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Notes for teachers

Washing soda is caustic. Be sure to use appropriate PPE.

Parts 1 and 2 of this series deal with pigments. Combine with part 3 to produce a working paint.

It can be a lot of fun do do both parts 1 and 2 in combination with a field trip where students can gather their own materials. Be sure they gather enough to have plenty of pigment left over from this lesson if continuing on to part 3.

Encourage students to think about how they can capture the colors they see around them. Student-gathered pigment material is the ideal, but materials can be provided if needed.

List of Materials

Mortars and Pestles for grinding
Disposable cups (at least 8 per group)
1 liter/quart Jars or containers (4 per group)
Coffee filters (at least 12 per group)
Dehydrator (optional)
Raw plant materials for pigments: leaves, flowers, fruits, etc. (Things like red cabbage, avocado pits, dandelion root, beets, and tea are great)
Solvents for extracting plant pigments: vinegar, acetone, rubbing alcohol, denatured alcohol, mineral spirits, etc.
Alum (potassium aluminum sulfate)
Washing soda (sodium carbonate)
pH strips
Dust masks
Safety glasses
Nitrile gloves
Hot plate and a pot for each group (can also use beakers and a bunsen)
Funnel or sieve (one per group)

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Standards

HS.U1: Scientists explain phenomena using evidence obtained from observations and or scientific investigations. Evidence may lead to developing models and or theories to make sense of phenomena. As new evidence is discovered, models and theories can be revised.

HS.U2: The knowledge produced by science is used in engineering and technologies to solve problems and/or create products.

8.P1U1.2 Obtain and evaluate information regarding how scientists identify substances based on unique physical and chemical properties.

Standards

Science and Engineering Practices

Analyze and interpret data
Planning and carrying out investigations
Obtaining, evaluating and communicating information
Constructing explanations and designing solutions
Engaging in argument from evidence

Cross Cutting Concepts

Patterns
Systems and system models
Structure and function
Stability and Change

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Objective(s):

In this lesson we will learn about ancient sources of color and the processes that turn plant colors into usable pigments.

We will explore extracting color from plants using an acid-base reaction. We will experiment with different sources of plant color and try extracting pigments with solvents.

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Agenda (lesson time)

Day 1: Research ancient pigments and dyes made from plants.

Day 2: Gather pigment materials and begin processing them.

Day 3-4: Discuss the chemistry of lake pigments, balance the chemical equation, and begin wet processing of plant specimens.

Day 4-5: Experiment with solvents on dry plant specimens.

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Intro/Driving Question/Opening

Upper Pictograph Cave, Great Basin National Park, Nevada.

Fremont Culture, 1000-1300 BCE

The following are images of pictographs in the southwest. Unlike petroglyphs, which are carved into the rock layers, pictographs are painted onto the surface of a rock.

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The Great Gallery, Horseshoe Canyon, Canyonlands, UT.

Barrier Canyon Style, Late Archaic Period 2000 BCE-500 CE

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Pictograph in Swallet Cave, Montezuma’s Well, Arizona.

Sinagua Culture 1050-1400 CE

Most ancient prehistoric paintings exist in caves around the world. Some are as old as 30,000 BCE. There are relatively few pictographs in the southwest, though examples do exist, as we’ve just seen.

DISCUSS:

Why do you think pictographs in the southwest are more rare?

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What is Paint?

Paint is made up of three components:

PIGMENT: Dry color powder. Insoluble in water. Can be sourced from minerals, plants, or animals, or be synthetically produced.

BINDER: The “glue” that holds the pigment particles together and sticks it to the surface being painted.

MEDIUM: Liquid that helps the paint be fluid and spreadable.

For example, a blue watercolor paint might be made of phthalo blue (a synthetic PIGMENT), gum arabic (a tree sap-based BINDER), and water, which is the MEDIUM that gives watercolors its name.

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Hands-on Activity Instructions Gathering Pigments

Divide students into groups of 3 or 4.
Begin with a little bit of research:

What plant pigments did ancient people use around the world? How did they prepare those pigments?

Gather plant pigment materials. Students should bring at least 2 different materials to test. They should have enough of each plant specimen to fill at least a 1 quart ziplock bag
Record initial observations about pigment materials

What is the desired or expected color for each specimen?
Where was each specimen found?

Divide each specimen in two parts–one for a wet process and one for dry.

Begin drying the half of the specimens intended for use in dry processing. Use a dehydrator or an oven, or leave them for a few days in a well-ventilated sunny place protected from wind.
Begin soaking the other half of each specimen for wet processing. Put specimens in jars and cover with water.

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Lake Pigments

Lake Pigment: a dye precipitated with metallic salts

Mordant: an inert metallic salt that binds with a dye and helps create colorfastness

Chemical Reaction of Lake Pigment: A reaction of an aqueous solution of Potash Alum KAl(SO₄)₂ and Washing Soda Na₂CO₃ which yields Aluminum Hydroxide precipitate Al(OH)₃, aqueous Sodium Sulfate NaSO₄, and Carbon Dioxide CO₂. The reaction is often described as “laking” a dye, wherein the color bonds with the mordant (aluminum hydroxide) and precipitates out as an insoluble powder.

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Hands-on Activity Instructions: Wet Process – Lake Pigments

Balance the chemical equation for the acid base reaction about to take place:

Al₂(SO₄)₃ + Na₂CO₃ + H₂O⟶ Al(OH)₃ + Na₂SO₄ + CO₂

Balanced:

Al₂(SO₄)₃ + 3Na₂CO₃ + 3H₂O⟶ 2Al(OH)₃ + 3Na₂SO₄ + 3CO₂

Step 1: Soak plant material overnight
Step 2: Make Tea/Dye

After overnight soak, heat the jars up to about 60-80ºC and steep, stirring frequently, for 30 minutes. The goal is to make a very strong and dark tea with the plant material. Each group record temperature and time throughout process. Remove from heat and let cool for 5 to 10 minutes until safe to touch.

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Wet Process Continued

Step 4 (continued):

Let the solution precipitate overnight.

Step 5: Rinse and dry

Strain precipitated solution through a coffee filter. Discard liquid and keep the residual solids in the filter. Rinse and repeat until water runs clear.
Flatten out filter, label with group name and plant material and set out to dry.

Step 6: Repeat steps 1-5 with other plant specimens.

Optional: If desired, adjust the hue by soaking copper or iron in vinegar and using this vinegar instead of water for initial soak of plant material.

Step 3: Sieve

Pour warm liquid dye into a coffee filter lined funnel or sieve to strain out the solid plant parts. This can be expedited by not dumping all the solids from the jar into the filter.

Step 4: Reaction

For each liter of dye solution, you will need approximately 10g of alum and 5g of washing soda. Mix these separate with equal mass hot water to dissolve.
Pour alum solution into the dye and stir (acid). Note the pH. Then pour in washing soda solution (base). Be cautious as this will bubble. Note the pH. If the pH is not neutral (7), make more acid or base solution and adjust. Record observations for each pigment.

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Colorfastness

Fugitive pigment: a pigment that changes colors over time

Colorfast pigment: a pigment that retains its same color over time

Something to consider:

Arizona is known for its hot sunny days. Have you noticed how colors on signs sometimes fade or change in the intense sunlight? Why do you think that is?

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Hands-on Activity Instructions: Testing Other Solvents

Crush the dried plant specimens. This can be done with a mortar, though it may be easy to rub it with fingers.
Divide up each specimen into 4 cups, enough to test each 4 different solvents
Test the plant-solvent mixture by dipping a paintbrush and painting a little streak on heavy paper. Record results every 5 minutes for each plant specimen
Make observations

Which plants had the strongest colors? Weakest?
Which solvents produced the strongest colors? Weakest?

Photograph the page for documentation. Check again in a week. Which colors are fugitive? Does the solvent make a difference?

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Assessment

By the end, student groups should have at least 2 pigments ready to mix with binders. They should have a scientific journal detailing their experiments and hypotheses. They should be able to explain the acid-base reaction that results in the precipitation of the pigment. Students should be able to identify the effects of different solvents on the resultant color of the pigments.

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Differentiation

Simplify the process by removing the testing of solvents.

Provide students an exact recipe to follow for pigment extraction. These can be easily found online.

Remediation

Extension/Enrichment

Use stoichiometry to calculate the exact amount of alum and washing soda needed for a complete reaction. Does this yield a perfectly neutral pH solution? Based on the resultant pH, can you calculate exactly how much extra alum or washing soda needed to create a neutral pH?

Introduce students to Chromatography and the different chemicals that make up plant colors. Intro to color chemistry

Revisit the plant pigments after several weeks. Have the colors changed? Do different solvents react differently with plants?

Invite students to study the history of traditional Native dyes and extract pigments from these plants. Link to PDF book on Navajo Dyes

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