The search for chametz is performed by the light of a single candle. A torch may not be used for this purpose since it is too large to be put into holes or crevices.
Moreover, since one might be worried that a burning torch could start a fire easily in his house, he will probably not be careful to conduct a thorough search. If one used a torch rather than a candle, the search should be conducted again with a candle, although the blessing is not repeated.
The candle used for the search should not be made of tallow, for a person may fear that the candle will drip on food and render it unfit for kosher consumption.
Similarly, it should not be made of meat fat, for he might fear that it will drip on dairy food. Moreover, one should not use an oil lamp, for one might fear that it will drip and stain his clothes. In all these cases, his fears may cause him to be less than careful in his search.
The initial letters of the Hebrew words for tallow, meat fat, oil, and torch [chelev, shuman, shemen, and avukah] form an acronym for the Aramaic word chashasha - fear. As noted, we do not use any of these to conduct the search since we fear that their use will prevent us from being thorough.
One should use a candle made of wax, for it gives bright light and if it drips, it will not stain nor will it render food unfit for kosher consumption. If one nevertheless used a different type of candle, e.g., one of those mentioned previously, the search need not be repeated.
If one has no wax candle, he should use any candle he can find, provided that it is not a torch. A candle with two wicks is considered a torch. Similarly, if one holds two candles together, even if he melts them together, it is considered to be a torch, and the search is invalid.