Breeding Chickens: Candling Eggs 

Candling is the term used to describe shining a bright light through the shell of an egg to observe the developing embryo inside. By candling an egg you can confirm that the egg is viable and will produce a chick. Infertile eggs can then be removed and discarded to avoid the bad eggs popping in the incubator.
The term 'candling' of course, refers to a time before electric light bulbs when candles were used to see inside the egg. 

You can first candle the eggs when they have had at least a few days incubation, usually about 5-7 when the air sac inside will have begun to develop. There is little point in candling after about 17 days as the chick will be almost fully formed and you will not be able to see through the egg anyway.

It is perfectly safe to remove eggs from the incubator to candle them. The centre of a chicken egg will not cool down enough in the space of 15 or 20 minutes to cause any damage to the embryo inside.
As long as you are careful not to overheat the egg with the candling lamp the embryo inside won't be harmed by candling.

When you shine the candling lamp into a developing egg you will clearly see the developing embryo inside. Depending on what stage of development the embryo is at you will clearly see details such as blood vessels, the pores in the egg shell, the air sac, and the shape of the developing chick. You may be able to see the developing chick move depending on the sage of development it's at. If the egg candles clear, it is infertile and can be discarded.

With some practice you will eventually be able to tell what stage of development and incubation the egg is at just by observing the egg with a candling lamp.

When candling your eggs-

• Take special care to handle the egg carefully and avoid cracking or damaging it. Once the egg is cracked, the embryo will die.
• Dont allow the egg to cool excessively, a few minutes out of the incuabtor is fine but dont take any longer than you have to
• Dont overheat the egg with the candling lamp. A purpose made candling lamp is best used but if you make your own, take care not to overheat the eggs with the heat of the lamp.
• If an embryo has clearly stopped developing then discard the egg without delay. A build up of gas will cause the egg to pop and the resulting mess is extremely unpleasant.

 See also ~ incubation problems