Having a light that you can properly see into the egg makes a difference. I use a big flash light and wrap my hands around the light to create a seal. I candled mine yesterday to 18 days and they were hard to see into but I could still see veins at the top by the air bubble!
You definitely don’t want to clean off an exploded egg right under a broody hen.
Look at this photo, you will see that there are no blood vessel growth, so the egg is definitely dead.
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What Does A Rotten Egg Look Like When Candled?
When candling eggs, having a light that you can properly see into the egg makes a difference.
So, use a big flash light and wrap your hands around the light to create a seal, and check the edges, if there no sign of blood vessels at all at day 7-18 then the egg is rotten.
Rotten eggs will typically have that opaque look to them, from all of the bacteria growing inside. But the edges would still show blood vessels.
You will be able to see blood vessels from some angles, all the way up until the day the chick hatches. And usually to see it, you’ll want to shine the light right in the tip of the large end, and look down the sides of the shell. With white eggs, it is VERY easy to see. There’s no way you can miss them. Sometimes I have problems on my green and brown eggs, but never the white ones.