Why is hot wood ash more fluid than cold ash? When I remove ash from my fireplace I do so by scraping it away through holes about 5 millimeters wide in the fireplace floor. Whenever the ash is hot, it definitely feels much more fluid than when it is cold. 
So it seems to me that interparticle friction diminishes with temperature. However googling around I found this paper those conclusion is that up to 500°C there is no significant effect of temperature on the flow properties of different fine powders.
So I wonder. What mechanisms could be involved in the ash becoming more fluid when hot? Is it a case of fluidization due to interaction with hot air when scraping? Or something else ? 
 A: Wood ash is most mineral. Opinions seem to differ about the composition, but it's things like calcium and potassium carbonates, phosphates and oxides.
The minerals in wood are distributed throughout the wood, so as the organic material burns away you are left with a very fine network of aggregated particles of the minerals. Googling will find you lots of electron micrographs showing the morphology. I found this nice picture from Getty Images:

This structure is mostly air and has a very low density and is highly friable. That means under an applied stress it will flow almost like a liquid so it's easy to push it through the holes you mention.
The trouble is that the minerals in the ash are soluble or slightly soluble in water, and they somewhat hygroscopic. That means when left to cool under ambient conditions the ash absorbs water from the air and restructures to form a considerably denser morphology that doesn't flow as well and tends to stick together.
So it isn't the temperature that is the primary cause of the change. If you put some of the hot ash into a vaccum chamber and left it to cool there then it should retain its light fluffy texture. Sadly this is not a practical way of cleaning your fireplace.
