I have been burning incense vertically, in the same direction that most people have been burning.
First, I light up the incense, place it on a holder and enjoy. In the end, I would clean up incense ash with a wet cloth around the incense holder. Alternatively, I could use an incense tray to collect the ash and tip them off the bin.
The horizontal burn
One day, I decided to lay the incense down to burn. I lit it up while working on something else.
After 30 minutes, the incense had finished burning. And I looked at the ash, and I found something unusual: the ash form was not continuous. There are gaps between them.
Initially, I thought it could be the breeze but there was no wind in my room. So I changed to a different room with a different incense tray.
After 30 minutes, there was a gap again, but in different position.
Out of curiosity, I decided to try different length and thickness of different incense to see if there would be any gaps between the ashes.
If the incense left ashes after burning, why there were gaps among them?
Did the incense burn completely without ash? Or
Did the ash move? If the ash moved, how could it move?
To get a better understanding, I filmed a 30-minute video and condensed them 37 seconds. Have a look, you may find it is interesting.
Here is the single incense I film indoor
So here are the reasons why the incense ashes seem to "disappear" (*)
If you watch the video, you will notice the ash did not disappear, it was "pulled" down by the burning part- the hottest point.
The heat energy moves from a higher to a lower temperature. It means the "pulled" force will move with the same direction of the fire.
Incense is not perfect straight
During drying, some incense may slightly bend. When we burn this not-so-straight bending incense, there is a good chance it will break at the bended or curved point. When the ash break and is pulled down, it will create a small gap in between.
Incense made with resin wood will have different density. Some parts will have more resin than others. The heat energy produced will be different when burning different material. So the "pulling" force is not consistent. If it is stronger the the molecule bonding of the ash, the ash will break and create gaps
Why did you only see these gaps in horizontal burn?
The molecule bonds of ashes were weaken by heat and they will be pulled down by gravity. You will not be likely to see a whole incense in ash form vertically.
(*) Please note: The above is my thought only and I could be wrong. If you have a better explanation, kindly email me. With your permission, I will post your answer and credit with your name