How to Candle Eggs

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Learn how to candle eggs by placing a bright light behind your eggs to determine the age of your eggs and find imperfections.

Not all of an egg’s qualities may be determined by looking at the outside of the egg. Detecting such things as hairline cracks, thin spots, and double yolks requires candling the eggs. You can learn how to candle eggs yourself. Egg candling means examining the contents of an egg by placing a bright light behind it, although I’d be surprised if anyone still uses candles.

Egg Candling Devices

Poultry-supply outlets offer various handheld devices designed specifically for the purpose. Most of them look like small flashlights with a plug-in cord. Although a battery-operated egg candler is available for candling eggs under a broody hen when no power is available, a small flashlight works at least as well as, if not better than, either a plug-in or battery-operated egg candler.

All you need is bright light that comes through an opening smaller than the diameter of the eggs you want to candle. If you have a too-big flashlight, cut a hole in a piece of cardboard and tape it over the business end of the flashlight, or tape a short piece of empty toilet-paper or paper-towel tube to the end in such a way that light only comes through the narrowed opening.

Such egg candling devices work best in a dark room. Hold the egg at a slight angle, large end to the light. Making sure your fingers don’t block the light, turn the egg until either you see something or you’re certain there’s nothing to see.

More expensive, but fun and easy to use, is a battery-operated OvaScope consisting of two parts: a tabletop egg candler fitted with a black plastic hood that looks something like a microscope. Although the candler may be used alone, with the scopelike hood surrounding the egg and candler, ambient light is blocked to make the contents easier to view — which means that with the OvaScope you don’t have to candle your eggs in a closet. The candling cylinder on which the egg is mounted has an adjustment wheel that lets you rotate the egg while you’re examining it. The scope also slightly magnifies an egg, which is handy when candling smaller eggs.

What to Look For When Egg Candling

White-shelled eggs are easier to candle than eggs with colored shells, which is why white eggs have become the industry standard. Similarly, plain-shelled eggs are easier to candle than eggs with speckled shells. Double yolks are more difficult to see in colored or speckled eggs, but the unusually large size of a double yolker is already a pretty good indication of what’s inside.

An egg blood spot is also difficult to detect in an egg with a speckled shell. It appears as a small, dark dot on the egg yolk, or sometimes within the white. A blood spot is not harmful, but is certainly not appetizing to find in a boiled or fried egg. Since an egg blood spot can be hereditary, watch for it if you’re hatching replacement layers for table egg production.

Hairline cracks and other shell imperfections are easy to spot on any egg. Cracks appear as white veins in the shell. Since cracks open the way for bacteria to enter, eliminate these eggs from your selection for hatching. Cracks are more likely to occur in eggs with thin shells.

Determining the Age of Eggs

Besides letting you easily see shell imperfections and double yolks, egg candling lets you estimate the age of eggs, which is important if you discover a hidden nest full of eggs and have no idea how long they’ve been there. The size of an egg’s air cell increases as an egg ages. A freshly laid egg has no air cell at all, but as the egg cools and its contents shrink, the air space develops. Then, as moisture evaporates from within the egg, the inner shell membrane pulls away from the outer shell membrane, and the air cell grows.

Just how fast the air space grows depends on the porosity of the shell and on the temperature and humidity the egg has been subjected to. The cell of a freshly laid cool egg is no more than 1/8 inch (3 mm) deep. From then on, the larger the cell, the older the egg. In a nest full of found eggs, egg candling will allow you to see a progression in air cell sizes based on how long ago or how recently each egg was laid.

This excerpt has been reprinted with permission from Hatching & Brooding Your Own Chicks by Gail Damerow, published by Storey Publishing, 2013.