibers like wool and silk than plant-based fibers like cotton or linen. The scale-like protein molecules in wool fibers provide a lot of active "sites" to which pigment molecules can attach. Cotton, flax, and other plant fibers, on the other hand, are made mostly of smoother cellulose, which has few sites available to combine with color molecules. But, don't take our word for it. Invite your students to carefully observe different types of fibers and fabrics under magnifying lenses. (You may want to include synthetic fabrics as well. Pigments will bind differently to each type of fiber.) Consider numbering each sample and have students, notebooks in hand, describe and/or draw each specimen and compare it with others. (You may want to reveal the origin of each type of fiber.) Which do they predict will more readily "take" a dye and hang onto it? Have them explain their responses and then give them an opportunity to test their assumptions.
Mordants: The Ties That Bind
To help dyes bind better to fibers, dyers typically use different mordants, most of which are salts of metals like aluminum, iron, tin, or chrome. The word mordant actually comes from the Latin mordre, meaning "to bite," because early dyers thought these substances enabled the dye to get a better bite on fibers. Some dyes will bind well without mordants, but they may be more likely to fade over time. Most plants will produce different colors depending on the mordant used. We only recommend one type of mordant—alum—for classroom use. You can also often affect dye colors by changing the pH of the dye solution, by making it more acidic with white vinegar, for example. Challenge your students to try to affect the outcome of dyeing sessions by changing these types of variables: amount of mordant (or no mordant) or vinegar in the dye bath, quantity of plant material, soaking time, water temperature, and so on.
Students can also design investigations on the mordant effects of other other pantry products, such as sugar and table salt, alone and in combination. Dyeing fabric in brine from pickled beets, which contains sugar and vinegar, or juice from canned beets, which are usually sweetened and salted, produces a truer and more colorfast result than the water from boiled beets.)
A Colorful History
Since prehistoric times, humans from across the globe have used plant pigments to enrich their lives. Historians and scientists believe that prehistoric animal skins and cave paintings dating back to 15,000 B.C. were dyed with plant pigments. They've discovered examples of early dyed fibers in Egypt dating to around 2000 B.C., and Chinese records revealing even earlier use of plants as fabric dyes. Ancient Britons, called Picts, used woad, a plant, to dye their bodies blue and frighten enemies in battle, while the British marched against the Americans in their well-known red coats dyed with madder root.
Getting their hands dirty with dyes made from flowers, grass, berries, and roots can allow students to experience the wonder likely felt by the ancients who first discovered them. You can use the experience to pique their curiosity about the significance of dye colors to various cultures, and the history of dye technology throughout the millennia. Ask your young explorers to imagine and discuss how they think early humans might have discovered that they could use plant pigments to color their bodies, hair, crafts, animal skins, and fabrics. Encourage exploration through library and Internet research, or better yet, ask an experienced craft dyer or folklife educator to speak to your class. Students might feature their discoveries in a story or a play, develop a presentation for younger students, or otherwise demonstrate what they've learned.
Chromatography: Revealing Hidden Hues
Most leaves look green because they contain the green pigment, chlorophyll, which is a vital component in photosynthesis. Other colors also exist, but are masked by the strong green pigment. In temperate areas, shorter days and longer nights trigger deciduous trees to release a hormone that restricts the flow of nutrients to leaves in preparation for winter dormancy, causing chlorophyll to break down. Other formerly masked colors are then revealed, resulting in brilliant autumn foliage. The yellow and orange pigments are carotenoids. The red pigment, anthocyanin, is formed when nights cool and sugars are trapped in the leaves. (Scientists still don't fully understand the role of these substances, though some have been found to provide a natural "sunscreen" to protect plant DNA from degrading UV rays.)
Your students can uncover "hidden" colors in leaves through a process called chromatography. In chromatography, a solvent carries pigments up a paper strip, with some will "dropping out" of the solution sooner than others. Because each pigment has a characteristic rate of movement, they will separate from each other on the strip, typically as green, yellow, and light orange bands.
1. Invite students to collect and keep track of leaves from different types of trees and locations (e.g., shade and sun).
2. Crush the leaves and put each type in a test tube or small, lidded glass jar. Carefully add enough of the solvent acetone (you can use nail polish remover) to create a milkshake consistency (for a test tube, this will be about 2 cm). Cover and let the mixtures sit for 24 hours to allow pigment molecules to dissolve in the acetone.
3. Make narrow strips from the middle of a coffee filter, then place one end in each solution and let it sit overnight.
4. Remove the strips and let them dry, making sure they don't get mixed up.
Ask, What conclusions, inferences, and new questions do you have? Did some leaves have the same mix of pigments? Were they similar in other ways? Did sunlight seem to make a difference? Would you expect to get the same results all year round? How might the colors in tree leaves compare with those in garden plant leaves, such as spinach? (Find out!)
Dye Hunters in the Kitchen
Color makes things more appealing, including foods, and food coloring is ubiquitous on grocery store shelves. A quick survey of ingredient labels shows that everything from breakfast cereal to hot dogs has some hue included to help it compete with other products! Further student scrutiny can yield information about ancient food colorings that are used to this day, including annato, saffron, turmeric, extracts of carrots (beta-carotene), beet powder, fruit juices, paprika, hibiscus flowers, and caramel color.
Before synthetic pigments were available, colorful fabric tended to be expensive and hard to come by. The same is true for food colorings. Our ancestors' records suggest that specially colored food was mostly for the well-to-do, although one favorite of the Colonial American masses -- pancakes -- were tinted pink with beet juice! Most food colors were derived from edible berries, flowers, and even medicinal lichens. Some food dyes weren't even edible, but people tried all kinds of combinations in the quest for fashionable foods (For details, visit Elise Fleming's Web page on Food Coloring Agents.)
Natural food dyes generally impart the flavor of their source (not always a good thing!), but synthetic food colorings have no flavor. However, synthetic food coloring has been the object of investigation by consumer advocates for decades because of their potential (and established) health risks. This issue can provide fertile ground for more student research into the history of food marketing and safety issues. To conclude their analysis, suggest that the class holds a cook-off to compare natural food dyes (known to be absolutely safe) with synthetic ones. The event might feature baked goods, herb teas, ice cream, or other snacks tinted with fruit and vegetable juices or edible flowers (e.g., rose petals, violas, nasturtiums) as well as bottled food coloring.
Dyeing Across the Curriculum
Fran Ludwig, Science Consultant from Lexington, MA, reports that the plant-dyeing students tried during a study of colonial crafts sparked lots of questions worthy of classroom investigation: What happens if we leave it in the dye bath longer? Will dyes work differently in different types of fabrics? What flowers might make good dyes? Will different parts of the same plant produce different colors? Can we dye other natural materials like wood or shells? Questions like these can lead to fruitful investigations that allow students to think and act like scientists as they explore the mysteries of plants and colors. Consider having an initial dyeing experience with your class, then asking students: What do you wonder about colors from plants? What variables do you think influence dyeing? Which would you like to investigate?
Colorful Curriculum Choices
In addition to the science investigations suggested above, dyeing with plants provides opportunities for integrating learning across the curriculum. For instance, consider having students tackle the following:
Write about or generate a list of the ways in which plant colors enrich our lives.
Write or tell stories depicting how early humans might have discovered plant dyes.
Research the history of dyeing blue jeans. Were they ever dyed with natural plant pigments?
Observe and describe how "natural" colors compare with synthetic colors.
Grow some fiber plants like cotton or flax, and research how fabrics are made from them.
Learn about the meaning and use of colors by different cultures.
Find out about synthetic dyeing in the textile industry. Research why some people promote undyed or naturally dyed fabrics as environmentally "friendly."
Discover which plants in your area were used as dyes by indigenous people.
Create a plant dye recipe book.
Arrange your own dyed materials according to the color spectrum.
Create products from your dyed yarn or fabric. Weave on simple looms, create friendship bracelets, make God's Eyes or make a sampler featuring dye plants matched with the pieces of yarn or material dyed from each of them.
Apply plant colors directly (as with the pounded flower print activity) to create natural paintings. (Donna Hayes' sixth graders in Aurora, OH, create watercolor-like paintings using their fingers to press and rub flower petals, leaves, rotten logs, and other plant-based materials on drawing paper.)
Create plant color indicators for acids and bases using your homemade plant dyes. Dip a white coffee filter strip into one of your plant dyes, then dry it, and dip it in vinegar, and observe the resulting color. Do the same, but this time dip the strip in a baking soda solution. If they turned different colors, you can use plant-dyed strips like litmus paper.