For deep-frying, choose a deep, heavy pot with a thick bottom; a thin-bottomed pot cannot retain heat, so the oil temperature drops sharply with each batch. Do not fill the pot more than halfway with oil. Food makes the oil foam and rise, and the empty space above it helps prevent overflow. The oil should be deep enough for the pieces to float without touching the bottom, and at least 5 cm deep.
The pot should have a thick bottom and walls. Cast iron, enameled cast iron, and thick-bottomed steel pots retain heat; when food is added, the oil temperature drops less and recovers faster. In a thin aluminum pot, the oil temperature can drop by 30–40°C / 54–72°F in a single batch, and the food absorbs oil during that time. Cast iron’s weight also keeps the pot stable on the stove. The pot should be deep; a wide, shallow pan is not suitable for deep-frying because it takes much more oil to reach the same depth and leaves less room to prevent overflow.
The fill limit is fundamental to safety. Oil should never fill more than half the pot; one-third full is even safer. When food goes into the oil, its water turns to steam and the oil foams and rises; a wet or frozen batch can make the oil rise several centimeters at once. In a pot filled to the rim, this surge can send oil onto the stove, where it may catch fire if it reaches a flame. On an electric stove, spilled oil can land on the hot surface, smoke, and ignite.
Oil depth depends on the food. For potatoes, breaded foods, and fried dough to float without touching the bottom of the pot, the oil needs to be at least 5 cm deep; if a piece rests on the bottom, that side burns while the top stays pale. A greater volume of oil keeps the temperature steadier; in a home kitchen, 1–1.5 liters of oil in a thick 3–4-liter pot is enough for most frying. Trying to deep-fry with too little oil makes it cool with each batch, leaving the food greasy. The wider the pot, the more oil it takes to reach the same depth; a narrow, deep pot achieves that depth with less oil.
The pot’s position and stability matter too. Place it on a rear burner with its bottom sitting flat on the stove; an unstable pot or one extending beyond the burner can tip over. Do not turn the handle toward the edge of the stove or a walkway; an elbow bump could spill hot oil. Keep paper towels, cloths, and plastic containers away from the pot. Do not place a glass of water or a wet sponge next to it either; spilled water can mix with the oil. Keep a matching metal lid nearby. If the oil catches fire, slide the lid over the pot, turn off the burner, do not move the pot, and do not pour water on it. Leave the lid on until the pot has cooled.
A few tools make deep-frying easier. A wire skimmer, the wide perforated utensil commonly called a spider, lifts food quickly from the oil while letting the oil drain off. Long-handled tongs keep your hands away from the oil. A clip-on frying thermometer continuously displays the oil temperature. Transfer fried foods to a wire rack set over a tray, rather than paper towels, so the underside stays crisp too. When frying is done, leave the pot on the stove to cool, and strain the oil once it has cooled.