As the mass of the solid fuel is melted and consumed, the candle grows shorter. Portions of the wick that are not evaporating the liquid fuel are themselves consumed in the flame, limiting the exposed length of the wick.
Usage
Prior to the domestication of electricity, candles were a common source of lighting, before, and later in addition to, the oil lamp. Due to local availability and the cost of resources, for several centuries up to the 19th century candles were more common in northern Europe, and olive oil lamps more common in southern Europe and around the Mediterranean Sea. Makers of candles were known as chandlers. Today, candles are usually used for their aesthetic value, particularly to set a soft, warm, or romantic ambience, and for emergency lighting during electrical power failures. Scented candles are common in aromatherapy. Small candles are often placed on birthday cakes.