I've been making candles for 9 years and have never had this happen. I have white curtains in my candle making room and the only thing on those is dust.:rolleyes2 I don't have a furnace I have centeral heating and cooling but my mother does and she has black stuff on her registers that she said comes from the furnace. But that is the only place she or I have ever seen it. I don't believe its the candle making.