How backflow incense works, and why the smoke falls

Backflow incense looks like a trick and it is not one. The smoke behaves exactly as smoke should: it is a fluid, and fluids move according to how heavy they are compared with what is around them. Ordinary incense smoke rises because it is hot. Backflow smoke falls because by the time you see it, it has cooled down and it is still carrying a lot of solid matter. Everything else is plumbing.

Smoke is a fluid, and this one is heavy

Smoke is not a substance in its own right. It is air with two things suspended in it: fine solid particles of carbon and ash, and droplets of organic matter that boiled off the burning material and condensed again as they cooled. The whole mixture has a density, and whether it rises or falls depends only on whether that density is lower or higher than the air in the room.

Hot air is less dense than cool air, which is why most smoke rises. But the particles and droplets are far heavier than air, and there are enough of them in incense smoke to matter. Cool a well loaded smoke down to room temperature and the mixture ends up slightly heavier than the air it is sitting in. At that point gravity takes over and it runs downhill like water.

That is the entire mechanism. Nothing is pushing the smoke down. It is falling.

Why ordinary incense smoke goes up

At the burning tip of any incense the gases are hot. A smouldering cone has no proper flame, but the ember still runs at several hundred degrees, which is hot enough that the gas around it expands, becomes much less dense than the room, and lifts. It carries the particles with it in a rising plume.

By the time that plume has cooled enough to be heavier than the room, it has travelled a long way up, mixed with a great deal of clean air and diluted to the point where you cannot see it any more. The sinking still happens, in principle. It just happens in slow motion, somewhere near your ceiling, with far too little smoke left in it to look like anything.

To get a visible fall you have to cool the smoke quickly, before it has risen and before it has diluted. That is what the hole in the cone is for.

The hole through the middle of the cone

A backflow cone has a bore a few millimetres across running from the base up to near the tip. An ordinary cone is solid. This is the only difference that matters, and it is why cones bored through the middle are not interchangeable with ordinary ones.

When the cone is lit, the tip smoulders and produces smoke at the top as usual. But smoke generated inside the body of the cone, just under the burning front, has a second route available: down the bore. The bore is narrow, its walls are unburnt cone and then cool ceramic, and the smoke gives up heat to them fast. By the time it reaches the bottom it is close to room temperature and still fully loaded with particles.

Cool and loaded is the condition for falling. The cone is a small heat exchanger that happens to be made of the fuel.

What the burner actually does

The well that the cone sits in has a hole in the bottom, and that hole opens into a channel moulded into the body of the burner. On a crescent shaped burner the channel runs down the inside of the curve and lets the smoke out lower down, so the fall starts part way along the piece rather than at the base of the cone.

The channel does two jobs. It takes more heat out of the smoke, because glazed ceramic is a decent heat sink and the smoke is in contact with it for the whole length of the run. And it keeps the falling smoke together. A column of dense smoke that stays in one piece keeps falling. One that spreads out mixes with room air, loses its density advantage and stops being visible.

That is why the exit is usually a narrow slot or a small mouth rather than a wide opening, and why the height of the drop below that exit matters. Give the smoke somewhere to fall and it falls further.

Why it looks like a waterfall rather than a mist

A dense fluid spreading along the ground underneath a lighter one is called a gravity current, and it is a shape you have seen before. Fog settling into the bottom of a valley, cold air spilling out of an open chest freezer and the smoke off a backflow burner are all the same flow.

A gravity current needs a continuous body of dense fluid to work, which is part of why these burners take a minute or two to get going. The cone has to establish a proper ember, the channel has to warm up and fill, and only then is there enough smoke in one place to form a column rather than a few wisps. Once the column exists it tends to persist, because it is moving as one body rather than mixing away at its edges.

When the column reaches a flat surface it stops falling and spreads sideways in a thin layer, which is the pooling you see around the base. The layer is thin because it has run out of anywhere else to go, not because anything is containing it.

The forces involved are tiny

It is worth being clear about scale. The density difference driving all of this is small, well under one per cent, and the flow it produces is slow: a few centimetres per second. There is no pump and no pressure to speak of.

That is why the effect is so easy to break. A window on the latch, a door opening at the other end of the room, a radiator under the shelf, a laptop fan, or simply walking past will produce air movement far stronger than the flow you are trying to watch. A still room is not an aesthetic preference here. It is a requirement.

It is also why the burner has to stand on something flat and clear. The smoke needs a continuous surface to run down and then across. Put one on a cloth with a fold in it, or hard against the back of a shelf, and the column breaks up on the obstruction and the whole thing turns into haze.

Questions people actually ask

Why does backflow incense smoke go down instead of up?

Because by the time it leaves the burner it has cooled to roughly room temperature while still carrying a lot of solid particles, which makes the mixture slightly heavier than the air around it. It falls for the same reason cold air spills out of an open freezer.

Can I use normal incense cones in a backflow burner?

No. Backflow cones have a hole bored up the middle so smoke can travel down through the cone and into the burner's channel. A solid cone gives the smoke no route down, so it simply rises off the top.

Does a backflow burner need to be a particular shape?

It needs a hole under the cone, a channel to carry the smoke down and a drop below the exit. Beyond that the shape only changes how the fall looks, and a taller piece gives a longer visible column.