Candling Chicken Eggs Should Be Done When Using Artificial Means Like an Incubator to Hatch Eggs, Both Prior to Incubation and During the Incubation Process.
The University of Nebraska – Lincoln, uses some great terminology that’s too good not to share.
What Is Candling?
Candling chicken eggs is the act of shining a bright intense light through the chicken egg in an attempt to see what is happening inside.
When done properly, it allows us to “see” inside the egg and some of the different parts of an egg without hurting or disturbing the baby chicken growing inside.
Candling chicken eggs is done prior to incubation to look for small hair line cracks that will compromise the integrity of the shell. These cracks are a concern because they are a doorway for bacteria to enter as well as potentially causing an excess loss of moisture within the shell.
Both of these events can prevent fertilization of the egg or lead to death of a developing embryo. I've heard that small hairline cracks can be sealed with beeswax or non-toxic white glue although I’m not aware of its success rate. Just to be safe, I’d recommend that you don’t incubate any eggs with cracks in them.
Candling is also done during the incubation period to confirm fertilization and the viability of the embryo during the growth process.
How to Candle
Candling chicken eggs can be done using a flashlight with an adjustable beam like a MAGLITE, flashlight or by buying or building a candling device. Any of these methods should work fine.
You’ll want to hold your egg gently with clean dry hands. Make the room as dark as possible for best results and shine your light source through the egg. If you're using a bulb that gets really hot, don't hold it near the egg for too long. It can harm the growing embryo.
On new eggs prior to incubation, you won’t be able to tell much other than if the egg shell has cracks in it. On days 7, 13, and 18 as long as the embryos are viable, you should be able to watch them grow and mature before your eyes.
When to Candle
You’ll begin candling during incubation, usually starting on the 7th day.
If the embryo is alive and growing you should be able to see indications of growth like the blood vessels in the picture above moving toward the air sac, possibly an outline of the embryo, and maybe even the beginnings of movement.
If the egg shows no changes in development compared to when you first candled it, it is likely unfertilized (a yolker).
And is not going to hatch. =(
It won’t hurt to leave it in there for a few more days. Just put a small identifying mark on it and check it again later.
The second time you’ll candle will be on the 13th day of incubation. If there are still no changes in the eggs you checked before, you should remove them from the incubator and throw them away.
Do NOT eat eggs that have been in an incubator.
On day 13 if you see any eggs that were developing before, but have ceased to develop (quitters) and show a blood ring (a ring that runs around the outside of the shell, very noticeable) you can mark them and leave them until the 18th day.
This is usually the final time you’ll candle prior to lock-down. If the quitters haven’t changed, remove them and throw away. At this point, the rest should be winners. Lock-down your incubator and relax, you should be seeing some baby chickens in the next 3 days or so.
If you’re doing a lot of incubating, you may want to build your own candling device. You can find directions for a homemade candler here.
Candling is more of an art than a science, but it’s great to know if you’re incubating eggs. The more you do it, the easier it will get.