eggshells

An eggshell is the calcareous outer shell of an egg, produced by the female of birds and some reptiles. In chickens, the shell is comprised mainly of calcium carbonate, usually around 95% by weight, with small amounts of calcium phosphate and trace minerals. The shell is covered by a protective cuticle that helps keep out bacteria.

The shell is porous, with thousands of microscopic pores that allow gas exchange for the developing embryo while helping prevent desiccation. The inner surface is lined by membranes that can separate from the shell when cleaned. The overall strength and thickness of the shell vary with species, age, nutrition, and environmental conditions.

In production, eggshells are typically discarded as waste, though large quantities are recycled. They can be collected, cleaned, and dried, then ground into a powder that is used as a calcium supplement in human nutrition or in poultry feed, and as a soil amendment to raise calcium content and reduce soil acidity. Ground shells can also be used in crafts and as a garden mulch component.

Safety and handling: Raw eggshells can harbor Salmonella. To reduce risk, rinse, dry, and bake shells (for example, at 180°C for 10–15 minutes) to sterilize before grinding into a fine powder. Store dried shells in a dry place to prevent mold growth. In gardening, crushed shells are sometimes spread around plants to deter pests and improve soil structure.

Other uses include incorporating shells into composting systems and, historically, as a source of lime for soil treatment. Some products utilize the eggshell membrane for cosmetic or wound-care-related applications, though evidence and regulations vary by country.