When you burn wood or coal, you get ashes, right? In a fireplace, or at the campground. The ashes consist of the non-combustible minerals that were part of the fuel.
If the fire burned out on its own, then pretty much all of the carbon has gotten burned out too.
If you poke around in the ashes, they are usually light and fluffy.
That’s what happens in an open fire, for the most part, because there is no forced air added.
But, if you add forced air, the fire gets hotter. This higher temperature tends to melt the ashes into a glassy sort of material that we call “clinker”. Still NO heat value.
We I was a kid, everybody in the neighborhood heated with coal. we had lots of clinker to contend with. The only use we had for it was to throw onto the driveway in the winter to get the car unstuck.
I, too, do a fair amount of forging. We actaully have clinker contests, to see who can get the biggest solid “donut” out of the forge after a day’s work.
Our success varies a lot with the coal itself. Some of it has a lot of ash (AKA clinker, in a forge) and some not so much.
Moral of this tale (from my perspective):
If your grate is getting very hot, let’s say over 1800 degrees F. or so, you will get clinker.
If your grate only runs at 1000 to 1200 or less, you won’t get much clinker.
These temps are only approximate, just to make the point that clinker = hot, ashes = not so hot.