What Makes Cork Buoyant?

Cork was first studied under the microscope in the 1660s. In fact, it was one of the first materials that Robert Hooke, inventor of the microscope, studied. This is likely because cork bark has been a well-known anomaly in the natural world for millennia due to its flexible, lightweight nature and regenerative ability. What Robert Hooke discovered when he looked under the microscope helped to explain the differences between cork bark and most other naturally occurring materials. 

When you look at cork under the microscope, you will see an irregular honeycomb structure that looks quite different from the rectangular shape of most wood cells. This wavy honeycomb structure allows the cells to behave like corrugated cardboard, creating the flexibility that cork is renowned for. In addition, since the cells are closed, they do not absorb liquid, explaining why cork has been the preferred closure material dating back to ancient times. 

Notably, only 15% of cork cells are solid material. The remaining 85% is comprised of trapped air. This makes cork incredibly lightweight and buoyant. The trapped air in the cork's cellular structure means that the cork has a density of 15% water, allowing it to hold several times its own weight while staying afloat.

In addition to making cork flexible and buoyant, cork's cellular structure is the reason for its remarkable thermal properties. This has created markets for cork insulation in weight-sensitive industries like shipbuilding, aerospace, and commercial building applications.