Key concepts
Chemistry
Water
Saturation
Absorption

Introduction
Have you ever wondered what art and science have in common? Although art draws on emotions and science uses rational thought, science and art both demand creativity and excellent observational skills. Most techniques used by artists even have interesting scientific explanations. This activity explores just one: painting with water-based paint on wet surfaces. See how science can help you become a more versatile artist!

Background
A fresco is a wall-painting technique that uses water-based paint on wet lime plaster. The technique is a marvelous work of chemistry—but it’s not without risk! First extreme heat above 800 degrees Celsius (around 1,500 degrees Fahrenheit) is applied to limestone, or CaCO3, a sedimentary rock found in warm, shallow marine waters. This breaks the rock down into carbon dioxide—CO2, the gas we exhale—and quicklime, or CaO. Quicklime, a toxic substance in itself, is then dissolved in water to create slaked lime, or Ca(OH)2, a substance that can cause chemical burns. Microscopic particles, such as sand, are then mixed in to form the plaster used as a surface on which to paint.

These particles play an essential role because they create air pockets inside the mixture, which allows carbon dioxide to creep in and react with the slaked lime to form limestone—CaCO3, the substance we started with! Before letting the plaster dry painters apply water-based paint. This is a mixture of water and colorful particles called pigments that can be applied onto the wet plaster, which absorbs the water carrying the pigments in. Then the water evaporates, the plaster sets and the pigments become a part of the wall—a fresco is born!

Because the pigments penetrate into the plaster, frescos are durable. We can still see remnants of frescos made around 1500 B.C. on the Greek island Crete. Other great examples are found in the ruins of the ancient Roman city of Pompeii or in Italian Renaissance works, such as Michelangelo's frescos on the ceiling of the Sistine Chapel in Vatican City.

In this activity you will investigate painting with water-based paint on wet surfaces. We will replace the toxic lime plaster used in traditional frescos with a homemade cornstarch mixture that is also fun to play with: Oobleck.

Materials

  • Pie pan
  • Measuring cup
  • Cornstarch
  • Water
  • Measuring cup
  • Liquid food coloring or watercolor paint (One color is enough to do the activity but several will yield a more colorful painting.)
  • Small bowl to mix food coloring or paint with water (one for each color you use)
  • Paintbrush (one for each color)
  • Sheet of watercolor or construction paper (preferably a light color)
  • Cloth or paper towels
  • Work surface where you can use food coloring or paint
  • Sponge (optional)
  • Fork (optional)


Preparation

Note: Prepare to get messy and be careful not to get any Oobleck or paint in your eyes. Always wash your hands after handling Oobleck or paint.

  • Slowly mix two cups of cornstarch with one cup of water in the pie pan. If you have a small pan, use half as much. You can use your hands or a fork for mixing. The result should be a gluelike mixture that slowly oozes between your fingers if you pick it up. Add small amounts of water if the substance seems too dry. This mixture is often called Oobleck.
  • Once you are done playing with the paste leave it in the middle of the pan and let it sit for at least 15 minutes. Watch how it slowly oozes out and fills the pan.
  • Mix a few drops of food coloring or watercolor paint with a bit of water in a small bowl. This is your water-based paint.
  • Just before you start the activity let water run over the paper or wet it with a sponge so its surface is wet (but not soaked trough).


Procedure

  • In a moment you will paint two lines—one on the wet paste in the pie pan and the other on the wet paper. Do you expect the lines to look sharp and crisp or soft and spread out?
  • Take a paintbrush, pick up some paint and paint a line on the wet paper. What happens? Why would this be?
  • Pick up some more paint and paint a line on the wet paste in the pie pan. What happens now? Why would this look different?
  • Go ahead and paint whatever you feel like on the wet paste and on the wet paper. Is the effect of the water-based paint on wet paper and on wet paste different? Why would this be?
  • Once you are done painting let both paintings dry and observe the results. Can you explain why these techniques create such different results?
  • Extra: Use red cabbage juice instead of water to make the paste and to wet your paper. Use white vinegar and/or a mixture of baking soda and water as paint. Do you see how these transparent paints create color? Can you find more substances in the house to paint on this cabbage juice–soaked base?
  • Extra: Explore how water-based paint works on different types of dry and wet paper. Is printer paper different from construction paper, watercolor paper or cardboard? What happens if you first make all these types of paper wet? How can you combine both techniques?

Observations and results
Were your lines soft and spread out on wet paper, and sharp and crisp on the wet paste? That is expected!

Even though the paste is wet it is not saturated; it can absorb more water. As a result the water-based paint penetrates the paste carrying the colorful pigments with it. This leaves a crisp, well-defined line on the surface.

Painting on wet paper is slightly different. The small air pockets between the paper fibers quickly fill with water. Once the paper is saturated it can no longer absorb any more moisture. Water-based paint applied on a saturated surface floats on top of the paper, carrying the pigments with it. The result is a soft line with crinkly edges.

In both cases the water evaporates, leaving the pigments as well as the cornstarch or paper behind.

Cleanup
Wash all equipment (and any soiled work surface) with soapy water and wipe them clean. Discard the Oobleck paste in the trash. Do not forget to wash your hands once you are done.

More to explore
It’s a Solid... It’s a Liquid... It’s Oobleck!, from Scientific American
Folded or Flat Paper Towel: Which One Absorbs More Water?, from Scientific American
Cabbage Chemistry—Finding Acids and Bases, from Scientific American
Science Activities for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies