Candle wicks are generally made out of cotton that has been chemically treated to make it burn more slowly than if it were left untreated. Some wicks contain other materials at their cores to make the wicks either burn cooler or to make the wick more rigid.
A thin strand of zinc is sometimes used in the center of a candle wick to make the wick stand up better.
A cotton core wick has a tough cotton strand in the center that the rest of the cotton of the wick is woven around.
A wick with a paper core is sometimes used as an alternative to a wick with a zinc core. These have a rigid strand of paper at the center.
A wick that is made from braided cotton has nothing at its core, giving it no rigidity.
Wicks once had lead cores, though this practice stopped in the 1970s. Never burn candles made in the 1970s or earlier because of the possibility of lead in the wicks.