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Notes for teachers

Do Part 1 or Part 2 of Ancient Paints first. Otherwise, purchase powdered pigment in preparation for this lesson.

If combining with Parts 1 or 2, your students will want to collaborate as a class to choose the best pigment materials. This will require that some groups share with others and that enough is prepared for testing.

Lime is caustic. Be sure to use PPE.

This lesson will guide students in the creation of a casein binder (milk paint). If desired, this lesson can be extended to test out other binder materials.

List of Materials

Prepared Pigments (see Part 1 or Part 2 of lesson series)
Watercolor paper, cut into ⅛ size swatches
Paint Brushes
Skim Milk (1 gallon for demonstration. Additional for individual groups)
White Vinegar and other acids
Lime, and other alkali materials (borax, baking soda, ammonia, etc.)
Additional raw materials for binders: eggs, dry tree sap, beeswax, honey etc.
Large pot
Beakers for each group
Stirring rods
Disposable cups
Fine mesh strainer
Cheesecloth
Rubber gloves
Safety glasses
N95 masks
pH strips
Mortars and Pestles for grinding

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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.

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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Objectives:

In this lesson we will learn about ancient rock paintings, and the technology that has allowed the marks to persist for tens of thousands of years.

We will explore

The role of binders in paint
The effect of pH on proteins in milk (and other substances)

At the end of this lesson, you will have a paint you can use to make your own long-lasting marks

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Agenda

Day 1 : Introduce topic, discuss paint materials, and begin research on binders. Begin gathering material

Day 2: Begin demonstration of casein extraction (must sit overnight). Discuss chemistry of casein and the role of acid in precipitation of the milk solids

Day 3: Finish preparing casein and test with pigments. Discuss role of alkali in formation of milk paints.

Day 4-6: Students begin alternate methods of casein extraction.

Day 6-8: Experiment with other binding materials students bring in (egg white, egg yolk, cactus pads, dandelion mash, wheat paste, tree sap, honey, wax, etc.)

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Intro/Driving Question/Opening

Have you ever drawn pictures with sidewalk chalk? How long does it last? What happens to the chalk after a rainstorm?

DISCUSS:

How do you think you could make your sidewalk chalk drawing permanent?

Today we will look at paintings that have endured for thousands of years.

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Cueva de las Manos, Perito Moreno, Argentina, Mesoamerica.

7,300 BCE and 700 CE

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Hand prints in Pettakere Cave at Leang-Leang Prehistoric Site, Maros

30,000-40,000 BCE

By Cahyo - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=31360759

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Chauvet Cave, France 30,000 BCE

Charcoal

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What is Paint?

Paint is made up of three components:

PIGMENT: Color. Usually added to paint in dry powdered form. Can be mineral (ocher), synthetic (phthalo), plant-based (indigo), or animal based (cochineal)

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: Initial Research

Divide students into groups of 3 or 4
Begin with researching historical binders

What are 3 different binders used in ancient times? Why do you think these were chosen?
What is the role of binders in paint?
What possible natural binder materials do you have around you?

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Demonstration Instructions: Prepare Casein Control Group

Prepare casein (2 day process):

The link below will connect you to a common recipe for preparing milk paint. Read carefully and repeat to demonstrate for students.

https://www.earthpigments.com/milk-paint-with-lime/

Test the pH at each stage of the experiment:

Milk before adding vinegar
Milk after adding vinegar
Quark and whey after overnight soak
Quark after rinsing
Lime and water mixture
Quark with lime mixture

Discuss: What is the role of the acid? What is the role of the base?

Have students mix a small amount of their pigments into a tablespoon of the paint base and test on paper swatches as a control group. Label this the control group, and record observations.

Notes:

Materials needed for this demo:

Large pot
1 gallon skim milk
2 cups white vinegar
cheesecloth
strainer
¾ cup lime

Be sure students use appropriate PPE when dealing with lime- gloves, mask, and goggles.

The recipe will yield about 1.5 quarts of milk paint base. Make the base as a class, then divide this among the groups as a control for their research.

Be sure to refrigerate the milk paint after it has been prepared to extend its life. Otherwise it will begin to smell within a day or two depending on temperature in the classroom.

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Chemistry of Casein production

Discuss: Have you ever seen spoiled milk? What does it look like? What do you think is going on?

Vocabulary:

colloid
lactic acid
solution
suspension
micelle
hydrophobic
hydrophilic
hydrogen bond

Milk is made up partly of large particles of casein (a protein) called micelles. These micelles have electric charges that keep them from sticking together and falling out of solution. When milk goes bad, bacteria eat the sugars in the milk and produce ACID, called lactic acid. This acid interrupts the electric charges that keep the micelles floating, and they clump together and fall out of suspension.

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The large particles of casein in milk are called MICELLES, and are actually made up of many small particles of casein proteins, each with slightly different properties. The hydrophobic casein proteins stay in the middle of the particle, and the hydrophilic casein proteins are on the outside. These hydrophilic particles have an electric charge that helps them stick to water, which is what creates a colloidal suspension.

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On the left you can see how the large casein micelles repel each other because they have the same charge, like how two negative terminals of a magnet repel.

On the right, you can see how the positive ions in an acid destroy the repulsion between micelles, causing them to collapse into each other.

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The role of the Lime (alkaline material)

Review:

What is the binder in milk paint?

casein, from milk

What is the medium of milk paint?

water, not milk

How are casein proteins held in a colloidal suspension in the milk?

electrical charges on the outside of the micelles

What does acid do to the colloidal suspension of milk?

collapses the micellar structure and removes its solubility through interrupting electric charges

We now have individual casein proteins without their micellar structure, which have a net neutral charge. In other words, there is no electrical charge that helps these proteins bond to water. In this state, casein is INSOLUBLE.

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The role of the Lime (alkaline material)

In the presence of a strong alkali (base), certain molecular groups on the casein protein become ionized, which allows them to form hydrogen bonds. These bonds allow the casein to become SOLUBLE in water, which turns it into a smooth gel-like glue.

Fun Fact: You may have found in your research that ancient pigments were sometimes mixed with urine, feces, spit, blood, or other bodily fluids. The decomposition of urine and feces produces ammonia, which is a strong base. Do you think this may have been used as an ingredient similar to how we are using lime?

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Hands-on Activity Instructions: Test Alternate Acids and Bases

With the prepared milk paint as a control, have student groups test out other combinations of acids and bases. Record the pH at each step of the process. Mix in pigment and test on paper swatch.
Record observations for each combination.

What pH works best for the acid? The base?
Do the different acids and bases make a difference in the final outcome of the paint?

smoothness, drying time, paintability, dried texture, etc.

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Hands-on Activity Instructions: Test other binders

Test out other binders

Albumen is a protein in eggs that is a historical binder. Try using the yolk, or the white, or both mixed together. Be sure to strain the membranes out first.
Tree sap can work well if it is thoroughly dried and ground to powder. Traditionally, people use gum arabic, but other saps may work.
Honey is messy and sticky, but historically was used as an additive to albumen paint. What does it do in other paint mixes? What about milk paint?
Think of natural substances that form sticky gels, and try using them as binding substances.

In all tests, mix with pigment, paint on a swatch, and record observations.

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Assessment

Student should have a working paint at the end of this lesson.

They should be able to explain the role of pH in the production of milk paint

They should be able to show additional binding materials and whether or not they are viable alternatives.

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Differentiation

Skip the chemistry lesson with the milk paint and jump straight into testing alternative binder materials. Albumen is a very easy to prepare binder, and makes for a good alternative exploration.

Remediation

Extension/Enrichment

Continue testing binders by making combinations of binding materials. Casein+lime, albumen+wax, etc.

Students can dig more into the chemistry of casein and find other methods of preparation. What happens when you polymerize the proteins with heat?

When we rinse the quark, we throw out the whey. Whey is full of proteins, however. Is there a way to use those proteins in making a paint binder?