1 cup milk (we used 2%)
4 tsp white vinegar
Stovetop or microwave to warm the milk
Pot or microwave safe bowl
Molds for shaping
Skewer to make a hole
Acrylic paint or permanent markers for color
Time: 30 minutes to create; 2 days to dry
How to do it:
Heat one cup of milk until it starts to steam. On the stovetop, this will take a few minutes on medium high heat. Once you see steam rising from the milk, remove it from the heat. Once you’ve removed it from the heat, add the 4 teaspoons of white vinegar.
Gently stir it. You’ll see the milk start to sour right away – what you’re doing here is turning the milk into curds and whey! Stir for about a minute or two (gently) to get all the curds to form.
Pour into a strainer to remove the whey. Mushing the curds down with a spoon will help – you want to remove as much liquid as possible.
To keep removing more liquid, remove the curds from the strainer onto a paper towels, and carefully pat and press to remove more whey. You’ll need a few paper towels for this; replace them with dry ones to keep removing more liquid.
Once you’ve done this you’ll find it forms into a ball.
Now you can shape it! You can squish into the shape you want, or into a mold. We shaped ours into a heart. If you’d like to hang it from a string, use a skewer to make a hole.
Let it dry – this takes about two days. If you’re not using a mold, after one day turn it over to let the bottom dry. Once it’s completely dry, if there are rough edges you can clean them up with a bit of sandpaper or a nail file. It’s hard plastic now, so it will take a bit of work! Then you can add color using permanent markers or acrylic paint.
What’s the Science?
This is a chemistry experiment working with polymers. Polymers are molecules that have formed a regular chain structure. Milk contains molecules of a protein called Casein. During the reaction between warm milk and acid (the vinegar), the casein molecules unfold and form long chains called a polymer. The polymer can be molded and shaped which makes it a plastic. In this case it’s called casein plastic or milk plastic because of the type of molecules that created the plastic.
Before synthetic plastics were invented around 1945, this was a common way to make plastic, and was used to make jewelry, buttons, and other things. This type of plastic has been around since the beginning of the 20th century. Click here to read more about casein plastic.