Polymers: Turning milk into plastic

When Queen Mary of the United Kingdom stepped out for the evening near the turn of the 20th century, she would sometimes do so wearing jewelry not made out of precious metals or gems but out of milk.

Milk was used to make one of the first plastics invented, and during the first half of the past century, people made jewelry, beads, combs, and many other decorative items out of what was known as casein plastic.

You can make milk plastic yourself by mixing hot milk with vinegar. All you need is some milk and the means to heat it, white vinegar, a heat-resistant bowl, a mesh strainer or coffee filter, and paper towels.

First, heat one cup of milk on a stove-top or in a microwave until it is steaming, but not boiling.

Transfer it to a heat-resistant bowl, add four tablespoons of vinegar, and stir. The acid in the vinegar will cause the caseins– protein molecules in the milk – to unfold and organize themselves into long chains called polymers, which makes the milk curdle.

As you stir, you should see these curds appear in the milk as white clumps. Once you start to see lots of curds, pour the liquid through a strainer or coffee filter. Using a paper towel, squeeze out the remaining liquid.

You should be able to get a 2-inch-wide disk of wet casein dough. Using your hands or a small cookie cutter, mold this into any shape you like. You can even add glitter or food coloring to the soft plastic. When you’re done, let it sit on a paper towel. After 48 hours, you’ll have an ornament fit for a queen. You can color your creation with paint or markers.

Milk plastic is too brittle for use as anything other than jewelry or decorations, but in recent years scientists have been working on ways to improve the structural defects to make a plastic that can quickly break down in a landfill.

This column first appeared in the June 11, 2018 edition of The Christian Science Monitor Weekly magazine as part of the Monitor's occasional Science at Home series.