The main function of an eggshell is to protect the embryo of an oviparous animal; as such, it sounds very logical to think that eggshells should be strong. The thickness of an average table eggshell ranges from 0.35–0.40 mm, which makes it very vulnerable to breakage when it hits a hard surface. Moreover, the eggshell is covered with 7,000 to 17,000 tiny pores that allow air and moisture to pass through. Nevertheless, despite this seemingly fragile appearance, eggshells are surprisingly stronger than we think.

The composition of the eggshell contributes to its strength. About 95% of the dry shell is made up of calcium carbonate (CaCO₃ ) crystals. In addition to the way calcium compounds are organized in the shell, these compounds have very strong binding properties.

The structure of the eggshell gives it much sturdiness than meets the eye. They are 3D-oval structures, with the upper side forming a relatively narrower arch than the lower. The strength of the oval body and the arch-shape lies in their ability to distribute pressure evenly across the surface, allowing the shell to withstand considerable force. Try crushing an egg through applying pressure with your interlocked fingers*, it will reveal considerable resistance before breaking.

You can conduct the following simple experiment to test the sturdiness of the eggshells firsthand. First, pick five or six eggs of approximately similar sizes; make sure there are no cracks on the shells. Then, distribute these eggs in a balanced manner around an egg tray. Now, gradually add weight by placing some flat objects such as books and magazines on your egg base. You will observe how eggs can carry weights heavier than you might expect.

The eggshell structural strength has inspired architects and biotics researchers to create robust yet lightweight structures. Even historically, arches have been widely used and hundreds of monumental arches have survived to testify the durability of this ingenious design.

Quick Facts

• Eggshell are bumpy and grainy in texture; the smoother the eggshell, the less easily it is likely to fracture.
• As the hen ages, it lays larger eggs that contain the same fixed amount of calcium; as such, large eggs are more fragile than small ones.
• The shell is semipermeable to allow oxygen, carbon dioxide, and moisture to pass through. However, it is protected against bacteria and dust by a shiny coating layer known as the bloom, as well as an inner membrane.

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