Only thick-shelled eggs are used for balut making because these can withstand stresses of egg placement and removal in cylindrical baskets called "toong". These Iare open on both ends, 34 inches high and 21 inches in diameter; spaces around are filled with rice hull up to 4 inches from the brim. Ideally, eggs made into balut should not be older than 5 days from the time these phase are laid by ducks.
Heat is needed to develop the embryos. Roast or heat palay to a temperature of 107oF or 43oC in an iron vat or cauldron. Remove palay when you can still hold the palay in your hand when you remove it.
Egg bags are then placed in the toong; these are alternated with heated palay bags. The number of heated palay bags is one for every egg bag. However, place two heated palay bags on the bottom and two on the top level of the toong to ensure heat conservation. For every toong containing 10 layers of eggs, you would need 13 bags of roasted palay.
Each toong can hold 10 bags to tikbo. Cover with jusi sacks to conserve heat further. Candling is the process of holding egg against the hole of a lighted box in a dark room to separate infertile eggs from fertile one. Infertile eggs are called penoy; these are also boiled like balut but fetch a lower price.
First candling is done on the 11th day after eggs are placed in toong. Candling is again done on the 17th day to separate eggs with dead embryos (abnoy) and those that are ready to be sold as balut. Eggs with weak embryos take 18 to 20 days to be released; these are hard-boiled and sold.
Eggs intended for hatching are left in the balutan for 28 days when duckling will hatch. After 20 days, palay bags are not heated anymore since embryos can generate enough heat to keep them warm.
When using kerosene or electric incubators for hatching duck eggs, maintain a temperature of 100°f and humidity from 55°f to 60°f. Do not hatch duck and hen's eggs together in one incubator as duck eggs require a temperature of lof but a higher rate of humidity. A pan of water kept in the bottom of the incubator helps maintain humidity level. During incubation period, turn eggs at least 3 to 4 times a day to obtain better percentage of hatchability.
Clean hatching eggs with slightly moist, clean rag before storing to prevent contamination of the developing embryo, or newly hatched chicks.