Candling eggs is the process of shining a bright light through an eggshell to see what’s inside. The light passes through the shell and illuminates the egg’s contents, revealing whether an embryo is developing, whether the egg is fertile at all, or whether there are defects like cracks or blood spots. The technique gets its name from the candles originally used as the light source, though today most people use high-powered flashlights or purpose-built candling lamps.
How the Light Reveals What’s Inside
Eggshells are slightly porous and semi-translucent. When you hold a focused light source against one end of the egg in a dark room, the light passes through the shell and illuminates the interior. An infertile egg or a freshly laid egg glows evenly, with only the faint shadow of the yolk visible. A developing embryo, on the other hand, blocks much of that light. By about the midpoint of incubation, the embryo and its surrounding membranes form a large, dark mass filling roughly two-thirds of the egg.
The color of what you see matters too. A healthy, growing embryo gives the egg a reddish tinge from the network of blood vessels spreading through the membranes. If the embryo has died, those blood vessels lose their red color and appear very dark, almost black. That color shift is one of the most reliable signs of whether things are progressing normally.
Why People Candle Eggs
Candling serves two very different purposes depending on the context. For anyone hatching eggs at home or on a farm, it’s the primary way to monitor embryo development and catch problems early. You can identify infertile eggs, spot embryos that stopped developing, and track whether humidity levels inside the incubator are correct by watching how the air cell grows over time.
In commercial egg production, candling is part of the USDA grading process. Before eggs reach grocery store shelves, they’re inspected under light for cracks, blood spots, meat spots, and yolk defects. Eggs graded AA or A can have no more than 1 percent with blood spots larger than 1/8 inch in diameter or air cells deeper than 3/8 inch. Cracked eggs glow noticeably brighter along the fracture lines than the rest of the shell, making them easy to catch. Mottled areas, where the shell is thinner in spots, also show up as more translucent patches under the light.
What You See at Each Stage of Incubation
For chicken eggs, which take 21 days to hatch, the first useful candling happens around day 7. Before that, there isn’t enough development to see clearly. At day 7, a fertile egg shows a small dark spot (the embryo) with a web of blood vessels radiating outward, almost like a tiny spider. An infertile egg still looks clear with no veins at all.
By day 10 to 14, the embryo is much larger. The dark mass fills a significant portion of the egg, and you can sometimes see movement. The air cell at the wide end of the egg is visibly bigger than it was at the start. Chicken eggs lose 12 to 14 percent of their total weight through evaporation during incubation, and that moisture loss is what makes the air cell grow. If the air cell is too large, humidity in the incubator is too low. If it’s too small, humidity is too high, and the chick may not have enough space to position itself for hatching.
Around day 18, the chick fills nearly the entire egg. Movement slows down because there’s simply no room left. At this point, most of the egg appears dark except for the air cell. This is the last time most people candle before hatch day, since the eggs shouldn’t be disturbed during the final stretch.
Signs of a Problem
Three common issues show up during candling. A “clear” egg at day 7 or later means the egg was never fertilized or the embryo died so early that it left no visible trace. These eggs should be removed from the incubator since they can harbor bacteria and eventually burst.
A blood ring is a red or dark circle visible inside the egg, with no veins extending outward from it. This means the embryo died in the first few days, and the blood that had started forming vessels pooled into a ring shape. There’s no saving an egg with a blood ring.
A “quitter” is an embryo that developed for a while and then stopped. You’ll see a dark mass but no movement, and the blood vessels will look dark or black rather than the healthy reddish color of a living embryo. Some quitters are obvious, while others can be tricky to distinguish from viable eggs, which is why candling at multiple points during incubation gives you more confidence.
Equipment and Technique
You don’t need specialized gear to candle eggs, but light intensity matters. A bright, focused flashlight with at least 1,500 lumens works well, especially for eggs with dark or thick shells. The small LED candlers that come with some incubators often aren’t strong enough to penetrate deeply pigmented shells from breeds like Marans or Welsummers. A concentrated beam works better than a wide flood because it pushes more light through a small area of shell.
To candle, hold the egg at a slight angle with the wide end (where the air cell sits) against the light source. Cup your hand around the junction of the egg and the light to block ambient light from leaking in. Do this in a dark room. Gently rotate the egg to get different views of the interior. The whole process should take only a few seconds per egg. Handle them carefully, wash your hands first, and avoid leaving eggs out of the incubator for extended periods.
Candling Different Species
The basic technique is the same for all poultry, but timelines and visibility vary. Duck eggs incubate for about 28 days instead of 21, so the development milestones shift later. Duck eggshells tend to be smoother and more translucent than chicken eggshells, which can actually make them easier to candle despite being larger. Quail eggs are tiny and have heavily speckled shells that make candling more challenging. Turkey and goose eggs follow the same principles as chicken eggs but with longer incubation periods of 28 and 30 days respectively.
Shell color has a bigger impact on visibility than egg size. White and light-colored shells let through the most light, giving you the clearest view. Dark brown or olive-colored shells require a stronger light source and more patience. With very dark shells, you may only be able to see clear signs of development later in incubation, around day 10 or beyond, rather than catching early details at day 7.