Terra cotta oil lamps have been used in the Middle East, Southeast Asia and across the world for thousands of years. Their outer decorations may differ, but all terra cotta lamps work basically the same way: one end of a long, absorbent wick is dipped into a bowl of oil, and the other end is set on fire. Due to a physical phenomenon known as capillary action, the flame's consumption of oil on one end of the wick draws more oil up from the bowl.
In festivals like Diwali, where millions of lamps must be improvised in a short amount of time, terra cotta lamps do not have to be covered. However, traditional terra cotta lamps have an elongated pitcher or "gravy bowl" shape with a permanent cover. Once in place, the cover leaves two openings: one at the elongated end (where the wick ultimately goes) and one in the center of the cover to allow for the adding of fuel. (Figure 1)
Made from linen, flax, papyrus or hemp, wicks are long bundles of fiber that have been chemically treated to become flame resistant. When the wick is dipped in the oil, the liquid's surface tension causes the molecules to draw up through the fibers against the force of gravity. This is called capillary action. Once the oil reaches throughout the entire wick, the user holds a flame to the exposed end to ignite the oil contained between the fibers.
By consuming the oil, this small flame in effect "empties" the end of the wick. To fill this void, capillary action draws more oil up into the tip, which is consumed as well. The lamp will continue to burn as long as there is oil to keep moving up the wick.
Due to the intense heat of the burning oil, the fibers on the tip of the wick will become charred. However, the wick itself is not consumed by the flames.
Flammable oils are found throughout nature. While most modern oil lamps use kerosene, a refined form of petroleum, peoples from antiquity used both animal oils and those extracted from certain kinds of plants. In ancient Rome, Greece and Western Europe, whale oil, fish oil and petroleum that oozed on the earth's surface were popular lamp fuels; African and Asian civilizations relied on vegetable oils (e.g., castor oil, peanut oil and nettle oil).