Black smoke is from incomplete combustion of an organic (nonmetal) compound and this results in what is commonly known as soot, or basically gaseous carbon(amorphic – no defined shape whereas most forms of carbon have a rigid/crystalline structure)
as for the state of the flame that is all based upon temperature of the room and flame as well as any sort of air density/movement, and the ingredients in the candle can cause it to sparkle. Also the way the candle melts and how much of the wick is exposed.
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