A peculiar thing happens when you have a combustion source (candles, kerosene heater, fireplace, wood stove, etc.) in a building that is poorly insulated. The particulates in the air will be attracted to the cold spots on the walls or ceilings. I have seen this with framing. After a heating season there are “shadows” of all the framing members showing up on the sheetrock or plaster walls. It washes off, but it is a good indication that the insulation is severely lacking.
Well, yesterday, on a site visit, I saw a great illustration of this occurrence. The nail heads that were under the plaster were showing up as black spots on the wall. This was a very good sign that the insulation retrofit already underway was a good decision!