Egg quality is judged by the size of the air pocket at the top of the egg and the stiffness of the white of the egg. It is usually determined by candling (Passing a bright light through a small hole in the "candler" through the shell of the egg). White eggs are easily candled. Brown eggs are more difficult.The smaller the air pocket and the stiffer the white, the higher the quality. Because the shell is porous, the white can evaporate over time, increasing the size of the air pocket. Commercial eggs are usually sprayed with a fine mist of oil to prolong shelf life.