Summary

In this lesson students learn about the chemistry of oil and acrylic paints. They make their own paint, and complete an experiment to collect qualitative and quantitative data through a series of tests. Students will also apply the concepts of physical and chemical change to the results of this controlled experiment.

Grade Level

Middle School

NGSS Alignment

This lesson will help prepare your students to meet the performance expectations in the following standards:

• MS-ETS1-2: Evaluate competing design solutions using a systematic process to determine how well they meet the criteria and constraints of the problem.
• Science and Engineering Practice - Using Mathematics and Computational Thinking - Decide when to use qualitative vs. quantitative data.

Objectives

By the end of this lesson, students should be able to

• Complete a controlled experiment to make two different types of paint and complete a variety of tests on the samples.
• Explain the difference between acrylic paint drying through evaporation (physical change) and oil paint drying through oxidation (chemical change).

Chemistry Topics

This lesson supports students’ understanding of

• Mixtures
• Physical Properties
• Chemical Properties
• Physical Change (Evaporation)
• Chemical Change (Oxidation)
• Scientific Method
• Polymers

Time

Teacher Preparation: 60 minutes

Lesson:

• Engage: 20 minutes
• Explore: 45 minutes
• Explain: 20 minutes
• Elaborate: 20 minutes
• Evaluate: 20 minutes

Materials

• Safety and Housekeeping:
  • Sealable, metal disposal can, such as an old paint can for all oil containing waste
  • Table covers to prevent paint stains (old newspapers, cardboard, disposable table clothes)
  • Aprons or old clothing
  • Gloves (optional)
• Making paint (per group):
  • 4-5 gram (heaping teaspoon) Ultramarine blue pigment: available for purchased online and at art supply stores
    • Example 1
    • Example 2
      Note: Other pigments may also be used. Ultramarine blue was chosen because of its interesting history in painting and safety profile. If other pigments are selected, make sure they are safe for student use.
  • 3-4ml Refined flax seed oil (pure): available found online and in health food stores (alternative: Safflower oil)
  • 4-5 gram (heaping teaspoon) Acrylic binder: emulsion of acrylic polymer and water, which forms a matrix holding the pigment in place.
    • Example 1
    • Example 2
  • 2 Plastic forks
  • 2 Plastic knives
  • 2 Plastic teaspoons
  • 1 Droppers
  • 2 Paper Plates topped with clear packing tape
• Testing the paint (per group):
  • 2 Identical paint brushes
  • 6 Cotton swabs
  • 1 Sheet of Canvas Paper
  • Scissors
  • Clock or stopwatches

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
• We are using flax seed oil, which can be thought of as food grade linseed oil. Linseed oil is flammable, and flax seed oil should be treated with equal caution. Paper towels and rags used to clean up linseed oil often spontaneously combust. To dispose of paper towels and rags containing linseed oil, submerged them in water in a metal paint can and carefully seal the can. The can should then be disposed of at local hazardous waste disposal center. This article and news video review this danger, and can be used as a powerful tool to remind students of the importance of proper chemical disposal.
• Ultramarine blue pigment selected for use in this lesson has low toxicity. If you modify this activity, please research the health impacts and safety of any other pigments that are brought into the classroom. This website provides a list of some toxic pigments to avoid.
• Do not breathe in pigment powder.

Teacher Notes

• Background: Beginning painters are faced with the decision between purchasing oil or acrylic paints. Both paint types have advantages and disadvantages. This lesson teaches students about the chemistry of the two types of paint, asks them to mix their own oil and acrylic paint, and guides them through an experiment comparing the drying times of their paints.
  
• Oil paints consist of pigment particles suspended in a drying oil, like linseed oil. A drying oil hardens when its components react with oxygen in the air and polymerize to form a tough solid. In this activity, flax seed oil is used as a drying oil. A thinner may be added to make the oil paint easier to store and spread, but will not be necessary in this activity.
• Oil paints do not change color when they dry, and can provide rich, deep colors. Solvents are required to clean oil paint off brushes once the paint has dried.
• Pure flax seed oil used in this lesson is very slow to dry. It should be possible to clean equipment and surfaces with soap and water, and not need any solvents.
• Acrylic paints consist of pigment particles suspended in an acrylic polymer emulsion. Acrylic paints dry quickly, but before they dry they are water soluble. This allows for simple cleanup.
  
• A comprehensive overview of the history of acrylic emulsions can be found at the American Chemical Society National Historic Chemical Landmarks website.
  
• Make sure that you do not tell the students about the difference in drying time between the two paints as they will discover this through their experiment.
  
• The pigment chosen for this activity is ultramarine blue, a deep blue pigment. It was originally obtained by grinding the rock lapis lazuli into powder. It was the most expensive blue pigment, and painters reserved it for the most precious parts of their paintings. In the early 1800s, scientists devised a way to produce the same color artificially. Synthetic ultramarine is inexpensive and nontoxic. It is now used in many types of artist’s paints and wall paints, makeup, and even in laundry detergent as a “bluing” agent that brightens up faded whites.
  
• Artists typically mix on a glass plate. Watch glasses work, but must be cleaned while the paint is still wet, or else a solvent will be needed to remove the oil paint. A simple, disposable solution is to apply clear packing tape or plastic wrap to part of a paper plate and have students mix on top of the tape/plastic wrap. This will prevent the oil from soaking into the paper plate.
• Engage: Ask the students to provide examples of when they have used paints (pre-lab question 1 from student handout), what they were painting, and what type of paint was used.
• Next, read the following scenario:
  • Frida and Andy enjoy painting at school. As a middle school graduation present, their uncle offers to buy them their own paints and painting supplies. Andy considers asking for acrylic paints, because he uses them in art class. Frida is interested in trying oil paints because many great masterpieces were created with oil paints.
• Ask students if they know anything about acrylic and oil paints. Review the basic information about acrylic and oil paint from the student handout. **Do not mention drying time.
• Explore: Students make oil and acrylic versions of ultramarine blue paint. The instructions in this activity create a paint that is good for immediate use, and does not store well. If you intended to store the paint for future use, stabilizers and binders need to be added.
• Each student group completes an experiment to compare the drying time of the two paints. Students define the independent variable (paint type), dependent variable (drying time), and controlled variables (type of paper, thickness of paint layer, temperature, humidity, convection).
• Students should complete this “Explore” section, which includes making and using the paint, in the same period, since the acrylic paint will dry by the next class period.
• The students have written directions for making and testing the paint, but it will be valuable to show them these images beforehand to help better prepare them for the lab experience.

Photos: Left: Label surfaces (paper plates covered in tape). Right: Approximately ½ tsp. of pigment (1.5g)

Photos: Left: Break apart pigment to make powder as fine as possible fine. Right: Oil added to pigment.

Photos: Smooth paint samples created. Note that the Acrylic sample is more timing consuming to create.

Photos: Sample tests prepared to compare drying times.

• Explain: Each group presents their qualitative and drying experiment results to the class, and summarizes their findings (Analysis question 1). The class collaborates to create a chart showing the advantages and disadvantages of each type of paint. Students take notes on class results to use in the Evaluate section.

Experimental results to help the teacher anticipate what the class will discover:

• The oil paint remains wet for days, although there are obvious changes in the first ten minutes. The pure flax seed oil used in this lesson is very slow to dry. Other versions of linseed oil have impurities that may speed drying time. The acrylic paint shows evidence of drying at 2 minutes. At 5 minutes it is mostly dry with only a few wisps of color rubbing off, and at 10 minutes it is dry. The class results vary, since each group uses different amounts of oil and acrylic binder, but the pattern is clear. When students report their results they should share how much oil and acrylic binder they used.
• Elaborate: In questions 3 and 4 of the “Analysis” section, students learn that acrylic paint drying is a physical change, as water evaporates from the mixture quickly. They learn that oil paint “drying” is actually a chemical reaction that occurs as the oil is exposed to air, and this process takes longer.
  
• Evaluate: Re-read the scenario from the “Engage” section. Students write a letter to Frida and Andy recommending one type of paint. They use all of the data collected by the class to explain their choice.
  
• Extend: This lesson can easily be expanded to test the drying time of commercial or homemade tempera and watercolor paint. Students could also engage in an inquiry investigation of other variables, such as type of surface, method of application (brush, roller, sponge, spray), ratio of ingredients, and types of oil used.