The candler should be set on a box or table at a convenient height (about 38 to 44 inches from the floor), so the light will not shine directly into the eyes of the operator. In candling, the egg is held in a slanting position with the large end against the hole in the candler. The egg is grasped by the small end and, while held between the thumb and tips of the first two fingers, is turned quickly to the right or left. This moves the contents of the egg and throws the yolk nearer the shell. Because of the color of their shells, brown eggs are more difficult to candle than white eggs.
To do a reasonable job, an extensive knowledge of candling is not necessary, particularly if the eggs are all relatively fresh. One should be able to distinguish a fresh egg from a stale egg and detect such abnormalities as bloody whites, blood spots, meat spots, and cracked shells. In a fresh egg, the air space is plainly visible and moves freely. The white is thin and clear. In a stale egg, the air space is plainly visible and moves freely. The white is thin.
Most newly laid eggs are good quality. Eggs not over two or three days old, if held under good conditions, will meet the specifications for Grade A. The only eggs to be removed by candling are those with bloody whites, blood or meat spots, and cracked shells.
Candling is a way of checking the fertility of an egg and the development of the embryo, with the use of a light source in a darkened room. In a darkened room, carefully hold the egg up to the light to observe the contents of the egg.
The embryo is located at the large end of the egg, where blood vessels will be present under the surface if the egg is fertile. The embryo appears as a dark spot which becomes larger as the incubation period continues.
The egg will appear to have a black spot which as the embryo grows and incubation continues will grow larger until light will only pass through the air cell end of the egg.
Eggs appear clear.
If the egg was fertile but the embryo has died then you will see a blood ring around the yolk or possibly a dark spot dried to the inside of the shell depending on when the embryo stopped growing.
Note that dark or brown shelled eggs are more difficult to candle than white or pale shelled eggs.
When To Candle
Candling can be done at any time, although day 8 onwards is usually when the embryo is more easily identified.
Day 3 of incubation (usually pale shelled eggs).
Day 5/6 of incubation (usually dark shelled eggs).
Between day 8 - 12 of incubation (embryo more easily identified).
3 days prior to hatching.
How To Candle
To candle, darken the room. Eggs can be out of the incubator (or away from a broody hen) for up to 20-30 minutes before starting to cool down inside, so don't rush. You can GENTLY roll the egg on the candler to get the best view.