On of the factors that causes container candles to produce soot is turbulent air flow. Candles in any type of enclosure have to fight to regulate the way the air passes in and out of the container. As the flame heats up the air inside the jar candle or other container it rises to the top and out. At the same time, cooler air is being drawn into the container to replace the rising hot air. In an unregulated environment the cool and hot air have to fight get past each other. This causes turbulent air inside the container which makes the flame dance and flicker and put off soot. I’m sure you have seen little round metal or ceramic disks called candle cappers. These have a large hole in the center and smaller holes around the outside. Other than being decorative these have a purpose which is to regulate the air flow in jar candles. The hot air come up through the center hole and the cooler air fills back into the jar through the smaller outer holes. You will notice a big difference in how still the flame stands still if you use one of these.
Another instance where the air flow problem occurs is in hurricanes. Many wedding decorators and brides use hurricane candle holders for their wedding decorations. A small tip that makes a big difference is to place 3 pennies (in the shape of a triangle) under the base of the hurricane shade. This allows cool air to flow in the bottom and hot air to go out the top.