Question

I work at a public library and we recently had a sidewalk chalk hour for families. Someone gave us a tip that if we brush milk with a paintbrush over the finished chalk drawings, it helps seal them and makes them last longer. It has been several weeks, two heavy rainstorms, and a hail storm, and all of the drawings we brushed with milk are still there! The ones we didn't brush with milk are long gone. What are the chemical properties of milk that make it do this to sidewalk chalk?

Answer 1

Milk acts as a sealing agent for chalk drawings due to the protein casein, which interacts with chalk to form a protective layer. Elements such as calcium in both milk and chalk enhance this durability, making the art more resistant to weather.

Step-by-step explanation:

The reason milk can act to preserve chalk drawings is due to its chemical composition. Milk contains a protein called casein, which acts as a binder. When milk is applied to chalk, the casein protein interacts with the chalk, forming a protective layer that resists water.

This is similar to how egg is used as a binder in traditional tempera paintings. The calcium found in both the milk and the chalk (calcium carbonate) likely contributes to this reaction, as calcium can form strong bonds with the carbonate in the chalk, further enhancing the durability of the drawing when exposed to rain or hail.

It's also worth noting that while chalk is a form of limestone, which is calcium carbonate, it can also contain various binders and pigments for color. These additional materials combined with the protective casein in milk can create a more lasting artwork. Chalk, whether white or colored with additional pigments like iron oxide or carbon, is more vulnerable to the elements without this added protection.